Re: [PATCH v10 08/15] KVM: arm64: define kvm_arch_gmem_supports_no_direct_map()

From: "David Hildenbrand (Arm)" <david@kernel.org>
To: "Kalyazin, Nikita" <kalyazin@amazon.co.uk>,
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Subject: Re: [PATCH v10 08/15] KVM: arm64: define kvm_arch_gmem_supports_no_direct_map()
Date: Thu, 5 Mar 2026 20:08:35 +0100	[thread overview]
Message-ID: <b1a79a08-d99a-492a-8baf-e83127154a5a@kernel.org> (raw)
In-Reply-To: <20260126164445.11867-9-kalyazin@amazon.com>

On 1/26/26 17:50, Kalyazin, Nikita wrote:
> From: Patrick Roy <patrick.roy@linux.dev>
> 
> Support for GUEST_MEMFD_FLAG_NO_DIRECT_MAP on arm64 depends on 1) direct
> map manipulations at 4k granularity being possible, and 2) FEAT_S2FWB.
> 
> 1) is met whenever the direct map is set up at 4k granularity (e.g. not
>  with huge/gigantic pages) at boottime, as due to ARM's
> break-before-make semantics, breaking huge mappings into 4k mappings in
> the direct map is not possible (BBM would require temporary invalidation
> of the entire huge mapping, even if only a 4k subrange should be zapped,
> which will probably crash the kernel). However, the current default for
> rodata_full is true, which forces a 4k direct map.
> 
> 2) is required to allow KVM to elide cache coherency operations when
> installing stage 2 page tables, which require the direct map to be
> entry for the newly mapped memory to be present (which it will not be,
> as guest_memfd would have removed direct map entries in
> kvm_gmem_get_pfn()).
> 
> Cc: Will Deacon <will@kernel.org>
> Signed-off-by: Patrick Roy <patrick.roy@linux.dev>
> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
> ---

Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David