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

From: Marc Zyngier <maz@kernel.org>
To: Mark Brown <broonie@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Jonathan Corbet <corbet@lwn.net>, Shuah Khan <shuah@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, Dave Martin <Dave.Martin@arm.com>,
	kvmarm@lists.linux.dev, linux-doc@vger.kernel.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v6 1/5] KVM: arm64: Share all userspace hardened thread data with the hypervisor
Date: Tue, 02 Apr 2024 15:53:33 +0100	[thread overview]
Message-ID: <86h6gju87m.wl-maz@kernel.org> (raw)
In-Reply-To: <fb54d7b0-9c83-4a0c-a08b-b722c9381ca7@sirena.org.uk>

On Tue, 02 Apr 2024 15:34:27 +0100,
Mark Brown <broonie@kernel.org> wrote:
> 
> [1  <text/plain; us-ascii (quoted-printable)>]
> On Sun, Mar 31, 2024 at 11:00:41AM +0100, Marc Zyngier wrote:
> > Mark Brown <broonie@kernel.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.

Sure, if your intent is for this code not to be merged.

Because it means this series assumes a different data life cycle, and
the review effort spent on it will be invalidated once you move to the
per-CPU state.

	M.

-- 
Without deviation from the norm, progress is not possible.