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Re: [PATCH 2/3] arm: Implement reference counting for overlapping regions
Hello,
On 04/11/2025 10:21, Orzel, Michal wrote:
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?
Perhaps overlap is a bit misleading/confusing in this context and
instead I should have said inclusive regions. Reference counting is used
in this context to prevent us from attempting to remove a portion of the
region whilst it is still in use by other regions.
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<harry.ramsey@xxxxxxx>
---
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?
I will remove this as I do not think it is necessary to understand the
permission attributes.
+ 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?
Unless I am mistaken, inside `page.h` we define 8 unique regions which
have their own sharability/permissions, so if `prlar.reg.ai` does not
match, the sharability/permissions are incorrect. Thus we should not
require a seperate check.
+
+ 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?
That would be correct, I will update the commit message to provide
additional context especially with regards to what we mean by allocating
inclusive regions.
+ {
+ 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?
Sorry, this is a mistake and I will update the comment to reflect to
better reflect our implementation with reference counting.
- 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
Thanks for the feedback, I will address the rest of these changes in v2.
Thanks,
Harry Ramsey.
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