linux-kernel - Re: [PATCH 3/3] KVM: arm64: Failback on unsupported huge page sizes

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Date:   Mon, 26 Oct 2020 10:04:08 +1100
From:   Gavin Shan <gshan@...hat.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        will@...nel.org, alexandru.elisei@....com
Subject: Re: [PATCH 3/3] KVM: arm64: Failback on unsupported huge page sizes

Hi Marc,

On 10/25/20 9:48 PM, Marc Zyngier wrote:
> On Sun, 25 Oct 2020 01:27:39 +0100,
> Gavin Shan <gshan@...hat.com> wrote:
>>
>> The huge page could be mapped through multiple contiguous PMDs or PTEs.
>> The corresponding huge page sizes aren't supported by the page table
>> walker currently.
>>
>> This fails the unsupported huge page sizes to the near one. Otherwise,
>> the guest can't boot successfully: CONT_PMD_SHIFT and CONT_PTE_SHIFT
>> fail back to PMD_SHIFT and PAGE_SHIFT separately.
>>
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>> ---
>>   arch/arm64/kvm/mmu.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 0f51585adc04..81cbdc368246 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -793,12 +793,20 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   		vma_shift = PMD_SHIFT;
>>   #endif
>>   
>> +	if (vma_shift == CONT_PMD_SHIFT)
>> +		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;
>>   	}
>>   
>> +	if (vma_shift == CONT_PTE_SHIFT) {
>> +		force_pte = true;
>> +		vma_shift = PAGE_SHIFT;
>> +	}
>> +
>>   	vma_pagesize = 1UL << vma_shift;
>>   	if (vma_pagesize == PMD_SIZE || vma_pagesize == PUD_SIZE)
>>   		fault_ipa &= ~(vma_pagesize - 1);
> 
> Yup, nice catch. However, I think we should take this opportunity to
> rationalise the logic here, and catch future discrepancies (should
> someone add contiguous PUD or something similarly silly). How about
> something like this (untested):
> 

Yeah, I started the work to support contiguous PMDs/PTEs, but I'm not
sure when I can post the patches for review as my time becomes a bit
fragmented recently. At least, I need focus on "async page fault" in
the coming weeks :)

Thanks for the suggested code and it worked for me. I'll post v2 to
integrate them. However, I would like to drop PATCH[1] and PATCH[2]
as I really don't have strong reasons to have them.

Thanks,
Gavin

> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index cc323d96c9d4..d9a13a8a82e0 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -787,14 +787,31 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>   		vma_shift = PAGE_SHIFT;
>   	}
>   
> -	if (vma_shift == PUD_SHIFT &&
> -	    !fault_supports_stage2_huge_mapping(memslot, hva, PUD_SIZE))
> -	       vma_shift = PMD_SHIFT;
> +	switch (vma_shift) {
> +	case PUD_SHIFT:
> +		if (fault_supports_stage2_huge_mapping(memslot, hva, PUD_SIZE))
> +			break;
> +		fallthrough;
>   
> -	if (vma_shift == PMD_SHIFT &&
> -	    !fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) {
> -		force_pte = true;
> +	case CONT_PMD_SHIFT:
> +		vma_shift = PMD_SHIFT;
> +		fallthrough;
> +
> +	case PMD_SHIFT:
> +		if (fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE))
> +			break;
> +		fallthrough;
> +
> +	case CONT_PTE_SHIFT:
>   		vma_shift = PAGE_SHIFT;
> +		force_pte = true;
> +		fallthrough;
> +
> +	case PAGE_SHIFT:
> +		break;
> +
> +	default:
> +		WARN_ONCE(1, "Unknown vma_shift %d", vma_shift);
>   	}
>   
>   	vma_pagesize = 1UL << vma_shift;
>