linux-kernel - Re: [PATCH v9 4/6] KVM: arm64: New function to determine hardware cache management support

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Message-ID: <7ccee3be-6438-462d-8dce-efa58238f28d@redhat.com>
Date: Fri, 4 Jul 2025 16:10:30 +0200
From: David Hildenbrand <david@...hat.com>
To: ankita@...dia.com, jgg@...dia.com, maz@...nel.org,
 oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com,
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Cc: aniketa@...dia.com, cjia@...dia.com, kwankhede@...dia.com,
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Subject: Re: [PATCH v9 4/6] KVM: arm64: New function to determine hardware
 cache management support

On 21.06.25 06:21, ankita@...dia.com wrote:
> From: Ankit Agrawal <ankita@...dia.com>
> 
> VM_PFNMAP VMA's are allowed to contain PTE's which point to physical
> addresses that does not have a struct page and may not be in the kernel
> direct map.
> 
> However ARM64 KVM relies on a simple conversion from physaddr to a
> kernel virtual address when it does cache maintenance as the CMO
> instructions work on virtual addresses. This simple approach does not
> work for physical addresses from VM_PFNMAP since those addresses may
> not have a kernel virtual address, or it may be difficult to find it.
> 
> Fortunately if the ARM64 CPU has two features, S2FWB and CACHE DIC,
> then KVM no longer needs to do cache flushing and NOP's all the
> CMOs. This has the effect of no longer requiring a KVA for addresses
> mapped into the S2.
> 
> Add a new function, kvm_arch_supports_cacheable_pfnmap(), to report
> this capability. From a core prespective it means the arch can accept
> a cachable VM_PFNMAP as a memslot. From an ARM64 perspective it means
> that no KVA is required.
> 
> CC: Jason Gunthorpe <jgg@...dia.com>
> CC: David Hildenbrand <david@...hat.com>
> CC: Donald Dutile <ddutile@...hat.com>
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Ankit Agrawal <ankita@...dia.com>
> ---

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb