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

Marc Zyngier
2024-04-02T14:53:36Z

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.