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

[Dave Martin <Dave dot Martin at arm dot com>]

On Thu, Oct 12, 2017 at 12:28:32PM +0100, Marc Zyngier wrote:
> On 12/10/17 12:04, Dave Martin wrote:
> > 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?
> Dunno. It just feels weird. But if you are sure that it won't break,
> fine by me. I guess we'll find out pretty quickly how this fares,
> specially with older toolchains.

I thought this was why the kernel doesn't support building with -O0.
There are many instances of this in the series, not just here.

Let me know if you feel this isn't good enough though.

Do you have any other comments on this patch?

Cheers
---Dave