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

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Date:   Mon, 26 Oct 2020 08:55:25 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Gavin Shan <gshan@...hat.com>
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

On 2020-10-25 23:04, Gavin Shan wrote:
> 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.

Yes, please drop these patches, and focus on the actual bug fix.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...