[PATCH 9/9] KVM: arm64: np-guest CMOs with PMD_SIZE fixmap

Vincent Donnefort
2025-02-28T10:26:15Z

With the introduction of stage-2 huge mappings in the pKVM hypervisor,
guest pages CMO is needed for PMD_SIZE size. Fixmap only supports
PAGE_SIZE and iterating over the huge-page is time consuming (mostly due
to TLBI on hyp_fixmap_unmap) which is a problem for EL2 latency.

Introduce a shared PMD_SIZE fixmap (hyp_fixblock_map/hyp_fixblock_unmap)
to improve guest page CMOs when stage-2 huge mappings are installed.

On a Pixel6, the iterative solution resulted in a latency of ~700us,
while the PMD_SIZE fixmap reduces it to ~100us.

Because of the horrendous private range allocation that would be
necessary, this is disabled for 64KiB pages systems.

Suggested-by: Quentin Perret <qperret@google.com>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 1b43bcd2a679..2888b5d03757 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -59,6 +59,11 @@ typedef u64 kvm_pte_t;

#define KVM_PHYS_INVALID (-1ULL)

+#define KVM_PTE_TYPE BIT(1)
+#define KVM_PTE_TYPE_BLOCK 0
+#define KVM_PTE_TYPE_PAGE 1
+#define KVM_PTE_TYPE_TABLE 1
+
#define KVM_PTE_LEAF_ATTR_LO GENMASK(11, 2)

#define KVM_PTE_LEAF_ATTR_LO_S1_ATTRIDX GENMASK(4, 2)
diff --git a/arch/arm64/kvm/hyp/include/nvhe/mm.h b/arch/arm64/kvm/hyp/include/nvhe/mm.h
index 230e4f2527de..b0c72bc2d5ba 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/mm.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/mm.h
@@ -13,9 +13,11 @@
extern struct kvm_pgtable pkvm_pgtable;
extern hyp_spinlock_t pkvm_pgd_lock;

-int hyp_create_pcpu_fixmap(void);
+int hyp_create_fixmap(void);
void *hyp_fixmap_map(phys_addr_t phys);
void hyp_fixmap_unmap(void);
+void *hyp_fixblock_map(phys_addr_t phys);
+void hyp_fixblock_unmap(void);

int hyp_create_idmap(u32 hyp_va_bits);
int hyp_map_vectors(void);
diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 271893eff021..d27ce31370aa 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -220,25 +220,64 @@ static void guest_s2_put_page(void *addr)
hyp_put_page(&current_vm->pool, addr);
}

+static void *__fixmap_guest_page(void *va, size_t *size)
+{
+ if (IS_ALIGNED(*size, PMD_SIZE)) {
+ void *addr = hyp_fixblock_map(__hyp_pa(va));
+
+ if (addr)
+ return addr;
+
+ *size = PAGE_SIZE;
+ }
+
+ if (IS_ALIGNED(*size, PAGE_SIZE))
+ return hyp_fixmap_map(__hyp_pa(va));
+
+ WARN_ON(1);
+
+ return NULL;
+}
+
+static void __fixunmap_guest_page(size_t size)
+{
+ switch (size) {
+ case PAGE_SIZE:
+ hyp_fixmap_unmap();
+ break;
+ case PMD_SIZE:
+ hyp_fixblock_unmap();
+ break;
+ default:
+ WARN_ON(1);
+ }
+}
+
static void clean_dcache_guest_page(void *va, size_t size)
{
while (size) {
- __clean_dcache_guest_page(hyp_fixmap_map(__hyp_pa(va)),
- PAGE_SIZE);
- hyp_fixmap_unmap();
- va += PAGE_SIZE;
- size -= PAGE_SIZE;
+ size_t fixmap_size = size == PMD_SIZE ? size : PAGE_SIZE;
+ void *addr = __fixmap_guest_page(va, &fixmap_size);
+
+ __clean_dcache_guest_page(addr, fixmap_size);
+ __fixunmap_guest_page(fixmap_size);
+
+ size -= fixmap_size;
+ va += fixmap_size;
}
}

