Re: [RFC PATCH v3 2/2] KVM: arm64: Distinguish cases of memcache allocations completely

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On 2021/4/7 23:35, Alexandru Elisei wrote:
Hi Yanan,

On 3/26/21 3:16 AM, Yanan Wang wrote:
With a guest translation fault, the memcache pages are not needed if KVM
is only about to install a new leaf entry into the existing page table.
And with a guest permission fault, the memcache pages are also not needed
for a write_fault in dirty-logging time if KVM is only about to update
the existing leaf entry instead of collapsing a block entry into a table.

By comparing fault_granule and vma_pagesize, cases that require allocations
from memcache and cases that don't can be distinguished completely.

Signed-off-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
---
  arch/arm64/kvm/mmu.c | 25 ++++++++++++-------------
  1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 1eec9f63bc6f..05af40dc60c1 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -810,19 +810,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
  	gfn = fault_ipa >> PAGE_SHIFT;
  	mmap_read_unlock(current->mm);
- /*
-	 * 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.
-	 */
-	if (fault_status != FSC_PERM || (logging_active && write_fault)) {
-		ret = kvm_mmu_topup_memory_cache(memcache,
-						 kvm_mmu_cache_min_pages(kvm));
-		if (ret)
-			return ret;
-	}
-
  	mmu_seq = vcpu->kvm->mmu_notifier_seq;
  	/*
  	 * Ensure the read of mmu_notifier_seq happens before we call
@@ -880,6 +867,18 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
  	else if (cpus_have_const_cap(ARM64_HAS_CACHE_DIC))
  		prot |= KVM_PGTABLE_PROT_X;
+ /*
+	 * Allocations from the memcache are required only when granule of the
+	 * lookup level where the guest fault happened exceeds vma_pagesize,
+	 * which means new page tables will be created in the fault handlers.
+	 */
+	if (fault_granule > vma_pagesize) {
+		ret = kvm_mmu_topup_memory_cache(memcache,
+						 kvm_mmu_cache_min_pages(kvm));
+		if (ret)
+			return ret;
+	}
As I explained in v1 [1], this looks correct to me. I still think that someone
else should have a look, but if Marc decides to pick up this patch as-is, he can
add my Reviewed-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>.
Thanks again for this, Alex!

Hi Marc, Will,
Any thoughts about this patch?

Thanks,
Yanan
[1] https://lore.kernel.org/lkml/2c65bff2-be7f-b20c-9265-939bc73185b6@xxxxxxx/

Thanks,

Alex

+
  	/*
  	 * Under the premise of getting a FSC_PERM fault, we just need to relax
  	 * permissions only if vma_pagesize equals fault_granule. Otherwise,
.



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