On Fri, Aug 29, 2025 at 12:27:13PM -0700, Stefano Stabellini wrote:
> On Thu, 28 Aug 2025, [email protected] wrote:
> > From: Denis Mukhin <[email protected]>
> >
> > Introduce a driver framework to abstract UART emulators in the hypervisor.
> >
> > That allows for architecture-independent handling of virtual UARTs in the
> > console driver and simplifies enabling new UART emulators.
> >
> > The framework is built under CONFIG_VUART_FRAMEWORK, which will be
> > automatically enabled once the user enables any UART emulator.
> >
> > Current implementation supports maximum of one vUART of each kind per
> > domain.
> >
> > Use new domain_has_vuart() in the console driver code to check whether to
> > forward console input to the domain using vUART.
> >
> > Enable console forwarding over vUART for hardware domains with a vUART. That
> > enables console forwarding to dom0 on x86, since console can be forwarded
> > only
> > to Xen, dom0 and pvshim on x86 as of now.
> >
> > Note: existing vUARTs are deliberately *not* hooked to the new framework to
> > minimize the scope of the patch: vpl011 (i.e. SBSA) emulator and "vuart"
> > (i.e.
> > minimalistic MMIO-mapped dtuart for hwdoms on Arm) are kept unmodified.
> >
> > No functional changes for non-x86 architectures.
> >
> > Signed-off-by: Denis Mukhin <[email protected]>
> > ---
> > Changes since v4:
> > - addressed feedback
> > - Link to v4:
> > https://lore.kernel.org/xen-devel/[email protected]/
> > ---
> > xen/arch/arm/xen.lds.S | 1 +
> > xen/arch/ppc/xen.lds.S | 1 +
> > xen/arch/riscv/xen.lds.S | 1 +
> > xen/arch/x86/xen.lds.S | 1 +
> > xen/common/Kconfig | 2 +
> > xen/common/Makefile | 1 +
> > xen/common/emul/Kconfig | 6 ++
> > xen/common/emul/Makefile | 1 +
> > xen/common/emul/vuart/Kconfig | 6 ++
> > xen/common/emul/vuart/Makefile | 1 +
> > xen/common/emul/vuart/vuart.c | 156 +++++++++++++++++++++++++++++++++
> > xen/common/keyhandler.c | 3 +
> > xen/drivers/char/console.c | 6 +-
> > xen/include/xen/serial.h | 3 +
> > xen/include/xen/vuart.h | 116 ++++++++++++++++++++++++
> > xen/include/xen/xen.lds.h | 10 +++
> > 16 files changed, 314 insertions(+), 1 deletion(-)
> > create mode 100644 xen/common/emul/Kconfig
> > create mode 100644 xen/common/emul/Makefile
> > create mode 100644 xen/common/emul/vuart/Kconfig
> > create mode 100644 xen/common/emul/vuart/Makefile
> > create mode 100644 xen/common/emul/vuart/vuart.c
> > create mode 100644 xen/include/xen/vuart.h
> >
> > diff --git a/xen/arch/arm/xen.lds.S b/xen/arch/arm/xen.lds.S
> > index db17ff1efa98..cd05b18770f4 100644
> > --- a/xen/arch/arm/xen.lds.S
> > +++ b/xen/arch/arm/xen.lds.S
> > @@ -58,6 +58,7 @@ SECTIONS
> > *(.rodata)
> > *(.rodata.*)
> > VPCI_ARRAY
> > + VUART_ARRAY
> > *(.data.rel.ro)
> > *(.data.rel.ro.*)
> >
> > diff --git a/xen/arch/ppc/xen.lds.S b/xen/arch/ppc/xen.lds.S
> > index 1de0b77fc6b9..f9d4e5b0dcd8 100644
> > --- a/xen/arch/ppc/xen.lds.S
> > +++ b/xen/arch/ppc/xen.lds.S
> > @@ -52,6 +52,7 @@ SECTIONS
> > *(.rodata)
> > *(.rodata.*)
> > VPCI_ARRAY