static void invalidate_icache_guest_page(void *va, size_t size)
{
while (size) {
- __invalidate_icache_guest_page(hyp_fixmap_map(__hyp_pa(va)),
- PAGE_SIZE);
- hyp_fixmap_unmap();
- va += PAGE_SIZE;
- size -= PAGE_SIZE;
+ size_t fixmap_size = size == PMD_SIZE ? size : PAGE_SIZE;
+ void *addr = __fixmap_guest_page(va, &fixmap_size);
+
+ __invalidate_icache_guest_page(addr, fixmap_size);
+ __fixunmap_guest_page(fixmap_size);
+
+ size -= fixmap_size;
+ va += fixmap_size;
}
}

diff --git a/arch/arm64/kvm/hyp/nvhe/mm.c b/arch/arm64/kvm/hyp/nvhe/mm.c
index f41c7440b34b..e3b1bece8504 100644
--- a/arch/arm64/kvm/hyp/nvhe/mm.c
+++ b/arch/arm64/kvm/hyp/nvhe/mm.c
@@ -229,9 +229,8 @@ int hyp_map_vectors(void)
return 0;
}

-void *hyp_fixmap_map(phys_addr_t phys)
+static void *fixmap_map_slot(struct hyp_fixmap_slot *slot, phys_addr_t phys)
{
- struct hyp_fixmap_slot *slot = this_cpu_ptr(&fixmap_slots);
kvm_pte_t pte, *ptep = slot->ptep;

pte = *ptep;
@@ -243,10 +242,21 @@ void *hyp_fixmap_map(phys_addr_t phys)
return (void *)slot->addr;
}

+void *hyp_fixmap_map(phys_addr_t phys)
+{
+ return fixmap_map_slot(this_cpu_ptr(&fixmap_slots), phys);
+}
+
static void fixmap_clear_slot(struct hyp_fixmap_slot *slot)
{
kvm_pte_t *ptep = slot->ptep;
u64 addr = slot->addr;
+ u32 level;
+
+ if (FIELD_GET(KVM_PTE_TYPE, *ptep) == KVM_PTE_TYPE_PAGE)
+ level = KVM_PGTABLE_LAST_LEVEL;
+ else
+ level = KVM_PGTABLE_LAST_LEVEL - 1; /* create_fixblock() guarantees PMD level */

WRITE_ONCE(*ptep, *ptep & ~KVM_PTE_VALID);

@@ -260,7 +270,7 @@ static void fixmap_clear_slot(struct hyp_fixmap_slot *slot)
* https://lore.kernel.org/kvm/20221017115209.2099-1-will@kernel.org/T/#mf10dfbaf1eaef9274c581b81c53758918c1d0f03
*/
dsb(ishst);
- __tlbi_level(vale2is, __TLBI_VADDR(addr, 0), KVM_PGTABLE_LAST_LEVEL);
+ __tlbi_level(vale2is, __TLBI_VADDR(addr, 0), level);
dsb(ish);
isb();
}
@@ -273,9 +283,9 @@ void hyp_fixmap_unmap(void)
static int __create_fixmap_slot_cb(const struct kvm_pgtable_visit_ctx *ctx,
enum kvm_pgtable_walk_flags visit)
{
- struct hyp_fixmap_slot *slot = per_cpu_ptr(&fixmap_slots, (u64)ctx->arg);
+ struct hyp_fixmap_slot *slot = (struct hyp_fixmap_slot *)ctx->arg;

