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

Marc Zyngier
2017-10-12T11:28:32Z

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.

M.
--
Jazz is not dead. It just smells funny...