[PATCH v3 1/3] KVM: arm64: skip pKVM cache flushes for non cacheable mappings
Leonardo Bras
leo.bras at arm.com
Wed Jul 1 09:53:34 PDT 2026
On Wed, Jul 01, 2026 at 05:40:46PM +0100, Bradley Morgan wrote:
> On July 1, 2026 5:05:53 PM GMT+01:00, Leonardo Bras <leo.bras at arm.com>
> wrote:
> >On Wed, Jun 24, 2026 at 04:00:26PM +0000, Bradley Morgan wrote:
> >> pKVM keeps its own mapping list for stage 2 operations. Its flush path
> >> uses that list directly, so it lost the PTE attribute check done by the
> >> generic stage 2 walker.
> >>
> >> Record whether a mapping is cacheable and skip cache maintenance for
> >> mappings that are not cacheable.
> >>
> >> Fixes: e912efed485a ("KVM: arm64: Introduce the EL1 pKVM MMU")
> >> Signed-off-by: Bradley Morgan <include at grrlz.net>
> >> ---
> >> arch/arm64/kvm/pkvm.c | 51 ++++++++++++++++++++++++++++++++++---------
> >> 1 file changed, 41 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/arch/arm64/kvm/pkvm.c b/arch/arm64/kvm/pkvm.c
> >> index 428723b1b0f5..ca6e823028c2 100644
> >> --- a/arch/arm64/kvm/pkvm.c
> >> +++ b/arch/arm64/kvm/pkvm.c
> >> @@ -302,9 +302,32 @@ static u64 __pkvm_mapping_start(struct pkvm_mapping
> >*m)
> >> return m->gfn * PAGE_SIZE;
> >> }
> >>
> >> +#define PKVM_MAPPING_NR_PAGES_MASK GENMASK_ULL(47, 0)
> >> +#define PKVM_MAPPING_CACHEABLE BIT_ULL(48)
> >
> >Out of curiosity here, why do you choose to use bit 48 here instead of,
> >let's say, bit 63?
> >
> >(I know it makes absolutely no difference to inner working here, as there
> >should probably not be 2^48 pages in one mapping.)
> >
> >Thanks!
> >Leo
>
>
> sup Leo, here's a quote from maz
Hi Bradley,
>
> "This thing is already big enough, let's not add a bool right in the
> middle (use pahole to find out why this is bad).
I suppose you proposed to add a bool into a struct, maybe?
It would screw the struct alignment.
> Given that nr_pages
> is for a range, and that the minimum page size uses 12 bits, the
> largest number of pages you can have here is 56-12=48 bit wide. That's
> another 16 bits worth of flags you can use."
Humm, makes sense.
And since he mentions 16 bits worth of flags, you start by using the 48th
bit. Ok, got your rationale.
(I would possibly start with the 63, though, but that's more on personal
taste)
>
> this should just clarify things, any questions, feel more than free to ask!
>
> (btw V4 is coming soon)
Thanks!
Leo
>
> >> +
> >> +static u64 pkvm_mapping_nr_pages(struct pkvm_mapping *m)
> >> +{
> >> + return m->nr_pages & PKVM_MAPPING_NR_PAGES_MASK;
> >> +}
> >> +
> >> +static bool pkvm_mapping_is_cacheable(struct pkvm_mapping *m)
> >> +{
> >> + return m->nr_pages & PKVM_MAPPING_CACHEABLE;
> >> +}
> >> +
> >> +static void pkvm_mapping_set_nr_pages(struct pkvm_mapping *m, u64
> >nr_pages,
> >> + bool cacheable)
> >> +{
> >> + WARN_ON_ONCE(nr_pages & ~PKVM_MAPPING_NR_PAGES_MASK);
> >> +
> >> + m->nr_pages = nr_pages & PKVM_MAPPING_NR_PAGES_MASK;
> >> + if (cacheable)
> >> + m->nr_pages |= PKVM_MAPPING_CACHEABLE;
> >> +}
> >> +
> >> static u64 __pkvm_mapping_end(struct pkvm_mapping *m)
> >> {
> >> - return (m->gfn + m->nr_pages) * PAGE_SIZE - 1;
> >> + return (m->gfn + pkvm_mapping_nr_pages(m)) * PAGE_SIZE - 1;
> >> }
> >>
> >> INTERVAL_TREE_DEFINE(struct pkvm_mapping, node, u64, __subtree_last,