> > + VUART_ARRAY
> > *(.data.rel.ro)
> > *(.data.rel.ro.*)
> >
> > diff --git a/xen/arch/riscv/xen.lds.S b/xen/arch/riscv/xen.lds.S
> > index edcadff90bfe..59dcaa5fef9a 100644
> > --- a/xen/arch/riscv/xen.lds.S
> > +++ b/xen/arch/riscv/xen.lds.S
> > @@ -47,6 +47,7 @@ SECTIONS
> > *(.rodata)
> > *(.rodata.*)
> > VPCI_ARRAY
> > + VUART_ARRAY
> > *(.data.rel.ro)
> > *(.data.rel.ro.*)
> >
> > diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
> > index 966e514f2034..d877b93a6964 100644
> > --- a/xen/arch/x86/xen.lds.S
> > +++ b/xen/arch/x86/xen.lds.S
> > @@ -132,6 +132,7 @@ SECTIONS
> > *(.rodata)
> > *(.rodata.*)
> > VPCI_ARRAY
> > + VUART_ARRAY
> > *(.data.rel.ro)
> > *(.data.rel.ro.*)
> >
> > diff --git a/xen/common/Kconfig b/xen/common/Kconfig
> > index 76f9ce705f7a..78a32b69e2b2 100644
> > --- a/xen/common/Kconfig
> > +++ b/xen/common/Kconfig
> > @@ -676,4 +676,6 @@ config PM_STATS
> > Enable collection of performance management statistics to aid in
> > analyzing and tuning power/performance characteristics of the system
> >
> > +source "common/emul/Kconfig"
> > +
> > endmenu
> > diff --git a/xen/common/Makefile b/xen/common/Makefile
> > index c316957fcb36..c0734480ee4b 100644
> > --- a/xen/common/Makefile
> > +++ b/xen/common/Makefile
> > @@ -11,6 +11,7 @@ obj-$(filter-out $(CONFIG_X86),$(CONFIG_ACPI)) += device.o
> > obj-$(CONFIG_DEVICE_TREE_PARSE) += device-tree/
> > obj-$(CONFIG_IOREQ_SERVER) += dm.o
> > obj-y += domain.o
> > +obj-y += emul/
> > obj-y += event_2l.o
> > obj-y += event_channel.o
> > obj-$(CONFIG_EVTCHN_FIFO) += event_fifo.o
> > diff --git a/xen/common/emul/Kconfig b/xen/common/emul/Kconfig
> > new file mode 100644
> > index 000000000000..7c6764d1756b
> > --- /dev/null
> > +++ b/xen/common/emul/Kconfig
> > @@ -0,0 +1,6 @@
> > +menu "Domain Emulation Features"
> > + visible if EXPERT
> > +
> > +source "common/emul/vuart/Kconfig"
> > +
> > +endmenu
> > diff --git a/xen/common/emul/Makefile b/xen/common/emul/Makefile
> > new file mode 100644
> > index 000000000000..ae0b575c3901
> > --- /dev/null
> > +++ b/xen/common/emul/Makefile
> > @@ -0,0 +1 @@
> > +obj-$(CONFIG_VUART_FRAMEWORK) += vuart/
> > diff --git a/xen/common/emul/vuart/Kconfig b/xen/common/emul/vuart/Kconfig
> > new file mode 100644
> > index 000000000000..ce1b976b7da7
> > --- /dev/null
> > +++ b/xen/common/emul/vuart/Kconfig
> > @@ -0,0 +1,6 @@
> > +config VUART_FRAMEWORK
> > + bool
> > +
> > +menu "UART Emulation"
> > +
> > +endmenu
> > diff --git a/xen/common/emul/vuart/Makefile b/xen/common/emul/vuart/Makefile
> > new file mode 100644
> > index 000000000000..97f792dc6641
> > --- /dev/null
> > +++ b/xen/common/emul/vuart/Makefile
> > @@ -0,0 +1 @@
> > +obj-y += vuart.o
> > diff --git a/xen/common/emul/vuart/vuart.c b/xen/common/emul/vuart/vuart.c
> > new file mode 100644
> > index 000000000000..7b277d00d5c7
> > --- /dev/null
> > +++ b/xen/common/emul/vuart/vuart.c
> > @@ -0,0 +1,156 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +/*
> > + * UART emulator framework.