- if (!kvm_pte_valid(ctx->old) || ctx->level != KVM_PGTABLE_LAST_LEVEL)
+ if (!kvm_pte_valid(ctx->old) || (ctx->end - ctx->start) != kvm_granule_size(ctx->level))
return -EINVAL;

slot->addr = ctx->addr;
@@ -296,13 +306,73 @@ static int create_fixmap_slot(u64 addr, u64 cpu)
struct kvm_pgtable_walker walker = {
.cb = __create_fixmap_slot_cb,
.flags = KVM_PGTABLE_WALK_LEAF,
- .arg = (void *)cpu,
+ .arg = (void *)per_cpu_ptr(&fixmap_slots, cpu),
};

return kvm_pgtable_walk(&pkvm_pgtable, addr, PAGE_SIZE, &walker);
}

-int hyp_create_pcpu_fixmap(void)
+#ifndef CONFIG_ARM64_64K_PAGES
+static struct hyp_fixmap_slot hyp_fixblock_slot;
+static DEFINE_HYP_SPINLOCK(hyp_fixblock_lock);
+
+void *hyp_fixblock_map(phys_addr_t phys)
+{
+ hyp_spin_lock(&hyp_fixblock_lock);
+ return fixmap_map_slot(&hyp_fixblock_slot, phys);
+}
+
+void hyp_fixblock_unmap(void)
+{
+ fixmap_clear_slot(&hyp_fixblock_slot);
+ hyp_spin_unlock(&hyp_fixblock_lock);
+}
+
+static int create_fixblock(void)
+{
+ struct kvm_pgtable_walker walker = {
+ .cb = __create_fixmap_slot_cb,
+ .flags = KVM_PGTABLE_WALK_LEAF,
+ .arg = (void *)&hyp_fixblock_slot,
+ };
+ unsigned long addr;
+ phys_addr_t phys;
+ int ret, i;
+
+ /* Find a RAM phys address, PMD aligned */
+ for (i = 0; i < hyp_memblock_nr; i++) {
+ phys = ALIGN(hyp_memory[i].base, PMD_SIZE);
+ if (phys + PMD_SIZE < (hyp_memory[i].base + hyp_memory[i].size))
+ break;
+ }
+
+ if (i >= hyp_memblock_nr)
+ return -EINVAL;
+
+ hyp_spin_lock(&pkvm_pgd_lock);
+ addr = ALIGN(__io_map_base, PMD_SIZE);
+ ret = __pkvm_alloc_private_va_range(addr, PMD_SIZE);
+ if (ret)
+ goto unlock;
+
+ ret = kvm_pgtable_hyp_map(&pkvm_pgtable, addr, PMD_SIZE, phys, PAGE_HYP);
+ if (ret)
+ goto unlock;
+
+ ret = kvm_pgtable_walk(&pkvm_pgtable, addr, PMD_SIZE, &walker);
+
+unlock:
+ hyp_spin_unlock(&pkvm_pgd_lock);
+
+ return ret;
+}
+#else
+void hyp_fixblock_unmap(void) { WARN_ON(1); }
+void *hyp_fixblock_map(phys_addr_t phys) { return NULL; }
+static int create_fixblock(void) { return 0; }
+#endif
+
+int hyp_create_fixmap(void)
{
unsigned long addr, i;
int ret;
@@ -322,7 +392,7 @@ int hyp_create_pcpu_fixmap(void)
return ret;
}

- return 0;
+ return create_fixblock();
}

int hyp_create_idmap(u32 hyp_va_bits)
diff --git a/arch/arm64/kvm/hyp/nvhe/setup.c b/arch/arm64/kvm/hyp/nvhe/setup.c
index d62bcb5634a2..fb69cf5e6ea8 100644
--- a/arch/arm64/kvm/hyp/nvhe/setup.c
+++ b/arch/arm64/kvm/hyp/nvhe/setup.c
@@ -295,7 +295,7 @@ void __noreturn __pkvm_init_finalise(void)
if (ret)
goto out;

- ret = hyp_create_pcpu_fixmap();
+ ret = hyp_create_fixmap();
if (ret)
goto out;

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index df5cc74a7dd0..c351b4abd5db 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -11,12 +11,6 @@
#include <asm/kvm_pgtable.h>
#include <asm/stage2_pgtable.h>

-
-#define KVM_PTE_TYPE BIT(1)
-#define KVM_PTE_TYPE_BLOCK 0
-#define KVM_PTE_TYPE_PAGE 1
-#define KVM_PTE_TYPE_TABLE 1
-
struct kvm_pgtable_walk_data {
struct kvm_pgtable_walker *walker;