On Wed, Aug 23, 2023 at 02:29:02AM -0700, Mattias Nissler wrote:
> When DMA memory can't be directly accessed, as is the case when
> running the device model in a separate process without shareable DMA
> file descriptors, bounce buffering is used.
>
> It is not uncommon for device models to request mapping of several DMA
> regions at the same time. Examples include:
> * net devices, e.g. when transmitting a packet that is split across
> several TX descriptors (observed with igb)
> * USB host controllers, when handling a packet with multiple data TRBs
> (observed with xhci)
>
> Previously, qemu only provided a single bounce buffer and would fail DMA
> map requests while the buffer was already in use. In turn, this would
> cause DMA failures that ultimately manifest as hardware errors from the
> guest perspective.
>
> This change allocates DMA bounce buffers dynamically instead of
> supporting only a single buffer. Thus, multiple DMA mappings work
> correctly also when RAM can't be mmap()-ed.
>
> The total bounce buffer allocation size is limited by a new command line
> parameter. The default is 4096 bytes to match the previous maximum
> buffer size. It is expected that suitable limits will vary quite a bit
> in practice depending on device models and workloads.
>
> Signed-off-by: Mattias Nissler <mniss...@rivosinc.com>
> ---
> include/sysemu/sysemu.h | 2 +
> qemu-options.hx | 27 +++++++++++++
> softmmu/globals.c | 1 +
> softmmu/physmem.c | 84 +++++++++++++++++++++++------------------
> softmmu/vl.c | 6 +++
> 5 files changed, 83 insertions(+), 37 deletions(-)
>
> diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
> index 25be2a692e..c5dc93cb53 100644
> --- a/include/sysemu/sysemu.h
> +++ b/include/sysemu/sysemu.h
> @@ -61,6 +61,8 @@ extern int nb_option_roms;
> extern const char *prom_envs[MAX_PROM_ENVS];
> extern unsigned int nb_prom_envs;
>
> +extern uint64_t max_bounce_buffer_size;
> +
> /* serial ports */
>
> /* Return the Chardev for serial port i, or NULL if none */
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 29b98c3d4c..6071794237 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4959,6 +4959,33 @@ SRST
> ERST
> #endif
>
> +DEF("max-bounce-buffer-size", HAS_ARG,
> + QEMU_OPTION_max_bounce_buffer_size,
> + "-max-bounce-buffer-size size\n"
> + " DMA bounce buffer size limit in bytes (default=4096)\n",
> + QEMU_ARCH_ALL)
> +SRST
> +``-max-bounce-buffer-size size``
> + Set the limit in bytes for DMA bounce buffer allocations.
> +
> + DMA bounce buffers are used when device models request memory-mapped
> access
> + to memory regions that can't be directly mapped by the qemu process, so
> the
> + memory must read or written to a temporary local buffer for the device
> + model to work with. This is the case e.g. for I/O memory regions, and
> when
> + running in multi-process mode without shared access to memory.
> +
> + Whether bounce buffering is necessary depends heavily on the device model
> + implementation. Some devices use explicit DMA read and write operations
> + which do not require bounce buffers. Some devices, notably storage, will
> + retry a failed DMA map request after bounce buffer space becomes
> available
> + again. Most other devices will bail when encountering map request
> failures,
> + which will typically appear to the guest as a hardware error.
> +
> + Suitable bounce buffer size values depend on the workload and guest
> + configuration. A few kilobytes up to a few megabytes are common sizes
> + encountered in practice.
Does it mean that the default 4K size can still easily fail with some
device setup?
IIUC the whole point of limit here is to make sure the allocation is still
bounded, while 4K itself is not a hard limit. Making it bigger would be,
IMHO, nice if it should work with known configs which used to be broken.
> +ERST
> +
> DEFHEADING()
>
> DEFHEADING(Generic object creation:)
> diff --git a/softmmu/globals.c b/softmmu/globals.c
> index e83b5428d1..d3cc010717 100644
> --- a/softmmu/globals.c
> +++ b/softmmu/globals.c
> @@ -54,6 +54,7 @@ const char *prom_envs[MAX_PROM_ENVS];
> uint8_t *boot_splash_filedata;
> int only_migratable; /* turn it off unless user states otherwise */
> int icount_align_option;
> +uint64_t max_bounce_buffer_size = 4096;
>
> /* The bytes in qemu_uuid are in the order specified by RFC4122, _not_ in the
> * little-endian "wire format" described in the SMBIOS 2.6 specification.
