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

[Dev Jain]

On 06/07/26 12:57 am, Marc Zyngier wrote:
> On Sun, 05 Jul 2026 15:08:58 +0100,
> Dev Jain <dev.jain@xxxxxxx> 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.

Thanks for explaining.


> 
> 	M.
>