linux-kernel - Re: [PATCH v6 29/43] arm64: RME: Always use 4k pages for realms

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Message-ID: <7e03ad4a-ca82-4fe6-b789-1990dbbaf256@arm.com>
Date: Fri, 7 Feb 2025 17:05:39 +0000
From: Steven Price <steven.price@....com>
To: Gavin Shan <gshan@...hat.com>, kvm@...r.kernel.org, kvmarm@...ts.linux.dev
Cc: 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>,
 Suzuki K Poulose <suzuki.poulose@....com>, 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 v6 29/43] arm64: RME: Always use 4k pages for realms

On 02/02/2025 06:52, Gavin Shan wrote:
> On 12/13/24 1:55 AM, Steven Price wrote:
>> Always split up huge pages to avoid problems managing huge pages. There
>> are two issues currently:
>>
>> 1. The uABI for the VMM allows populating memory on 4k boundaries even
>>     if the underlying allocator (e.g. hugetlbfs) is using a larger page
>>     size. Using a memfd for private allocations will push this issue onto
>>     the VMM as it will need to respect the granularity of the allocator.
>>
>> 2. The guest is able to request arbitrary ranges to be remapped as
>>     shared. Again with a memfd approach it will be up to the VMM to deal
>>     with the complexity and either overmap (need the huge mapping and add
>>     an additional 'overlapping' shared mapping) or reject the request as
>>     invalid due to the use of a huge page allocator.
>>
>> For now just break everything down to 4k pages in the RMM controlled
>> stage 2.
>>
>> Signed-off-by: Steven Price <steven.price@....com>
>> ---
>>   arch/arm64/kvm/mmu.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index e88714903ce5..9ede143ccef1 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1603,6 +1603,10 @@ static int user_mem_abort(struct kvm_vcpu
>> *vcpu, phys_addr_t fault_ipa,
>>       if (logging_active) {
>>           force_pte = true;
>>           vma_shift = PAGE_SHIFT;
>> +    } else if (kvm_is_realm(kvm)) {
>> +        // Force PTE level mappings for realms
>> +        force_pte = true;
>> +        vma_shift = PAGE_SHIFT;
>>       } else {
>>           vma_shift = get_vma_page_shift(vma, hva);
>>       }
> 
> Since a memory abort is specific to a vCPU instead of a VM, so
> vcpu_is_rec()
> instead of kvm_is_realm() is more accurate for the check. Besides, it looks
> duplicate to the check added by "PATCH[20/43] arm64: RME: Runtime faulting
> of memory", which is as below.
> 
>        /* FIXME: We shouldn't need to disable this for realms */
>        if (vma_pagesize == PAGE_SIZE && !(force_pte || device ||
> kvm_is_realm(kvm))) {
>                                                                 
> ^^^^^^^^^^^^^^^^^
>                                                                  Can be
> dropped now.

Indeed, thanks for that - one less FIXME ;)

Thanks,
Steve

> Thanks,
> Gavin
>                 
>