linux-kernel - Re: [PATCH v4 7/7] KVM: arm64: Add support for creating PUD hugepages at stage 2

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Date:   Fri, 6 Jul 2018 15:23:03 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Punit Agrawal <punit.agrawal@....com>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        marc.zyngier@....com, Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 7/7] KVM: arm64: Add support for creating PUD hugepages
 at stage 2

Hi Punit,


On 06/07/18 15:12, Punit Agrawal wrote:
> Suzuki K Poulose <Suzuki.Poulose@....com> writes:
> 
>> Hi Punit,
>>
>> On 05/07/18 15:08, Punit Agrawal wrote:
>>> KVM only supports PMD hugepages at stage 2. Now that the various page
>>> handling routines are updated, extend the stage 2 fault handling to
>>> map in PUD hugepages.
>>>
>>> Addition of PUD hugepage support enables additional page sizes (e.g.,
>>> 1G with 4K granule) which can be useful on cores that support mapping
>>> larger block sizes in the TLB entries.
>>>
>>> Signed-off-by: Punit Agrawal <punit.agrawal@....com>
>>> Cc: Christoffer Dall <christoffer.dall@....com>
>>> Cc: Marc Zyngier <marc.zyngier@....com>
>>> Cc: Russell King <linux@...linux.org.uk>
>>> Cc: Catalin Marinas <catalin.marinas@....com>
>>> Cc: Will Deacon <will.deacon@....com>
>>
>>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>>> index 0c04c64e858c..5912210e94d9 100644
>>> --- a/virt/kvm/arm/mmu.c
>>> +++ b/virt/kvm/arm/mmu.c
>>> @@ -116,6 +116,25 @@ static void stage2_dissolve_pmd(struct kvm *kvm, phys_addr_t addr, pmd_t *pmd)
>>>    	put_page(virt_to_page(pmd));
>>>    }
>>>    +/**
>>> + * stage2_dissolve_pud() - clear and flush huge PUD entry
>>> + * @kvm:	pointer to kvm structure.
>>> + * @addr:	IPA
>>> + * @pud:	pud pointer for IPA
>>> + *
>>> + * Function clears a PUD entry, flushes addr 1st and 2nd stage TLBs. Marks all
>>> + * pages in the range dirty.
>>> + */
>>> +static void stage2_dissolve_pud(struct kvm *kvm, phys_addr_t addr, pud_t *pud)
>>> +{
>>> +	if (!pud_huge(*pud))
>>> +		return;
>>> +
>>> +	pud_clear(pud);
>>
>> You need to use the stage2_ accessors here. The stage2_dissolve_pmd() uses
>> "pmd_" helpers as the PTE entries (level 3) are always guaranteed to exist.
> 
> I've fixed this and other uses of the PUD helpers to go via the stage2_
> accessors.
> 
> I've still not quite come to terms with the lack of certain levels at
> stage 2 vis-a-vis stage 1. I'll be more careful about this going
> forward.

I accept that it can be quite confusing. Once we get level independent types
and table accessors this might be easier. For now, the general rule is stick
to stage2_ accessors whenever you deal with the stage2 table. Rest should be
left to the stage2 code to deal with it.

> 
>>> @@ -1572,7 +1631,18 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>    	if (exec_fault)
>>>    		invalidate_icache_guest_page(pfn, vma_pagesize);
>>>    -	if (hugetlb && vma_pagesize == PMD_SIZE) {
>>> +	if (hugetlb && vma_pagesize == PUD_SIZE) {
>>
>> I think we may need to check if the stage2 indeed has 3 levels of
>> tables to use stage2 PUD. Otherwise, fall back to PTE level mapping
>> or even PMD huge pages. Also, this cannot be triggered right now,
>> as we only get PUD hugepages with 4K and we are guaranteed to have
>> at least 3 levels with 40bit IPA. May be I can take care of it in
>> the Dynamic IPA series, when we run a guest with say 32bit IPA.
>> So for now, it is worth adding a comment here.
> 
> Good point. I've added the following comment.
> 
>      /*
>       * PUD level may not exist if the guest boots with two
>       * levels at Stage 2. This configuration is currently
>       * not supported due to IPA size supported by KVM.
>       *
>       * Revisit the assumptions about PUD levels when
>       * additional IPA sizes are supported by KVM.
>       */
> 

Yep, that looks fine to me.

Suzuki