On Thu, 2021-04-01 at 15:03 +0200, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevi...@redhat.com> writes:
>
> > Currently to support Intel->AMD migration, if CPU vendor is GenuineIntel,
> > we emulate the full 64 value for MSR_IA32_SYSENTER_{EIP|ESP}
> > msrs, and we also emulate the sysenter/sysexit instruction in long mode.
> >
> > (Emulator does still refuse to emulate sysenter in 64 bit mode, on the
> > ground that the code for that wasn't tested and likely has no users)
> >
> > However when virtual vmload/vmsave is enabled, the vmload instruction will
> > update these 32 bit msrs without triggering their msr intercept,
> > which will lead to having stale values in kvm's shadow copy of these msrs,
> > which relies on the intercept to be up to date.
> >
> > Fix/optimize this by doing the following:
> >
> > 1. Enable the MSR intercepts for SYSENTER MSRs iff vendor=GenuineIntel
> > (This is both a tiny optimization and also ensures that in case
> > the guest cpu vendor is AMD, the msrs will be 32 bit wide as
> > AMD defined).
> >
> > 2. Store only high 32 bit part of these msrs on interception and combine
> > it with hardware msr value on intercepted read/writes
> > iff vendor=GenuineIntel.
> >
> > 3. Disable vmload/vmsave virtualization if vendor=GenuineIntel.
> > (It is somewhat insane to set vendor=GenuineIntel and still enable
> > SVM for the guest but well whatever).
> > Then zero the high 32 bit parts when kvm intercepts and emulates vmload.
> >
> > Thanks a lot to Paulo Bonzini for helping me with fixing this in the most
>
> s/Paulo/Paolo/ :-)
Sorry about that!
>
> > correct way.
> >
> > This patch fixes nested migration of 32 bit nested guests, that was
> > broken because incorrect cached values of SYSENTER msrs were stored in
> > the migration stream if L1 changed these msrs with
> > vmload prior to L2 entry.
> >
> > Signed-off-by: Maxim Levitsky <mlevi...@redhat.com>
> > ---
> > arch/x86/kvm/svm/svm.c | 99 +++++++++++++++++++++++++++---------------
> > arch/x86/kvm/svm/svm.h | 6 +--
> > 2 files changed, 68 insertions(+), 37 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 271196400495..6c39b0cd6ec6 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -95,6 +95,8 @@ static const struct svm_direct_access_msrs {
> > } direct_access_msrs[MAX_DIRECT_ACCESS_MSRS] = {
> > { .index = MSR_STAR, .always = true },
> > { .index = MSR_IA32_SYSENTER_CS, .always = true },
> > + { .index = MSR_IA32_SYSENTER_EIP, .always = false },
> > + { .index = MSR_IA32_SYSENTER_ESP, .always = false },
> > #ifdef CONFIG_X86_64
> > { .index = MSR_GS_BASE, .always = true },
> > { .index = MSR_FS_BASE, .always = true },
> > @@ -1258,16 +1260,6 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
> > if (kvm_vcpu_apicv_active(vcpu))
> > avic_init_vmcb(svm);
> >
> > - /*
> > - * If hardware supports Virtual VMLOAD VMSAVE then enable it
> > - * in VMCB and clear intercepts to avoid #VMEXIT.
> > - */
> > - if (vls) {
> > - svm_clr_intercept(svm, INTERCEPT_VMLOAD);
> > - svm_clr_intercept(svm, INTERCEPT_VMSAVE);
> > - svm->vmcb->control.virt_ext |=
> > VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;
> > - }
> > -
> > if (vgif) {
> > svm_clr_intercept(svm, INTERCEPT_STGI);
> > svm_clr_intercept(svm, INTERCEPT_CLGI);
> > @@ -2133,9 +2125,11 @@ static int vmload_vmsave_interception(struct
> > kvm_vcpu *vcpu, bool vmload)
> >
> > ret = kvm_skip_emulated_instruction(vcpu);
> >
> > - if (vmload)
> > + if (vmload) {
> > nested_svm_vmloadsave(vmcb12, svm->vmcb);
> > - else
> > + svm->sysenter_eip_hi = 0;
> > + svm->sysenter_esp_hi = 0;
> > + } else
> > nested_svm_vmloadsave(svm->vmcb, vmcb12);
>
> Nitpicking: {} are now needed for both branches here.
I didn't knew about this rule, and I'll take this into
account next time. Thanks!
Best regards,
Maxim Levitsky