> >> @@ -350,7 +373,7 @@ static int __pkvm_pgtable_stage2_reclaim(struct
> >kvm_pgtable *pgt, u64 start, u64
> >> continue;
> >>
> >> page = pfn_to_page(mapping->pfn);
> >> - WARN_ON_ONCE(mapping->nr_pages != 1);
> >> + WARN_ON_ONCE(pkvm_mapping_nr_pages(mapping) != 1);
> >> unpin_user_pages_dirty_lock(&page, 1, true);
> >> account_locked_vm(kvm->mm, 1, false);
> >> pkvm_mapping_remove(mapping, &pgt->pkvm_mappings);
> >> @@ -369,7 +392,7 @@ static int __pkvm_pgtable_stage2_unshare(struct
> >kvm_pgtable *pgt, u64 start, u64
> >>
> >> for_each_mapping_in_range_safe(pgt, start, end, mapping) {
> >> ret = kvm_call_hyp_nvhe(__pkvm_host_unshare_guest, handle, mapping->gfn,
> >> - mapping->nr_pages);
> >> + pkvm_mapping_nr_pages(mapping));
> >> if (WARN_ON(ret))
> >> return ret;
> >> pkvm_mapping_remove(mapping, &pgt->pkvm_mappings);
> >> @@ -448,7 +471,7 @@ int pkvm_pgtable_stage2_map(struct kvm_pgtable *pgt,
> >u64 addr, u64 size,
> >> * permission faults are handled in the relax_perms() path.
> >> */
> >> if (mapping) {
> >> - if (size == (mapping->nr_pages * PAGE_SIZE))
> >> + if (size == (pkvm_mapping_nr_pages(mapping) * PAGE_SIZE))
> >> return -EAGAIN;
> >>
> >> /*
> >> @@ -472,7 +495,9 @@ int pkvm_pgtable_stage2_map(struct kvm_pgtable *pgt,
> >u64 addr, u64 size,
> >> swap(mapping, cache->mapping);
> >> mapping->gfn = gfn;
> >> mapping->pfn = pfn;
> >> - mapping->nr_pages = size / PAGE_SIZE;
> >> + pkvm_mapping_set_nr_pages(mapping, size / PAGE_SIZE,
> >> + !(prot & (KVM_PGTABLE_PROT_DEVICE |
> >> + KVM_PGTABLE_PROT_NORMAL_NC)));
> >> pkvm_mapping_insert(mapping, &pgt->pkvm_mappings);
> >>
> >> return ret;
> >> @@ -503,7 +528,7 @@ int pkvm_pgtable_stage2_wrprotect(struct kvm_pgtable
> >*pgt, u64 addr, u64 size)
> >> lockdep_assert_held(&kvm->mmu_lock);
> >> for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping) {
> >> ret = kvm_call_hyp_nvhe(__pkvm_host_wrprotect_guest, handle, mapping->gfn,
> >> - mapping->nr_pages);
> >> + pkvm_mapping_nr_pages(mapping));
> >> if (WARN_ON(ret))
> >> break;
> >> }
> >> @@ -517,9 +542,13 @@ int pkvm_pgtable_stage2_flush(struct kvm_pgtable
> >*pgt, u64 addr, u64 size)
> >> struct pkvm_mapping *mapping;
> >>
> >> lockdep_assert_held(&kvm->mmu_lock);
> >> - for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping)
> >> + for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping) {
> >> + if (!pkvm_mapping_is_cacheable(mapping))
> >> + continue;
> >> +
> >> __clean_dcache_guest_page(pfn_to_kaddr(mapping->pfn),
> >> - PAGE_SIZE * mapping->nr_pages);
> >> + PAGE_SIZE * pkvm_mapping_nr_pages(mapping));
> >> + }
> >>
> >> return 0;
> >> }
> >> @@ -536,8 +565,10 @@ bool pkvm_pgtable_stage2_test_clear_young(struct
> >kvm_pgtable *pgt, u64 addr, u64
> >>
> >> lockdep_assert_held(&kvm->mmu_lock);
> >> for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping)
> >> - young |= kvm_call_hyp_nvhe(__pkvm_host_test_clear_young_guest, handle, mapping->gfn,
> >> - mapping->nr_pages, mkold);
> >> + young |= kvm_call_hyp_nvhe(__pkvm_host_test_clear_young_guest,
> >> + handle, mapping->gfn,
> >> + pkvm_mapping_nr_pages(mapping),
> >> + mkold);
> >>
> >> return young;
> >> }
> >> --
> >> 2.53.0
> >>
> >
>
> Thanks!
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