linux-kernel - Re: [PATCH v6 1/5] KVM: arm64: Share all userspace hardened thread data with the hypervisor

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Date: Tue, 2 Apr 2024 15:34:27 +0100
From: Mark Brown <broonie@...nel.org>
To: Marc Zyngier <maz@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Dave Martin <Dave.Martin@....com>, kvmarm@...ts.linux.dev,
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v6 1/5] KVM: arm64: Share all userspace hardened thread
 data with the hypervisor

On Sun, Mar 31, 2024 at 11:00:41AM +0100, Marc Zyngier wrote:
> Mark Brown <broonie@...nel.org> wrote:

> > As part of the lazy FPSIMD state transitioning done by the hypervisor we
> > currently share the userpsace FPSIMD state in thread->uw.fpsimd_state with
> > the host. Since this struct is non-extensible userspace ABI we have to keep

> Using the same representation is just pure convenience, and nothing
> requires us to use the it in the kernel/hypervisor.

Indeed, the additional data seemed contained enough that it was a
reasonable tradeoff.

> > the definition as is but the addition of FPMR in the 2023 dpISA means that
> > we will want to share more storage with the host. To facilitate this
> > refactor the current code to share the entire thread->uw rather than just
> > the one field.

> So this increase the required sharing with EL2 from 528 bytes to
> 560. Not a huge deal, but definitely moving in the wrong direction. Is
> there any plans to add more stuff to this structure that wouldn't be
> *directly* relevant to the hypervisor?

I'm not aware of any current plans to extend this.

> > @@ -640,7 +641,7 @@ struct kvm_vcpu_arch {
> >  	struct kvm_guest_debug_arch vcpu_debug_state;
> >  	struct kvm_guest_debug_arch external_debug_state;
> >  
> > -	struct user_fpsimd_state *host_fpsimd_state;	/* hyp VA */
> > +	struct thread_struct_uw *host_uw;	/* hyp VA */
> >  	struct task_struct *parent_task;

> Well, this is going away, and you know it.

Sure, those patches are still in flight though.  It does seem reasonable
to target the current code.

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