On 24/10/2025 17:37, Harry Ramsey wrote:
> Implement reference counting to enable overlapping MPU regions.
> References are incremented and decremented in xen_mpumap_update_entry.
AFAICT, looking at the code, you would return -EINVAL early on overlap (i.e.
mpumap_contains_region() returning MPUMAP_REGION_OVERLAP). If so, can you
clearly explain what this change intend to do and why we need refcounting?
>
> A region will be destoryed if the reference count is 0 upon calling
s/destoryed/destroyed/
> destroy_xen_mappings and if the full region range is specified.
>
> Additionally XEN_MPUMAP_ENTRY_SHIFT and XEN_MPUMAP_ENTRY_SHIFT_ZERO are
> no longer hardcoded and defined inside asm-offsets.c.
>
> Signed-off-by: Harry Ramsey <[email protected]>
> ---
> xen/arch/arm/arm32/asm-offsets.c | 2 +
> xen/arch/arm/arm64/asm-offsets.c | 2 +
> xen/arch/arm/include/asm/arm32/mpu.h | 2 +
> xen/arch/arm/include/asm/arm64/mpu.h | 2 +
> xen/arch/arm/include/asm/mpu/regions.inc | 11 +++-
> xen/arch/arm/mpu/mm.c | 73 +++++++++++++++++++-----
> 6 files changed, 77 insertions(+), 15 deletions(-)
>
> diff --git a/xen/arch/arm/arm32/asm-offsets.c
> b/xen/arch/arm/arm32/asm-offsets.c
> index c203ce269d..951f8d03f3 100644
> --- a/xen/arch/arm/arm32/asm-offsets.c
> +++ b/xen/arch/arm/arm32/asm-offsets.c
> @@ -79,6 +79,8 @@ void __dummy__(void)
> #ifdef CONFIG_MPU
> DEFINE(XEN_MPUMAP_MASK_sizeof, sizeof(xen_mpumap_mask));
> DEFINE(XEN_MPUMAP_sizeof, sizeof(xen_mpumap));
> + DEFINE(XEN_MPUMAP_ENTRY_SHIFT, ilog2(sizeof(pr_t)));
> + DEFINE(XEN_MPUMAP_ENTRY_ZERO_OFFSET, sizeof(prbar_t) + sizeof(prlar_t));
> BLANK();
> #endif
> }
> diff --git a/xen/arch/arm/arm64/asm-offsets.c
> b/xen/arch/arm/arm64/asm-offsets.c
> index 320289b281..38a3894a3b 100644
> --- a/xen/arch/arm/arm64/asm-offsets.c
> +++ b/xen/arch/arm/arm64/asm-offsets.c
> @@ -73,6 +73,8 @@ void __dummy__(void)
> #ifdef CONFIG_MPU
> DEFINE(XEN_MPUMAP_MASK_sizeof, sizeof(xen_mpumap_mask));
> DEFINE(XEN_MPUMAP_sizeof, sizeof(xen_mpumap));
> + DEFINE(XEN_MPUMAP_ENTRY_SHIFT, ilog2(sizeof(pr_t)));
> + DEFINE(XEN_MPUMAP_ENTRY_ZERO_OFFSET, sizeof(prbar_t) + sizeof(prlar_t));
> BLANK();
> #endif
> }
> diff --git a/xen/arch/arm/include/asm/arm32/mpu.h
> b/xen/arch/arm/include/asm/arm32/mpu.h
> index 0a6930b3a0..137022d922 100644
> --- a/xen/arch/arm/include/asm/arm32/mpu.h
> +++ b/xen/arch/arm/include/asm/arm32/mpu.h
> @@ -39,6 +39,8 @@ typedef union {
> typedef struct {
> prbar_t prbar;
> prlar_t prlar;
> + uint8_t refcount;
> + uint8_t pad[7]; /* Pad structure to 16 Bytes */
> } pr_t;
>
> #endif /* __ASSEMBLY__ */
> diff --git a/xen/arch/arm/include/asm/arm64/mpu.h
> b/xen/arch/arm/include/asm/arm64/mpu.h
> index f0ce344e78..17f62ccaf6 100644
> --- a/xen/arch/arm/include/asm/arm64/mpu.h
> +++ b/xen/arch/arm/include/asm/arm64/mpu.h
> @@ -38,6 +38,8 @@ typedef union {
> typedef struct {
> prbar_t prbar;
> prlar_t prlar;
> + uint8_t refcount;
> + uint8_t pad[15]; /* Pad structure to 32 Bytes */
> } pr_t;
>
> #endif /* __ASSEMBLY__ */
> diff --git a/xen/arch/arm/include/asm/mpu/regions.inc
> b/xen/arch/arm/include/asm/mpu/regions.inc
> index 23fead3b21..0cdbb17bc3 100644
> --- a/xen/arch/arm/include/asm/mpu/regions.inc
> +++ b/xen/arch/arm/include/asm/mpu/regions.inc
> @@ -14,14 +14,12 @@
> #define PRLAR_ELx_EN 0x1
>
> #ifdef CONFIG_ARM_64
> -#define XEN_MPUMAP_ENTRY_SHIFT 0x4 /* 16 byte structure */
>
> .macro store_pair reg1, reg2, dst
> stp \reg1, \reg2, [\dst]
> .endm
>
> #else
> -#define XEN_MPUMAP_ENTRY_SHIFT 0x3 /* 8 byte structure */
>
> .macro store_pair reg1, reg2, dst
> strd \reg1, \reg2, [\dst]
> @@ -97,6 +95,15 @@
>
> 3:
>
> + /* Clear the rest of the xen_mpumap entry. */
> +#ifdef CONFIG_ARM_64
