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.mar...@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 _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
