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

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Message-ID: <20250704134740.GG1410929@nvidia.com>
Date: Fri, 4 Jul 2025 10:47:40 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: ankita@...dia.com
Cc: maz@...nel.org, oliver.upton@...ux.dev, joey.gouly@....com,
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Subject: Re: [PATCH v9 4/6] KVM: arm64: New function to determine hardware
 cache management support

On Sat, Jun 21, 2025 at 04:21:09AM +0000, 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>
> ---
>  arch/arm64/kvm/mmu.c     | 23 +++++++++++++++++++++++
>  include/linux/kvm_host.h |  2 ++
>  virt/kvm/kvm_main.c      |  5 +++++
>  3 files changed, 30 insertions(+)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason