Re: [PATCH v4] KVM: arm64: Record whether pKVM stage 2 mapping is cacheable

[Bradley Morgan]

On July 5, 2026 8:27:34 PM GMT+01:00, Marc Zyngier <maz at kernel.org> wrote:
>On Sun, 05 Jul 2026 15:08:58 +0100,
>Dev Jain <dev.jain at arm.com> wrote:
>> 
>> 
>> 
>> On 02/07/26 12:54 am, Bradley Morgan wrote:
>> > pKVM keeps its own mapping list for stage 2 operations. Its flush path
>> > uses that list directly, so it lost the PTE attribute check done by
>the
>> > generic stage 2 walker.
>> > 
>> > Record whether a mapping is cacheable and skip cache maintenance for
>> > mappings that are not cacheable.
>> > 
>> > Fixes: e912efed485a ("KVM: arm64: Introduce the EL1 pKVM MMU")
>> 
>> Is Fixes tag required? If I am reading correctly, Arm ARM says this:
>> 
>> "For VA-based cache maintenance instructions, the instruction operates
>on the
>> caches regardless of the memory type and cacheability attributes marked
>for
>> the memory address in the VMSA translation table entries. This means
>that
>> the effects of the cache maintenance instructions can apply regardless
>of:
>>   Whether the address accessed:
>>     Is Normal memory or Device memory.
>>     Has the Cacheable attribute or the Non-cacheable attribute."
>> 
>> So nothing goes wrong if we do dcache clean for non-cacheable
>> memory.
>
>Two things:
>
>- having to perform CMOs for something that is not *expected* to be
>  cacheable is both pointless and a contradiction of the intent
>
>- what you quote is about the nature of the *mapping*, and not the
>  memory that is being mapped. Cleaning a dirty cache line on an
>  unsuspecting MMIO endpoint is never going to end nicely. Just have a
>  try.
>
>My reading of all this is that a fix indeed is required, and therefore
>a Fixes tag *must* be present.
>
>	M.
>
>


Well, fair enough.

It's kind of two things.

1: A fix
2: a optimization 

I say it's a fix, because it's not fun, no.

And I Also say it's a optimization because wasting cycles isn't a good
idea, trust me.

Thanks!