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

[Alexandru Elisei]

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>.

[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,