> + stp xzr, xzr, [\base, #XEN_MPUMAP_ENTRY_ZERO_OFFSET]
> +#else
> + mov \prbar, #0
> + mov \prlar, #0
> + strd \prbar, \prlar, [\base, #XEN_MPUMAP_ENTRY_ZERO_OFFSET]
> +#endif
> +
> add \sel, \sel, #1
>
> 1:
> diff --git a/xen/arch/arm/mpu/mm.c b/xen/arch/arm/mpu/mm.c
> index a058db19ef..c5128244b7 100644
> --- a/xen/arch/arm/mpu/mm.c
> +++ b/xen/arch/arm/mpu/mm.c
> @@ -106,6 +106,7 @@ pr_t pr_of_addr(paddr_t base, paddr_t limit, unsigned int
> flags)
> region = (pr_t) {
> .prbar = prbar,
> .prlar = prlar,
> + .refcount = 0,
> };
>
> /* Set base address and limit address. */
> @@ -170,6 +171,37 @@ int mpumap_contains_region(pr_t *table, uint8_t
> nr_regions, paddr_t base,
> return MPUMAP_REGION_NOTFOUND;
> }
>
> +static bool is_mm_attr_match(pr_t *region, unsigned int attributes)
> +{
> + bool ret = true;
> +
> + if ( region->prbar.reg.ro != PAGE_RO_MASK(attributes) )
> + {
> + printk(XENLOG_WARNING
> + "Mismatched Access Permission attributes (%#x0 instead of
> %#x0)\n",
Why %#x0 and not %#x?
> + region->prbar.reg.ro, PAGE_RO_MASK(attributes));
> + ret = false;
> + }
> +
> + if ( region->prbar.reg.xn != PAGE_XN_MASK(attributes) )
> + {
> + printk(XENLOG_WARNING
> + "Mismatched Execute Never attributes (%#x instead of %#x)\n",
> + region->prbar.reg.xn, PAGE_XN_MASK(attributes));
> + ret = false;
> + }
> +
> + if ( region->prlar.reg.ai != PAGE_AI_MASK(attributes) )
> + {
> + printk(XENLOG_WARNING
> + "Mismatched Memory Attribute Index (%#x instead of %#x)\n",
> + region->prlar.reg.ai, PAGE_AI_MASK(attributes));
> + ret = false;
> + }
What about shareability?
> +
> + return ret;
> +}
> +
> /* Map a frame table to cover physical addresses ps through pe */
> void __init setup_frametable_mappings(paddr_t ps, paddr_t pe)
> {
> @@ -287,19 +319,19 @@ static int xen_mpumap_update_entry(paddr_t base,
> paddr_t limit,
> /* Currently we don't support modifying an existing entry. */
> if ( flags_has_page_present && (rc >= MPUMAP_REGION_FOUND) )
> {
> - printk("Modifying an existing entry is not supported\n");
> - return -EINVAL;
> - }
> + if ( !is_mm_attr_match(&xen_mpumap[idx], flags) )
Do I understand correctly that this change (not mentioned in commit msg) is here
so that when we call xen_mpumap_update_entry() with existing matching or
inclusive region we will increment refcount only if the attributes match?
> + {
> + printk("Modifying an existing entry is not supported\n");
> + return -EINVAL;
> + }
>
> - /*
> - * Currently, we only support removing/modifying a *WHOLE* MPU memory
> - * region. Part-region removal/modification is not supported as in the
> worst
> - * case it will leave two/three fragments behind.
> - */
Hmm, I think that we still don't support removing/modifying regions partially.
Why is this comment removed?
> - if ( rc == MPUMAP_REGION_INCLUSIVE )
> - {
> - printk("Part-region removal/modification is not supported\n");
> - return -EINVAL;
> + /* Check for overflow of refcount before incrementing. */
> + if ( xen_mpumap[idx].refcount == 0xFF )
> + {
> + printk("Cannot allocate region as it would cause reference
> overflow\n");
> + return -ENOENT;
> + }
> + xen_mpumap[idx].refcount += 1;
> }
>
> /* We are inserting a mapping => Create new region. */
> @@ -323,7 +355,22 @@ static int xen_mpumap_update_entry(paddr_t base, paddr_t
> limit,
> return -EINVAL;
> }
>
> - disable_mpu_region_from_index(idx);
> + if ( xen_mpumap[idx].refcount == 0 )
> + {
> + if (MPUMAP_REGION_FOUND == rc)
Missing spaces around ().
> + {
No need for brackets for single instruction
> + disable_mpu_region_from_index(idx);
> + }
> + else
> + {
> + printk("Cannot remove a partial region\n");
> + return -EINVAL;
> + }
> + }
> + else
> + {
Same here.
~Michal
