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

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Message-ID: <Z9sItt8BIgvbBY8M@arm.com>
Date: Wed, 19 Mar 2025 18:11:02 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>,
	Ankit Agrawal <ankita@...dia.com>,
	"joey.gouly@....com" <joey.gouly@....com>,
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	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags

On Wed, Mar 19, 2025 at 02:04:29PM -0300, Jason Gunthorpe wrote:
> 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)..

As I replied earlier, it might be worth doing both - fault on !FWB
hardware (or rather reject the memslot creation), cacheable S2
otherwise.

> > 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.

I came to the same conclusion, qemu wouldn't be able to pass any
KVM_MEMORY_* flag to request cacheable since it doesn't know what
attributes it got from VFIO. You need a side-channel communication
between the host VFIO driver and KVM. With Ankit's proposal, that's the
Stage 1 MMU attributes for the qemu mapping.

-- 
Catalin