linux-kernel - Re: [PATCH v9 14/30] KVM: arm64: Implement SME vector length configuration

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Message-ID: <CA+EHjTxQUADWmCbSgUiFcXz_BxDPLE+THHnF6Mwx73hnhc7OJw@mail.gmail.com>
Date: Mon, 12 Jan 2026 13:28:51 +0000
From: Fuad Tabba <tabba@...gle.com>
To: Mark Brown <broonie@...nel.org>
Cc: Marc Zyngier <maz@...nel.org>, 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>, Oliver Upton <oupton@...nel.org>, Dave Martin <Dave.Martin@....com>, 
	Mark Rutland <mark.rutland@....com>, Ben Horgan <ben.horgan@....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 v9 14/30] KVM: arm64: Implement SME vector length configuration

On Mon, 12 Jan 2026 at 13:27, Mark Brown <broonie@...nel.org> wrote:
>
> On Fri, Jan 09, 2026 at 03:59:00PM +0000, Fuad Tabba wrote:
> > On Tue, 23 Dec 2025 at 01:22, Mark Brown <broonie@...nel.org> wrote:
>
> > > +
> > > +#define vcpu_cur_sve_vl(vcpu) (vcpu_in_streaming_mode(vcpu) ? \
> > > +                              vcpu_sme_max_vl(vcpu) : vcpu_sve_max_vl(vcpu))
>
> > nit: This isn't really the current VL, but the current max VL. That
> > said, I don't think 'cur_max` is a better name. Maybe a comment or
> > something?
>
> It is the current VL for the hypervisor and what we present to
> userspace, EL1 can reduce the VL that it sees to something lower if the
> hardware supports that but as far as the hypervisor is concerned the VL
> is always whatever is configured at EL2.  We can obviously infer what
> the guest is doing internally but we never really interact with it.  The
> existing code doesn't really have a concept of current VL since with SVE
> only the hypervisor set VL is always the SVE VL, it often refers to the
> maximum VL when it means the VL the hypervisor works with.
>
> > > +       if (WARN_ON(vcpu->arch.sve_state || vcpu->arch.sme_state))
> > > +               return -EINVAL;
> > > +
>
> > Can this ever happen? kvm_arm_vcpu_vec_finalized() is checked above,
> > and vcpu->arch.{sve,sme}_state are only assigned in
> > kvm_vcpu_finalize_vec() immediately before setting the finalized flag.
>
> I don't expect it to, hence why it's a WARN_ON.

I understand. My point is, by code inspection we can demonstrate that
this isn't needed.

Cheers,
/fuad