> > + *
> > + * Copyright 2025 Ford Motor Company
> > + */
> > +
> > +#include <xen/err.h>
> > +#include <xen/sched.h>
> > +#include <xen/vuart.h>
> > +#include <xen/xvmalloc.h>
> > +
> > +#define for_each_emulator(e) \
> > + for ( e = vuart_array_start; e < vuart_array_end; e++ )
> > +
> > +extern const struct vuart_emulator vuart_array_start[];
> > +extern const struct vuart_emulator vuart_array_end[];
> > +
> > +static const struct vuart_emulator *
> > +vuart_match_by_compatible(struct domain *d, const char *compat)
> > +{
> > + const struct vuart_emulator *emulator;
> > +
> > + if ( d->console.vuart )
> > + return NULL;
> > +
> > + for_each_emulator(emulator)
> > + if ( emulator->compatible &&
> > + !strncmp(emulator->compatible, compat,
> > + strlen(emulator->compatible)) )
>
> strncmp will continue until the given count even if compat is shorter
I checked lib/strncmp.c. I'll flip the arguments, so that strncmp() will
return early if compat is shorter than emulator->compatible
>
>
> > + return emulator;
> > +
> > + return NULL;
> > +}
> > +
> > +static struct vuart *vuart_find_by_console_permission(const struct domain
> > *d)
> > +{
> > + struct vuart *vuart = d->console.vuart;
> > +
> > + ASSERT(d->console.input_allowed);
>
> This ASSERT looks suspicious as we haven't even checked that vuart!=NULL
> yet. I would remove it
Ack
>
>
> > + if ( !vuart || !vuart->emulator || !vuart->emulator->put_rx ||
> > + !(vuart->flags & VUART_CONSOLE_INPUT))
> > + return NULL;
> > +
> > + return vuart;
> > +}
> > +
> > +struct vuart *vuart_find_by_io_range(struct domain *d, unsigned long addr,
> > + unsigned long size)
> > +{
> > + struct vuart *vuart = d->console.vuart;
> > +
> > + if ( !vuart || !vuart->info )
> > + return NULL;
>
> You might as well call vuart_find_by_console_permission that has all the
> checks already
vuart_find_by_io_range() is used in the I/O hook to find the vUART by its
I/O port range.
Using vuart_find_by_console_permission() here may be problematic if the
vUART does not support/enable console forwarding.
I'll keep this check as is for now.
>
> > +
> > + if ( addr >= vuart->info->base_addr &&
> > + addr + size - 1 <= vuart->info->base_addr + vuart->info->size - 1
> > )
> > + return vuart;
> > +
> > + return NULL;
> > +}
> > +
> > +int vuart_init(struct domain *d, struct vuart_info *info)
> > +{
> > + const struct vuart_emulator *emulator;
> > + struct vuart *vuart;
> > + int rc;
> > +
> > + emulator = vuart_match_by_compatible(d, info->compatible);
> > + if ( !emulator )
> > + return -ENODEV;
> > +
> > + vuart = xzalloc(typeof(*vuart));
> > + if ( !vuart )
> > + return -ENOMEM;
> > +
> > + vuart->info = xvzalloc(typeof(*info));
> > + if ( !vuart->info )
> > + {
> > + rc = -ENOMEM;
> > + goto err_out;
> > + }
> > + memcpy(vuart->info, info, sizeof(*info));
>
> one thing to note is that the fields (strings) compatible and name are
> copied by address, I am not if it is OK but FYI
Thanks, will update vuart_info struct.
>
>
> > + vuart->vdev = emulator->alloc(d, vuart->info);
> > + if ( IS_ERR(vuart->vdev) )
> > + {
> > + rc = PTR_ERR(vuart->vdev);
> > + goto err_out;
>
> this path leads to vuart->info not being freed
Thanks
>
>
> > + }
> > +
> > + vuart->emulator = emulator;
> > + vuart->owner = d;
> > + vuart->flags |= VUART_CONSOLE_INPUT;
> > +
> > + d->console.input_allowed = true;
> > + d->console.vuart = vuart;
> > +
> > + return 0;
> > +
> > + err_out:
> > + XVFREE(vuart);
> > + return rc;
> > +}
> > +
> > +/*
> > + * Release any resources taken by UART emulators.
> > + *
> > + * NB: no flags are cleared, since currently exit() is called only during
> > + * domain destroy.
> > + */
> > +void vuart_deinit(struct domain *d)
> > +{
> > + struct vuart *vuart = d->console.vuart;
> > +
> > + if ( vuart )
> > + {
> > + vuart->emulator->free(vuart);
>
> should we pass vuart or vuart->vdev here? The emulator state is supposed
> to be vuart->vdev?
That should be vuart->vdev; thanks!
