Re: [RFC PATCH 3/4] arm64/sve: KVM: Ensure user SVE use traps after vcpu execution

Dave Martin
2017-11-23T14:31:36Z

On Wed, Nov 22, 2017 at 08:23:44PM +0100, Christoffer Dall wrote:
> On Fri, Nov 17, 2017 at 04:38:54PM +0000, Dave Martin wrote:
> > Currently, SVE use can remain untrapped if a KVM vcpu thread is
> > preempted inside the kernel and we then switch back to some user
> > thread.
> >
> > This patch ensures that SVE traps for userspace are enabled before
> > switching away from the vcpu thread.
>
> I don't really understand why KVM is any different then any other thread
> which could be using SVE that gets preempted?

The state of CPACR_EL1.ZEN is part of the per-task SVE state in the host,
and needs to be context switched. This is different from CPACR_EL1.FEN
which is always 0b11 in the host. KVM currently is unaware of the
context handling on flow in the host though, and corrupts the ZEN field
rather than saving/restoring it.

We could truly save/restore ZEN, but this feels like a misstep: firstly
this only applies to the VHE case so will be a bit ugly, and secondly
I expect context handling cleanup that makes KVM aware of the host
FPSIMD/SVE handling flow will make such save/restore unnecessary, in
any case, the affected ZEN bit is already recorded as the TIF_SVE flag,
so saving it is redundant.

> > In an attempt to preserve some clarity about why and when this is
> > needed, kvm_fpsimd_flush_cpu_state() is used as a hook for doing
> > this. This means that this function needs to be called after
> > exiting the vcpu instead of before entry:
>
> I don't understand why the former means the latter?

I preferred to keep sve_flush_cpu_state() as the "invalidate any SVE
context cached in the CPU" notification, but if we handle CPACR here
then we need to do this after running the vcpu -- because the hyp
switch code will corrupt the trap state anyway in the VHE case, as a
side effect of disabling traps on vcpu exit.

Alternatively, the hyp trap disable code could be changed to actually
_enable_ SVE traps in the VHE case, but I thought this was both
confusing and tends to hide the real rationale.

>
> > this patch moves the call
> > as appropriate. As a side-effect, this will avoid the call if vcpu
> > entry is shortcircuited by a signal etc.
> >
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > ---
> > arch/arm64/kernel/fpsimd.c | 2 ++
> > virt/kvm/arm/arm.c | 6 +++---
> > 2 files changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> > index 3dc8058..3b135eb 100644
> > --- a/arch/arm64/kernel/fpsimd.c
> > +++ b/arch/arm64/kernel/fpsimd.c
> > @@ -1083,6 +1083,8 @@ void sve_flush_cpu_state(void)
> >
> > if (last->st && last->sve_in_use)
> > fpsimd_flush_cpu_state();
> > +
> > + sve_user_disable();
> > }
> > #endif /* CONFIG_ARM64_SVE */
> >
> > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> > index 772bf74..554b157 100644
> > --- a/virt/kvm/arm/arm.c
> > +++ b/virt/kvm/arm/arm.c
> > @@ -651,9 +651,6 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> > */
> > preempt_disable();
> >
> > - /* Flush FP/SIMD state that can't survive guest entry/exit */
> > - kvm_fpsimd_flush_cpu_state();
> > -
> > kvm_pmu_flush_hwstate(vcpu);
> >
> > local_irq_disable();
> > @@ -754,6 +751,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> > guest_exit();
> > trace_kvm_exit(ret, kvm_vcpu_trap_get_class(vcpu), *vcpu_pc(vcpu));
> >
> > + /* Flush FP/SIMD state that can't survive guest entry/exit */
> > + kvm_fpsimd_flush_cpu_state();
> > +
>
> Could this be done in kvm_arch_vcpu_put() instead?

I think so -- I didn't want to take the VHE optimisation series into
account yet so I wasn't tracking a moving target, but I think this would
fit naturally there.

All of this is fairly confusing, so if there is a way to make it
clearer, I'd be happy to pick it up...

Cheers
---Dave