linux-kernel - Re: [PATCH v5 02/43] kvm: arm64: pgtable: Track the number of pages in the entry level

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Message-ID: <7d02ed25-7618-4920-8f35-9e480cdf709f@arm.com>
Date: Wed, 23 Oct 2024 15:35:35 +0100
From: Steven Price <steven.price@....com>
To: Gavin Shan <gshan@...hat.com>, kvm@...r.kernel.org, kvmarm@...ts.linux.dev
Cc: Suzuki K Poulose <suzuki.poulose@....com>,
 Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
 Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
 Oliver Upton <oliver.upton@...ux.dev>, Zenghui Yu <yuzenghui@...wei.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Joey Gouly <joey.gouly@....com>, Alexandru Elisei
 <alexandru.elisei@....com>, Christoffer Dall <christoffer.dall@....com>,
 Fuad Tabba <tabba@...gle.com>, linux-coco@...ts.linux.dev,
 Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
 Shanker Donthineni <sdonthineni@...dia.com>, Alper Gun
 <alpergun@...gle.com>, "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>
Subject: Re: [PATCH v5 02/43] kvm: arm64: pgtable: Track the number of pages
 in the entry level

On 23/10/2024 05:03, Gavin Shan wrote:
> On 10/5/24 1:27 AM, Steven Price wrote:
>> From: Suzuki K Poulose <suzuki.poulose@....com>
>>
>> Keep track of the number of pages allocated for the top level PGD,
>> rather than computing it every time (though we need it only twice now).
>> This will be used later by Arm CCA KVM changes.
>>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> Signed-off-by: Steven Price <steven.price@....com>
>> ---
>>   arch/arm64/include/asm/kvm_pgtable.h | 2 ++
>>   arch/arm64/kvm/hyp/pgtable.c         | 5 +++--
>>   2 files changed, 5 insertions(+), 2 deletions(-)
>>
> 
> If we really want to have the number of pages for the top level PGDs,
> the existing helpers kvm_pgtable_stage2_pgd_size() for the same purpose
> needs to replaced by (struct kvm_pgtable::pgd_pages << PAGE_SHIFT) and
> then removed.
> 
> The alternative would be just to use kvm_pgtable_stage2_pgd_size()
> instead of
> introducing struct kvm_pgtable::pgd_pages, which will be used in the slow
> paths where realm is created or destroyed.

I think just dropping this patch and using kvm_pgtable_stage2_pgd_size()
in the slow paths makes sense. I think originally there had been some
issue with the value being hard to obtain in the relevant path, but I
can't see any problem now.

Thanks,
Steve

>> diff --git a/arch/arm64/include/asm/kvm_pgtable.h
>> b/arch/arm64/include/asm/kvm_pgtable.h
>> index 03f4c3d7839c..25b512756200 100644
>> --- a/arch/arm64/include/asm/kvm_pgtable.h
>> +++ b/arch/arm64/include/asm/kvm_pgtable.h
>> @@ -404,6 +404,7 @@ static inline bool kvm_pgtable_walk_lock_held(void)
>>    * struct kvm_pgtable - KVM page-table.
>>    * @ia_bits:        Maximum input address size, in bits.
>>    * @start_level:    Level at which the page-table walk starts.
>> + * @pgd_pages:        Number of pages in the entry level of the
>> page-table.
>>    * @pgd:        Pointer to the first top-level entry of the page-table.
>>    * @mm_ops:        Memory management callbacks.
>>    * @mmu:        Stage-2 KVM MMU struct. Unused for stage-1 page-tables.
>> @@ -414,6 +415,7 @@ static inline bool kvm_pgtable_walk_lock_held(void)
>>   struct kvm_pgtable {
>>       u32                    ia_bits;
>>       s8                    start_level;
>> +    u8                    pgd_pages;
>>       kvm_pteref_t                pgd;
>>       struct kvm_pgtable_mm_ops        *mm_ops;
>>   diff --git a/arch/arm64/kvm/hyp/pgtable.c
>> b/arch/arm64/kvm/hyp/pgtable.c
>> index b11bcebac908..9e1be28c3dc9 100644
>> --- a/arch/arm64/kvm/hyp/pgtable.c
>> +++ b/arch/arm64/kvm/hyp/pgtable.c
>> @@ -1534,7 +1534,8 @@ int __kvm_pgtable_stage2_init(struct kvm_pgtable
>> *pgt, struct kvm_s2_mmu *mmu,
>>       u32 sl0 = FIELD_GET(VTCR_EL2_SL0_MASK, vtcr);
>>       s8 start_level = VTCR_EL2_TGRAN_SL0_BASE - sl0;
>>   -    pgd_sz = kvm_pgd_pages(ia_bits, start_level) * PAGE_SIZE;
>> +    pgt->pgd_pages = kvm_pgd_pages(ia_bits, start_level);
>> +    pgd_sz = pgt->pgd_pages * PAGE_SIZE;
>>       pgt->pgd = (kvm_pteref_t)mm_ops->zalloc_pages_exact(pgd_sz);
>>       if (!pgt->pgd)
>>           return -ENOMEM;
>> @@ -1586,7 +1587,7 @@ void kvm_pgtable_stage2_destroy(struct
>> kvm_pgtable *pgt)
>>       };
>>         WARN_ON(kvm_pgtable_walk(pgt, 0, BIT(pgt->ia_bits), &walker));
>> -    pgd_sz = kvm_pgd_pages(pgt->ia_bits, pgt->start_level) * PAGE_SIZE;
>> +    pgd_sz = pgt->pgd_pages * PAGE_SIZE;
>>       pgt->mm_ops->free_pages_exact(kvm_dereference_pteref(&walker,
>> pgt->pgd), pgd_sz);
>>       pgt->pgd = NULL;
>>   }
> 
> Thanks,
> Gavin
>