>
> > + XVFREE(vuart->info);
> > + }
> > +
> > + XVFREE(d->console.vuart);
> > +}
> > +
> > +void vuart_dump_state(const struct domain *d)
> > +{
> > + struct vuart *vuart = d->console.vuart;
> > +
> > + if ( vuart )
> > + vuart->emulator->dump_state(vuart);
>
> also here vuart->vdev?
Ack
>
>
> > +}
> > +
> > +/*
> > + * Put character to the *first* suitable emulated UART's FIFO.
> > + */
> > +int vuart_put_rx(struct domain *d, char c)
> > +{
> > + struct vuart *vuart = vuart_find_by_console_permission(d);
> > +
> > + return vuart ? vuart->emulator->put_rx(vuart, c) : -ENODEV;
>
> and here?
Ack
>
>
> > +}
> > +
> > +bool domain_has_vuart(const struct domain *d)
> > +{
> > + return vuart_find_by_console_permission(d);
> > +}
> > +
> > +/*
> > + * Local variables:
> > + * mode: C
> > + * c-file-style: "BSD"
> > + * c-basic-offset: 4
> > + * indent-tabs-mode: nil
> > + * End:
> > + */
> > diff --git a/xen/common/keyhandler.c b/xen/common/keyhandler.c
> > index cb6df2823b00..156e64d9eb58 100644
> > --- a/xen/common/keyhandler.c
> > +++ b/xen/common/keyhandler.c
> > @@ -22,6 +22,7 @@
> > #include <xen/mm.h>
> > #include <xen/watchdog.h>
> > #include <xen/init.h>
> > +#include <xen/vuart.h>
> > #include <asm/div64.h>
> >
> > static unsigned char keypress_key;
> > @@ -352,6 +353,8 @@ static void cf_check dump_domains(unsigned char key)
> > v->periodic_period / 1000000);
> > }
> > }
> > +
> > + vuart_dump_state(d);
> > }
> >
> > for_each_domain ( d )
> > diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
> > index 9bd5b4825da6..d5164897a776 100644
> > --- a/xen/drivers/char/console.c
> > +++ b/xen/drivers/char/console.c
> > @@ -33,6 +33,7 @@
> > #include <asm/setup.h>
> > #include <xen/sections.h>
> > #include <xen/consoled.h>
> > +#include <xen/vuart.h>
> >
> > #ifdef CONFIG_X86
> > #include <asm/guest.h>
> > @@ -596,11 +597,12 @@ static void __serial_rx(char c)
> > if ( !d )
> > return;
> >
> > - if ( is_hardware_domain(d) )
> > + if ( is_hardware_domain(d) && !domain_has_vuart(d) )
> > {
> > /*
> > * Deliver input to the hardware domain buffer, unless it is
> > * already full.
> > + * NB: must be the first check: hardware domain may have emulated
> > UART.
> > */
> > if ( (serial_rx_prod - serial_rx_cons) != SERIAL_RX_SIZE )
> > serial_rx_ring[SERIAL_RX_MASK(serial_rx_prod++)] = c;
> > @@ -611,6 +613,8 @@ static void __serial_rx(char c)
> > */
> > send_global_virq(VIRQ_CONSOLE);
> > }
> > + else if ( domain_has_vuart(d) )
> > + rc = vuart_put_rx(d, c);
> > #ifdef CONFIG_SBSA_VUART_CONSOLE
> > else
> > /* Deliver input to the emulated UART. */
> > diff --git a/xen/include/xen/serial.h b/xen/include/xen/serial.h
> > index 8e1844555208..d7e81f098359 100644
> > --- a/xen/include/xen/serial.h
> > +++ b/xen/include/xen/serial.h
> > @@ -36,6 +36,9 @@ struct vuart_info {
> > unsigned long data_off; /* Data register offset */
> > unsigned long status_off; /* Status register offset */
> > unsigned long status; /* Ready status value */
> > + unsigned int irq; /* Interrupt */
> > + const char *compatible; /* Compatible string */
> > + const char *name; /* User-friendly name */
> > };
> >
> > struct serial_port {
> > diff --git a/xen/include/xen/vuart.h b/xen/include/xen/vuart.h
> > new file mode 100644
> > index 000000000000..ca025b4179be
> > --- /dev/null
> > +++ b/xen/include/xen/vuart.h
> > @@ -0,0 +1,116 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +/*
> > + * UART emulator framework.
