| Message ID | bdd53f40-4e60-f3ae-7ec6-162198214953@siemens.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | i386/kvm: Do not sync nested state during runtime | expand |
> On 22 Jul 2019, at 7:00, Jan Kiszka <jan.kiszka@siemens.com> wrote: > > Writing the nested state e.g. after a vmport access can invalidate > important parts of the kernel-internal state, and it is not needed as > well. So leave this out from KVM_PUT_RUNTIME_STATE. > > Suggested-by: Paolo Bonzini <pbonzini@redhat.com> > Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com> As QEMU never modifies vCPU nested-state in userspace besides in vmload and vCPU creation, shouldn’t this be under KVM_PUT_FULL_STATE? Same as the call to kvm_arch_set_tsc_khz(). -Liran > --- > target/i386/kvm.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/target/i386/kvm.c b/target/i386/kvm.c > index ec7870c6af..da98b2cbca 100644 > --- a/target/i386/kvm.c > +++ b/target/i386/kvm.c > @@ -3577,12 +3577,12 @@ int kvm_arch_put_registers(CPUState *cpu, int level) > > assert(cpu_is_stopped(cpu) || qemu_cpu_is_self(cpu)); > > - ret = kvm_put_nested_state(x86_cpu); > - if (ret < 0) { > - return ret; > - } > - > if (level >= KVM_PUT_RESET_STATE) { > + ret = kvm_put_nested_state(x86_cpu); > + if (ret < 0) { > + return ret; > + } > + > ret = kvm_put_msr_feature_control(x86_cpu); > if (ret < 0) { > return ret; > -- > 2.16.4
On 22.07.19 11:44, Liran Alon wrote: > > >> On 22 Jul 2019, at 7:00, Jan Kiszka <jan.kiszka@siemens.com> wrote: >> >> Writing the nested state e.g. after a vmport access can invalidate >> important parts of the kernel-internal state, and it is not needed as >> well. So leave this out from KVM_PUT_RUNTIME_STATE. >> >> Suggested-by: Paolo Bonzini <pbonzini@redhat.com> >> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com> > > As QEMU never modifies vCPU nested-state in userspace besides in vmload and vCPU creation, > shouldn’t this be under KVM_PUT_FULL_STATE? Same as the call to kvm_arch_set_tsc_khz(). Reset is a relevant modification as well. If we do not write back a state that is disabling virtualization, we break. Jan
> On 22 Jul 2019, at 13:20, Jan Kiszka <jan.kiszka@siemens.com> wrote: > > On 22.07.19 11:44, Liran Alon wrote: >> >> >>> On 22 Jul 2019, at 7:00, Jan Kiszka <jan.kiszka@siemens.com> wrote: >>> >>> Writing the nested state e.g. after a vmport access can invalidate >>> important parts of the kernel-internal state, and it is not needed as >>> well. So leave this out from KVM_PUT_RUNTIME_STATE. >>> >>> Suggested-by: Paolo Bonzini <pbonzini@redhat.com> >>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com> >> >> As QEMU never modifies vCPU nested-state in userspace besides in vmload and vCPU creation, >> shouldn’t this be under KVM_PUT_FULL_STATE? Same as the call to kvm_arch_set_tsc_khz(). > > Reset is a relevant modification as well. If we do not write back a state that > is disabling virtualization, we break. > > Jan Currently QEMU writes to userspace maintained nested-state only at kvm_arch_init_vcpu() and when loading vmstate_nested_state vmstate subsection. kvm_arch_reset_vcpu() do not modify userspace maintained nested-state. I would expect KVM internal nested-state to be reset as part of KVM’s vmx_vcpu_reset(). Are we missing a call to vmx_leave_nested() there? -Liran
On 22.07.19 12:31, Liran Alon wrote: >> On 22 Jul 2019, at 13:20, Jan Kiszka <jan.kiszka@siemens.com> wrote: >> >> On 22.07.19 11:44, Liran Alon wrote: >>> >>> >>>> On 22 Jul 2019, at 7:00, Jan Kiszka <jan.kiszka@siemens.com> wrote: >>>> >>>> Writing the nested state e.g. after a vmport access can invalidate >>>> important parts of the kernel-internal state, and it is not needed as >>>> well. So leave this out from KVM_PUT_RUNTIME_STATE. >>>> >>>> Suggested-by: Paolo Bonzini <pbonzini@redhat.com> >>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com> >>> >>> As QEMU never modifies vCPU nested-state in userspace besides in vmload and vCPU creation, >>> shouldn’t this be under KVM_PUT_FULL_STATE? Same as the call to kvm_arch_set_tsc_khz(). >> >> Reset is a relevant modification as well. If we do not write back a state that >> is disabling virtualization, we break. >> >> Jan > > Currently QEMU writes to userspace maintained nested-state only at kvm_arch_init_vcpu() and > when loading vmstate_nested_state vmstate subsection. > kvm_arch_reset_vcpu() do not modify userspace maintained nested-state. Hmm, then we probably achieve that effect by clearing the related bit in CR4. If doing that implies an invalidation of the nested state by KVM, we are fine. Otherwise, I would expect userspace to reset the state to VMCLEAR and purge any traces of prior use. > > I would expect KVM internal nested-state to be reset as part of KVM’s vmx_vcpu_reset(). vmx_vcpu_reset is not issued on userspace initiated VM reset, only on INIT/SIPI cycles. It's misleading, and I remember myself running into that when I hacked on KVM back then. Jan > Are we missing a call to vmx_leave_nested() there? > > -Liran >
On 22/07/19 12:43, Jan Kiszka wrote: >> Currently QEMU writes to userspace maintained nested-state only at kvm_arch_init_vcpu() and >> when loading vmstate_nested_state vmstate subsection. >> kvm_arch_reset_vcpu() do not modify userspace maintained nested-state. > Hmm, then we probably achieve that effect by clearing the related bit in CR4. Almost: by clearing the VMX enable bit in MSR_IA32_FEATURE_CONTROL. Actually I think you contributed that. :) I think we could in principle skip that MSR write if env->nested_state != NULL, but it doesn't hurt either, and it makes sense that nested virt state goes together with MSR_IA32_FEATURE_CONTROL since the latter is indede nested virtualization related. Paolo > If doing that implies an invalidation of the nested state by KVM, we > are fine. Otherwise, I would expect userspace to reset the state to > VMCLEAR and purge any traces of prior use.
diff --git a/target/i386/kvm.c b/target/i386/kvm.c index ec7870c6af..da98b2cbca 100644 --- a/target/i386/kvm.c +++ b/target/i386/kvm.c @@ -3577,12 +3577,12 @@ int kvm_arch_put_registers(CPUState *cpu, int level) assert(cpu_is_stopped(cpu) || qemu_cpu_is_self(cpu)); - ret = kvm_put_nested_state(x86_cpu); - if (ret < 0) { - return ret; - } - if (level >= KVM_PUT_RESET_STATE) { + ret = kvm_put_nested_state(x86_cpu); + if (ret < 0) { + return ret; + } + ret = kvm_put_msr_feature_control(x86_cpu); if (ret < 0) { return ret;
Writing the nested state e.g. after a vmport access can invalidate important parts of the kernel-internal state, and it is not needed as well. So leave this out from KVM_PUT_RUNTIME_STATE. Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com> --- target/i386/kvm.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)