> diff --git a/softmmu/physmem.c b/softmmu/physmem.c
> index 3df73542e1..9f0fec0c8e 100644
> --- a/softmmu/physmem.c
> +++ b/softmmu/physmem.c
> @@ -50,6 +50,7 @@
> #include "sysemu/dma.h"
> #include "sysemu/hostmem.h"
> #include "sysemu/hw_accel.h"
> +#include "sysemu/sysemu.h"
> #include "sysemu/xen-mapcache.h"
> #include "trace/trace-root.h"
>
> @@ -2904,13 +2905,12 @@ void cpu_flush_icache_range(hwaddr start, hwaddr len)
>
> typedef struct {
> MemoryRegion *mr;
> - void *buffer;
> hwaddr addr;
> - hwaddr len;
> - bool in_use;
> + size_t len;
> + uint8_t buffer[];
> } BounceBuffer;
>
> -static BounceBuffer bounce;
> +static size_t bounce_buffer_size;
>
> typedef struct MapClient {
> QEMUBH *bh;
> @@ -2945,9 +2945,9 @@ void cpu_register_map_client(QEMUBH *bh)
> qemu_mutex_lock(&map_client_list_lock);
> client->bh = bh;
> QLIST_INSERT_HEAD(&map_client_list, client, link);
> - /* Write map_client_list before reading in_use. */
> + /* Write map_client_list before reading bounce_buffer_size. */
> smp_mb();
> - if (!qatomic_read(&bounce.in_use)) {
> + if (qatomic_read(&bounce_buffer_size) < max_bounce_buffer_size) {
> cpu_notify_map_clients_locked();
> }
> qemu_mutex_unlock(&map_client_list_lock);
> @@ -3076,31 +3076,35 @@ void *address_space_map(AddressSpace *as,
> RCU_READ_LOCK_GUARD();
> fv = address_space_to_flatview(as);
> mr = flatview_translate(fv, addr, &xlat, &l, is_write, attrs);
> + memory_region_ref(mr);
>
> if (!memory_access_is_direct(mr, is_write)) {
> - if (qatomic_xchg(&bounce.in_use, true)) {
> + size_t size = qatomic_add_fetch(&bounce_buffer_size, l);
> + if (size > max_bounce_buffer_size) {
> + size_t excess = size - max_bounce_buffer_size;
> + l -= excess;
> + qatomic_sub(&bounce_buffer_size, excess);
> + }
> +
> + if (l == 0) {
> *plen = 0;
> return NULL;
> }
> - /* Avoid unbounded allocations */
> - l = MIN(l, TARGET_PAGE_SIZE);
> - bounce.buffer = qemu_memalign(TARGET_PAGE_SIZE, l);
> - bounce.addr = addr;
> - bounce.len = l;
>
> - memory_region_ref(mr);
> - bounce.mr = mr;
> + BounceBuffer *bounce = g_malloc(l + sizeof(BounceBuffer));
Maybe g_malloc0() would be better?
I just checked that we had target page aligned allocations since the 1st
day (commit 6d16c2f88f2a). I didn't find any clue showing why it was done
like that, but I do have worry on whether any existing caller that may
implicitly relying on an address that is target page aligned. But maybe
not a major issue; I didn't see anything rely on that yet.
> + bounce->mr = mr;
> + bounce->addr = addr;
> + bounce->len = l;
> +
> if (!is_write) {
> flatview_read(fv, addr, MEMTXATTRS_UNSPECIFIED,
> - bounce.buffer, l);
> + bounce->buffer, l);
> }
>
> *plen = l;
> - return bounce.buffer;
> + return bounce->buffer;
> }
>
> -
> - memory_region_ref(mr);
> *plen = flatview_extend_translation(fv, addr, len, mr, xlat,
> l, is_write, attrs);
> fuzz_dma_read_cb(addr, *plen, mr);
> @@ -3114,31 +3118,37 @@ void *address_space_map(AddressSpace *as,
> void address_space_unmap(AddressSpace *as, void *buffer, hwaddr len,
> bool is_write, hwaddr access_len)
> {
> - if (buffer != bounce.buffer) {
> - MemoryRegion *mr;
> - ram_addr_t addr1;
> + MemoryRegion *mr;
> + ram_addr_t addr1;
> +
> + mr = memory_region_from_host(buffer, &addr1);
> + if (mr == NULL) {
> + /*
> + * Must be a bounce buffer (unless the caller passed a pointer which
> + * wasn't returned by address_space_map, which is illegal).
Is it possible to still have some kind of sanity check to make sure it's a
bounce buffer passed in, just in case of a caller bug? Or, the failure can
be weird..
> + */
> + BounceBuffer *bounce = container_of(buffer, BounceBuffer, buffer);
>
> - mr = memory_region_from_host(buffer, &addr1);
> - assert(mr != NULL);
> if (is_write) {
> - invalidate_and_set_dirty(mr, addr1, access_len);
> - }
> - if (xen_enabled()) {
> - xen_invalidate_map_cache_entry(buffer);
> + address_space_write(as, bounce->addr, MEMTXATTRS_UNSPECIFIED,
> + bounce->buffer, access_len);
> }
> - memory_region_unref(mr);
> +
> + memory_region_unref(bounce->mr);
> + qatomic_sub(&bounce_buffer_size, bounce->len);
> + /* Write bounce_buffer_size before reading map_client_list. */
> + smp_mb();
> + cpu_notify_map_clients();
> + g_free(bounce);
> return;
> }
> +
> + if (xen_enabled()) {
> + xen_invalidate_map_cache_entry(buffer);
> + }
> if (is_write) {
> - address_space_write(as, bounce.addr, MEMTXATTRS_UNSPECIFIED,
> - bounce.buffer, access_len);
> - }
> - qemu_vfree(bounce.buffer);
> - bounce.buffer = NULL;
> - memory_region_unref(bounce.mr);
> - /* Clear in_use before reading map_client_list. */
> - qatomic_set_mb(&bounce.in_use, false);
> - cpu_notify_map_clients();
> + invalidate_and_set_dirty(mr, addr1, access_len);
> + }
> }
>
> void *cpu_physical_memory_map(hwaddr addr,
> diff --git a/softmmu/vl.c b/softmmu/vl.c
> index b0b96f67fa..dbe52f5ea1 100644
> --- a/softmmu/vl.c
> +++ b/softmmu/vl.c
> @@ -3469,6 +3469,12 @@ void qemu_init(int argc, char **argv)
> exit(1);
> #endif
> break;
> + case QEMU_OPTION_max_bounce_buffer_size:
> + if (qemu_strtosz(optarg, NULL, &max_bounce_buffer_size) < 0)
> {
> + error_report("invalid -max-ounce-buffer-size value");
> + exit(1);
> + }
> + break;
PS: I had a vague memory that we do not recommend adding more qemu cmdline
options, but I don't know enough on the plan to say anything real.
> case QEMU_OPTION_object:
> object_option_parse(optarg);
> break;
> --
> 2.34.1
>
Thanks,
--
Peter Xu
