linux-kernel - Re: [PATCH v9 6/6] KVM: arm64: Expose new KVM cap for cacheable PFNMAP

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Message-ID: <29cc7d2a-b174-4ed3-ae1a-22db478c62ec@redhat.com>
Date: Fri, 4 Jul 2025 16:15:28 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>, ankita@...dia.com
Cc: maz@...nel.org, oliver.upton@...ux.dev, joey.gouly@....com,
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Subject: Re: [PATCH v9 6/6] KVM: arm64: Expose new KVM cap for cacheable
 PFNMAP

On 04.07.25 15:44, Jason Gunthorpe wrote:
> On Sat, Jun 21, 2025 at 04:21:11AM +0000, ankita@...dia.com wrote:
>> From: Ankit Agrawal <ankita@...dia.com>
>>
>> Introduce a new KVM capability to expose to the userspace whether
>> cacheable mapping of PFNMAP is supported.
>>
>> The ability to safely do the cacheable mapping of PFNMAP is contingent
>> on S2FWB and ARM64_HAS_CACHE_DIC. S2FWB allows KVM to avoid flushing
>> the D cache, ARM64_HAS_CACHE_DIC allows KVM to avoid flushing the icache
>> and turns icache_inval_pou() into a NOP. The cap would be false if
>> those requirements are missing and is checked by making use of
>> kvm_arch_supports_cacheable_pfnmap.
>>
>> This capability would allow userspace to discover the support.
>> It could for instance be used by userspace to prevent live-migration
>> across FWB and non-FWB hosts.
>>
>> CC: Catalin Marinas <catalin.marinas@....com>
>> CC: Jason Gunthorpe <jgg@...dia.com>
>> CC: Oliver Upton <oliver.upton@...ux.dev>
>> CC: David Hildenbrand <david@...hat.com>
>> Suggested-by: Marc Zyngier <maz@...nel.org>
>> Signed-off-by: Ankit Agrawal <ankita@...dia.com>
>> ---
>>   Documentation/virt/kvm/api.rst | 13 ++++++++++++-
>>   arch/arm64/kvm/arm.c           |  7 +++++++
>>   include/uapi/linux/kvm.h       |  1 +
>>   3 files changed, 20 insertions(+), 1 deletion(-)
> 
> I don't know if any VMM will ever use this, but it looks OK

So, should we defer it to the point where we actually have a use case?

I mean, patch #4 could be simplified by modifying arm64 code in patch #5 
only. No need for a common kvm_arch function etc.

-- 
Cheers,

David / dhildenb