linux-kernel - Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

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Message-ID: <20250319170429.GK9311@nvidia.com>
Date: Wed, 19 Mar 2025 14:04:29 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: Marc Zyngier <maz@...nel.org>,
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Subject: Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags

On Wed, Mar 19, 2025 at 12:01:29AM -0700, Oliver Upton wrote:

> You have a very good point that KVM is broken for cacheable PFNMAP'd
> crap since we demote to something non-cacheable, and maybe that
> deserves fixing first. Hopefully nobody notices that we've taken away
> the toys...

Fixing it is either faulting all access attempts or mapping it
cachable to the S2 (as this series is trying to do)..

> We still need to test the VMA flag here to select Normal-NC v. Device.

Yes

> I'm less worried about recovery and more focused on userspace being
> able to understand what happened. Otherwise we may get folks complaining
> about the ioctl failing "randomly" on certain machines.

Even today something like qemu doesn't know that VFIO has created a
cachable mmap, so it would need a bunch more work to feed everything
through as proposed here.

IMHO the semantic should be simple enough, when the memslot is created
determine if KVM can support it at all. non-struct page cacheable
memory requires FWB to support it today.

Maybe tomorrow KVM will learn how to cache flush non-struct page
memory and then everything can support it.

It just seems wrong to be making uAPI based around limitations of the
kernel implementation..

Jason