Re: [PATCH v3 3/3] KVM: arm64: top up stage 2 memcache for dirty logging faults

[Bradley Morgan]

On June 24, 2026 6:39:16 PM GMT+01:00, Bradley Morgan <include@xxxxxxxxx>
wrote:
>On June 24, 2026 5:00:28 PM GMT+01:00, Bradley Morgan <include@xxxxxxxxx>
>wrote:
>>Dirty logging forces new stage 2 mappings down to page size, but
>>it does not always remove an existing block mapping before the next
>>fault. Eager splitting is best effort and is disabled by default.
>>
>>A permission fault on such a block can still need a page table page
>>to install the smaller mapping. Top up the memcache for any permission
>>fault while dirty logging is active, not only for write faults.
>>
>>The issue was discovered [1] by Sashiko.
>>
>>Link:
>https://lore.kernel.org/all/59984F6D-06F2-4302-BDD7-92DF334E8FA0@xxxxxxxxx/T/#t
>[1]
>>
>>Fixes: 6f745f1bb5bf ("KVM: arm64: Convert user_mem_abort() to generic
>page-table API")
>>Cc: stable@xxxxxxxxxxxxxxx
>>Signed-off-by: Bradley Morgan <include@xxxxxxxxx>
>>---
>> arch/arm64/kvm/mmu.c | 9 ++++-----
>> 1 file changed, 4 insertions(+), 5 deletions(-)
>>
>>diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>>index 3f57f6825a33..8911e319e6fa 100644
>>--- a/arch/arm64/kvm/mmu.c
>>+++ b/arch/arm64/kvm/mmu.c
>>@@ -2122,13 +2122,12 @@ static int user_mem_abort(const struct
>kvm_s2_fault_desc *s2fd)
>> 	 * Permission faults just need to update the existing leaf entry,
>> 	 * and so normally don't require allocations from the memcache. The
>> 	 * only exception to this is when dirty logging is enabled at runtime
>>-	 * and a write fault needs to collapse a block entry into a table. With
>>-	 * pKVM, they may still need a fresh mapping object if the fault turns
>>-	 * page entries into a block entry.
>>+	 * and a fault needs to collapse a block entry into a table. With pKVM,
>>+	 * they may still need a fresh mapping object if the fault turns page
>>+	 * entries into a block entry.
>> 	 */
>> 	memcache = get_mmu_memcache(s2fd->vcpu);
>>-	if (!perm_fault || (memslot_is_logging(s2fd->memslot) &&
>>-			    kvm_is_write_fault(s2fd->vcpu))) {
>>+	if (!perm_fault || memslot_is_logging(s2fd->memslot)) {
>> 		ret = topup_mmu_memcache(s2fd->vcpu, memcache);
>> 		if (ret)
>> 			return ret;
>>
>
>Note: Patch 3 seems to conflict because of patch 2 (the comments)
>
>
>Oops! :(
>
>V4 (after people have their review go), will contain one commit (patch
>3) with the updated comments.
>
>Patch 1 and 2 applies as usual.
>
>Apologies for my messup. 
>
>Thanks!


Actually. Hmm.

I'll just drop patches 2 and 3, I'll do them at a later date, please
disregard patches 2 and 3, patch 1 doesn't rely on 2 and 3..

If you guys wanna have a look feel free! :)

Thanks!