linux-kernel - Re: [PATCH v5 7/7] KVM: arm64: Add support for creating PUD hugepages at stage 2

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Message-ID: <87lgahoidh.fsf@e105922-lin.cambridge.arm.com>
Date:   Wed, 11 Jul 2018 17:19:06 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Suzuki K Poulose <Suzuki.Poulose@....com>
Cc:     marc.zyngier@....com, Catalin Marinas <catalin.marinas@....com>,
        will.deacon@....com, Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5 7/7] KVM: arm64: Add support for creating PUD hugepages at stage 2

Suzuki K Poulose <Suzuki.Poulose@....com> writes:

> On 11/07/18 17:05, Punit Agrawal wrote:
>> Suzuki K Poulose <Suzuki.Poulose@....com> writes:
>>
>>> On 09/07/18 15:41, 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>
>>>> ---
>>>>    arch/arm/include/asm/kvm_mmu.h         | 19 +++++++
>>>>    arch/arm64/include/asm/kvm_mmu.h       | 15 +++++
>>>>    arch/arm64/include/asm/pgtable-hwdef.h |  2 +
>>>>    arch/arm64/include/asm/pgtable.h       |  2 +
>>>>    virt/kvm/arm/mmu.c                     | 78 ++++++++++++++++++++++++--
>>>>    5 files changed, 112 insertions(+), 4 deletions(-)
>>>>
>>
>> [...]
>>
>>>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>>>> index a6d3ac9d7c7a..d8e2497e5353 100644
>>>> --- a/virt/kvm/arm/mmu.c
>>>> +++ b/virt/kvm/arm/mmu.c
>>
>> [...]
>>
>>>> @@ -1100,6 +1139,7 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>>>>    			  phys_addr_t addr, const pte_t *new_pte,
>>>>    			  unsigned long flags)
>>>>    {
>>>> +	pud_t *pud;
>>>>    	pmd_t *pmd;
>>>>    	pte_t *pte, old_pte;
>>>>    	bool iomap = flags & KVM_S2PTE_FLAG_IS_IOMAP;
>>>> @@ -1108,6 +1148,22 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>>>>    	VM_BUG_ON(logging_active && !cache);
>>>>      	/* Create stage-2 page table mapping - Levels 0 and 1 */
>>>> +	pud = stage2_get_pud(kvm, cache, addr);
>>>> +	if (!pud) {
>>>> +		/*
>>>> +		 * Ignore calls from kvm_set_spte_hva for unallocated
>>>> +		 * address ranges.
>>>> +		 */
>>>> +		return 0;
>>>> +	}
>>>> +
>>>> +	/*
>>>> +	 * While dirty page logging - dissolve huge PUD, then continue
>>>> +	 * on to allocate page.
>>>
>>> Punit,
>>>
>>> We don't seem to allocate a page here for the PUD entry, in case if it is dissolved
>>> or empty (i.e, stage2_pud_none(*pud) is true.).
>>
>> I was trying to avoid duplicating the PUD allocation by reusing the
>> functionality in stage2_get_pmd().
>>
>> Does the below updated comment help?
>>
>> 	/*
>> 	 * While dirty page logging - dissolve huge PUD, it'll be
>> 	 * allocated in stage2_get_pmd().
>> 	 */
>>
>> The other option is to duplicate the stage2_pud_none() case from
>> stage2_get_pmd() here.
>
> I think the explicit check for stage2_pud_none() suits better here.
> That would make it explicit that we are tearing down the entries
> from top to bottom. Also, we may be able to short cut for case
> where we know we just allocated a PUD page and hence we need another
> PMD level page.

Ok, I'll add the PUD allocation code here.

>
> Also, you are missing the comment about the assumption that stage2 PUD
> level always exist with 4k fixed IPA.

Hmm... I'm quite sure I wrote a comment to that effect but can't find it
now. I'll include it in the next version.

Thanks,
Punit

>
> Cheers
> Suzuki
> _______________________________________________
> kvmarm mailing list
> kvmarm@...ts.cs.columbia.edu
> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

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