Re: [PATCH v2 1/2] KVM: arm64: skip pKVM cache flushes for non cacheable mappings

[Bradley Morgan]

On June 23, 2026 6:02:40 PM GMT+01:00, Marc Zyngier <maz at kernel.org> wrote:
>Bradley,
>
>Just a few things to keep in mind for your next contributions:
>
>- If you are sending more than a single patch, add a cover letter.
>
>- Don't send a v2 in reply to a v1. It messes the threading we are
>  relying on, and makes it hard to ignore replies to an older version.
>  Always send new series standalone.
>
>- Don't immediately send a V2, even if (especially if!) a bot is
>  pestering you. 34 minutes between versions is way too short (at
>  least a few days is the norm).
>
>On Tue, 23 Jun 2026 17:37:55 +0100,
>Bradley Morgan <include at grrlz.net> 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")
>> Cc: stable at vger.kernel.org
>
>What device memory gets mapped in an upstream pKVM guest that would
>require a backport to stable?
>
>> Signed-off-by: Bradley Morgan <include at grrlz.net>
>> ---
>> Changes in v2:
>> - Add patch 2 for the pKVM permission fault mapping cache bug.
>
>This is the sort of information that goes in the cover letter.
>
>> 
>>  arch/arm64/include/asm/kvm_pkvm.h | 1 +
>>  arch/arm64/kvm/pkvm.c             | 8 +++++++-
>>  2 files changed, 8 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/arm64/include/asm/kvm_pkvm.h
>b/arch/arm64/include/asm/kvm_pkvm.h
>> index 74fedd9c5ff0..d9dd8239910d 100644
>> --- a/arch/arm64/include/asm/kvm_pkvm.h
>> +++ b/arch/arm64/include/asm/kvm_pkvm.h
>> @@ -196,6 +196,7 @@ struct pkvm_mapping {
>>  	u64 gfn;
>>  	u64 pfn;
>>  	u64 nr_pages;
>> +	bool cacheable;
>
>Errr, no. That's a terrible idea.
>
>This thing is already big enough, let's not add a bool right in the
>middle (use pahole to find out why this is bad). Given that nr_pages
>is for a range, and that the minimum page size uses 12 bits, the
>largest number of pages you can have here is 56-12=48 bit wide. That's
>another 16 bits worth of flags you can use.
>
>Thanks,
>
>	M.
>
>

thanks.

I'll go and do V3 with another sashiko suggestion. I'll fix your path too.
I'll park V3 for a bit.

Thanks!