Re: [PATCH v3 22/28] arm64/sve: KVM: Prevent guests from using SVE

Dave Martin
2017-10-12T11:04:12Z

On Wed, Oct 11, 2017 at 05:28:06PM +0100, Marc Zyngier wrote:
> [+ Christoffer]
>
> On 10/10/17 19:38, Dave Martin wrote:
> > Until KVM has full SVE support, guests must not be allowed to
> > execute SVE instructions.
> >
> > This patch enables the necessary traps, and also ensures that the
> > traps are disabled again on exit from the guest so that the host
> > can still use SVE if it wants to.
> >
> > This patch introduces another instance of
> > __this_cpu_write(fpsimd_last_state, NULL), so this flush operation
> > is abstracted out as a separate helper fpsimd_flush_cpu_state().
> > Other instances are ported appropriately.
> >
> > As a side effect of this refactoring, a this_cpu_write() in
> > fpsimd_cpu_pm_notifier() is changed to __this_cpu_write(). This
> > should be fine, since cpu_pm_enter() is supposed to be called only
> > with interrupts disabled.
> >
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
> > Cc: Marc Zyngier <marc.zyngier@arm.com>
> > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > ---

[...]

> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index e923b58..674912d 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h

[...]

> > @@ -384,4 +385,14 @@ static inline void __cpu_init_stage2(void)

[...]

> > +static inline void kvm_fpsimd_flush_cpu_state(void)
> > +{
> > + if (system_supports_sve())
> > + sve_flush_cpu_state();
>
> Hmmm. How does this work if...

!IS_ENABLED(CONFIG_ARM64_SVE) implies !system_supports_sve(), so
if CONFIG_ARM64_SVE is not set, the call is optimised away.

[...]

> > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> > index a9cb794..6ae3703 100644
> > --- a/arch/arm64/kernel/fpsimd.c
> > +++ b/arch/arm64/kernel/fpsimd.c
> > @@ -1073,6 +1073,33 @@ void fpsimd_flush_task_state(struct task_struct *t)

[...]

> > +#ifdef CONFIG_ARM64_SVE
> > +void sve_flush_cpu_state(void)
> > +{
> > + struct fpsimd_state *const fpstate = __this_cpu_read(fpsimd_last_state);
> > + struct task_struct *tsk;
> > +
> > + if (!fpstate)
> > + return;
> > +
> > + tsk = container_of(fpstate, struct task_struct, thread.fpsimd_state);
> > + if (test_tsk_thread_flag(tsk, TIF_SVE))
> > + fpsimd_flush_cpu_state();
> > +}
> > +#endif /* CONFIG_ARM64_SVE */
>
> ... CONFIG_ARM64_SVE is not set? Fixing this should just be a matter of
> moving the #ifdef/#endif inside the function...

Because sve_flush_cpu_state() is not in the same compilation unit it
can't be static, and that means the compiler won't remove it
automatically if it's unused -- hence the #ifdef.

Because the call site is optimised away, there is no link failure.

Don't we rely on this sort of thing all over the place?

Cheers
---Dave