> > + *
> > + * Copyright 2025 Ford Motor Company
> > + */
> > +
> > +#ifndef XEN_VUART_H
> > +#define XEN_VUART_H
> > +
> > +#include <xen/serial.h>
> > +#include <public/xen.h>
> > +
> > +struct vuart_emulator;
> > +
> > +enum {
> > + VUART_CONSOLE_INPUT = 0U << 1, /** Physical console input forwarding.
> > */
>
> code style: single *
Ack
> This flag is zero, is that intended?
Will fix, thanks
>
>
> > +};
> > +
> > +
> > +/*
> > + * FIXME: #ifdef is temporary to avoid clash with
> > + * arch/arm/include/asm/domain.h
> > + */
> > +#ifdef CONFIG_VUART_FRAMEWORK
> > +struct vuart {
> > + const struct vuart_emulator *emulator;
> > + struct vuart_info *info;
> > + struct domain *owner;
> > + uint32_t flags;
> > + void *vdev;
> > +};
> > +#endif
> > +
> > +struct vuart_emulator {
> > + /* UART compatible string. Cannot be NULL or empty. */
> > + const char *compatible;
> > +
> > + /*
> > + * Allocate emulated UART state (RX/TX FIFOs, locks, initialize
> > registers,
> > + * hook I/O handlers, etc.)
> > + * Cannot be NULL.
> > + */
> > + void *(*alloc)(struct domain *d, const struct vuart_info *info);
> > +
> > + /*
> > + * Release resources used to emulate UART state (flush RX/TX FIFOs,
> > unhook
> > + * I/O handlers, etc.).
> > + * Cannot be NULL.
> > + */
> > + void (*free)(void *arg);
> > +
> > + /*
> > + * Print emulated UART state, including registers, on the console.
> > + * Can be NULL.
> > + */
> > + void (*dump_state)(void *arg);
> > +
> > + /*
> > + * Place character to the emulated RX FIFO.
> > + * Used to forward physical console input to the guest OS.
> > + * Can be NULL.
> > + */
> > + int (*put_rx)(void *arg, char c);
> > +};
> > +
> > +#define VUART_REGISTER(name, x) \
> > + static const struct vuart_emulator name##_entry \
> > + __used_section(".data.rel.ro.vuart") = x
> > +
> > +struct vuart *vuart_find_by_io_range(struct domain *d,
> > + unsigned long base_addr,
> > + unsigned long size);
> > +
> > +int vuart_put_rx(struct domain *d, char c);
> > +
> > +#ifdef CONFIG_VUART_FRAMEWORK
> > +
> > +int vuart_init(struct domain *d, struct vuart_info *info);
> > +void vuart_deinit(struct domain *d);
> > +void vuart_dump_state(const struct domain *d);
> > +bool domain_has_vuart(const struct domain *d);
> > +
> > +#else
> > +
> > +static inline int vuart_init(struct domain *d, struct vuart_info *info)
> > +{
> > + return 0;
> > +}
> > +
> > +static inline void vuart_deinit(struct domain *d)
> > +{
> > +}
> > +
> > +static inline void vuart_dump_state(const struct domain *d)
> > +{
> > +}
> > +
> > +static inline bool domain_has_vuart(const struct domain *d)
> > +{
> > + return false;
> > +}
> > +
> > +#endif /* CONFIG_VUART_FRAMEWORK */
> > +
> > +#endif /* XEN_VUART_H */
> > +
> > +/*
> > + * Local variables:
> > + * mode: C
> > + * c-file-style: "BSD"
> > + * c-basic-offset: 4
> > + * indent-tabs-mode: nil
> > + * End:
> > + */
> > +
> > diff --git a/xen/include/xen/xen.lds.h b/xen/include/xen/xen.lds.h
> > index b126dfe88792..2d65f32ddad3 100644
> > --- a/xen/include/xen/xen.lds.h
> > +++ b/xen/include/xen/xen.lds.h
> > @@ -194,4 +194,14 @@
> > #define VPCI_ARRAY
> > #endif
> >
> > +#ifdef CONFIG_VUART_FRAMEWORK
> > +#define VUART_ARRAY \
> > + . = ALIGN(POINTER_ALIGN); \
> > + vuart_array_start = .; \
> > + *(.data.rel.ro.vuart) \
> > + vuart_array_end = .;
> > +#else
> > +#define VUART_ARRAY
> > +#endif
> > +
> > #endif /* __XEN_LDS_H__ */
> > --
> > 2.51.0
> >
