linux-kernel - Re: [PATCH v8 06/29] KVM: arm64: Introduce non-UNDEF FGT control

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Message-ID: <d27a1c5a-9173-465b-90f9-fec528181ba7@sirena.org.uk>
Date: Fri, 19 Sep 2025 16:53:04 +0100
From: Mark Brown <broonie@...nel.org>
To: Marc Zyngier <maz@...nel.org>
Cc: Oliver Upton <oliver.upton@...ux.dev>, Joey Gouly <joey.gouly@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Will Deacon <will@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
	Dave Martin <Dave.Martin@....com>, Fuad Tabba <tabba@...gle.com>,
	Mark Rutland <mark.rutland@....com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
	Peter Maydell <peter.maydell@...aro.org>,
	Eric Auger <eric.auger@...hat.com>
Subject: Re: [PATCH v8 06/29] KVM: arm64: Introduce non-UNDEF FGT control

On Fri, Sep 19, 2025 at 04:14:49PM +0100, Marc Zyngier wrote:

> FGUs are uniform, because when something doesn't exist on a vcpu, it
> doesn't exist on *any* vcpu. Non-FGU use of FGTs, however, has to be
> more flexible because that's part of the emulation, and is actually
> pretty rare that we want to trap something at all times, on all vcpus.

> For the same reason, conflating the R and W registers doesn't work
> either. For the above example, I want to be able to trap write
> accesses to MDSCR_EL1, and not reads, just like the Ampere
> brain-damage.

> So please make this per-vcpu, decouple R and W FGTs, and convert the
> Ampere horror to this scheme.

OK, that makes more sense - it was a bit confusing that all the FGT
handling was done per VM not per vCPU without even any provision for
per-vCPU or distinct R/W stuff, it seemed strange.  Since the SME
requirement also ends up being per VM anyway I just tried to fit in with
what was there but if we don't want that behaviour it's certainly more
obvious to make it per-vCPU and to split read and write.

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