Re: [PATCH v12 09/16] KVM: arm64: define kvm_arch_gmem_supports_no_direct_map()

[Brendan Jackman]

On Tue Apr 21, 2026 at 4:55 PM UTC, Marc Zyngier wrote:
> On Fri, 10 Apr 2026 16:19:24 +0100,
> "Kalyazin, Nikita" <kalyazin@xxxxxxxxxxxx> wrote:
>> 
>> From: Patrick Roy <patrick.roy@xxxxxxxxx>
>> 
>> 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.
>
> Where is this 4kB requirement enforced? Or is it that you means
> "PAGE_SIZE"?

Yeah I believe this means PAGE_SIZE and that it's effectively enforced
by checking can_set_direct_map() in
kvm_arch_gmem_supports_no_direct_map().