[PATCH 2/2] KVM: arm64: Try PMD block mappings if PUD mappings are not supported

[Gavin Shan]

Hi Alexandru,

On 9/1/20 11:33 PM, Alexandru Elisei wrote:
> When userspace uses hugetlbfs for the VM memory, user_mem_abort() tries to
> use the same block size to map the faulting IPA in stage 2. If stage 2
> cannot use the same size mapping because the block size doesn't fit in the
> memslot or the memslot is not properly aligned, user_mem_abort() will fall
> back to a page mapping, regardless of the block size. We can do better for
> PUD backed hugetlbfs by checking if a PMD block mapping is possible before
> deciding to use a page.
> 
> vma_pagesize is an unsigned long, use 1UL instead of 1ULL when assigning
> its value.
> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
> ---
>   arch/arm64/kvm/mmu.c | 19 ++++++++++++++-----
>   1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 25e7dc52c086..f590f7355cda 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1871,15 +1871,24 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>   	else
>   		vma_shift = PAGE_SHIFT;
>   
> -	vma_pagesize = 1ULL << vma_shift;
>   	if (logging_active ||
> -	    (vma->vm_flags & VM_PFNMAP) ||
> -	    !fault_supports_stage2_huge_mapping(memslot, hva, vma_pagesize)) {
> +	    (vma->vm_flags & VM_PFNMAP)) {
>   		force_pte = true;
> -		vma_pagesize = PAGE_SIZE;
>   		vma_shift = PAGE_SHIFT;
>   	}
>  

It looks incorrect because @vma_pagesize wasn't initialized when
it's passed to fault_supports_stage2_huge_mapping() for the checking.
It's assumed you missed the following changes according to the commit
log:

    fault_supports_stage2_huge_mapping(memslot, hva, (1UL << vma_shift))
  
> +	if (vma_shift == PUD_SHIFT &&
> +	    !fault_supports_stage2_huge_mapping(memslot, hva, PUD_SIZE))
> +		vma_shift = PMD_SHIFT;
> +
> +	if (vma_shift == PMD_SHIFT &&
> +	    !fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) {
> +		force_pte = true;
> +		vma_shift = PAGE_SHIFT;
> +	}
> +
> +	vma_pagesize = 1UL << vma_shift;
> +>   	/*
>   	 * The stage2 has a minimum of 2 level table (For arm64 see
>   	 * kvm_arm_setup_stage2()). Hence, we are guaranteed that we can
> @@ -1889,7 +1898,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>   	 */
>   	if (vma_pagesize == PMD_SIZE ||
>   	    (vma_pagesize == PUD_SIZE && kvm_stage2_has_pmd(kvm)))
> -		gfn = (fault_ipa & huge_page_mask(hstate_vma(vma))) >> PAGE_SHIFT;
> +		gfn = (fault_ipa & ~(vma_pagesize - 1)) >> PAGE_SHIFT;
>   	mmap_read_unlock(current->mm);
>   
>   	/* We need minimum second+third level pages */
> 

Thanks,
Gavin