| Message ID | 20230411125718.2297768-4-aik@amd.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: SEV: Enable AMD SEV-ES DebugSwap | expand |
On Tue, Apr 11, 2023, Alexey Kardashevskiy wrote: > SVM/SEV enable debug registers intercepts to skip swapping DRs > on entering/exiting the guest. When the guest is in control of > debug registers (vcpu->guest_debug == 0), there is an optimisation to > reduce the number of context switches: intercepts are cleared and > the KVM_DEBUGREG_WONT_EXIT flag is set to tell KVM to do swapping > on guest enter/exit. > > The same code also executes for SEV-ES, however it has no effect as > - it always takes (vcpu->guest_debug == 0) branch; > - KVM_DEBUGREG_WONT_EXIT is set but DR7 intercept is not cleared; > - vcpu_enter_guest() writes DRs but VMRUN for SEV-ES swaps them > with the values from _encrypted_ VMSA. > > Be explicit about SEV-ES not supporting debug: > - return right away from dr_interception() and skip unnecessary processing; > - clear vcpu->guest_debug at SEV-ES' LAUNCH_UPDATE_VMSA if debugging > was already enabled; after that point the generic x86's > KVM_SET_GUEST_DEBUG ioctl disallows enabling debug. > > Add WARN_ON to kvm_x86::sync_dirty_debug_regs() (saves guest DRs on > guest exit) to signify that SEV-ES won't hit that path. > > Suggested-by: Sean Christopherson <seanjc@google.com> > Signed-off-by: Alexey Kardashevskiy <aik@amd.com> > Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com> > --- > Changes: > v5: > * new in the series > --- > arch/x86/kvm/svm/sev.c | 6 ++++++ > arch/x86/kvm/svm/svm.c | 10 +++++++++- > 2 files changed, 15 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > index 0f4761a57d86..b4365622222b 100644 > --- a/arch/x86/kvm/svm/sev.c > +++ b/arch/x86/kvm/svm/sev.c > @@ -639,6 +639,12 @@ static int __sev_launch_update_vmsa(struct kvm *kvm, struct kvm_vcpu *vcpu, > return ret; > > vcpu->arch.guest_state_protected = true; > + > + if (vcpu->guest_debug) > + pr_warn_ratelimited("guest_debug (%lx) not supported for SEV-ES", Note, this needs a newline in the printk, otherwise it'll get buffered until the next non-cont printk comes along (guess how many times I've been burned by this). > + vcpu->guest_debug); > + vcpu->guest_debug = 0; Argh, KVM's APIs can be quite frustrating. IIUC, guest_debug can never actually be consumed because, per Tom[*], "A guest can't run before the LAUNCH_UPDATE process is complete". But because the fact that the VM is an SEV-ES is communicated to KVM after KVM_CREATE_VM, userspace can do KVM_SET_GUEST_DEBUG before KVM_SEV_ES_INIT and before KVM_SEV_LAUNCH_UPDATE_VMSA, and thus get KVM into a state where guest_debug is non-zero for an SEV-ES guest. Blech. Instead of a ratelimited warn, can KVM get away with simply rejecting KVM_SEV_LAUNCH_UPDATE_VMSA if guest_debug is non-zero? That combo can't work, so it's seems unlikely userspace is relying on being able to do KVM_SET_GUEST_DEBUG. If we do "have" to keep this approach, I'm generally opposed to any kind of printk in KVM, but this one does seem to be justified since the most likely scenario is that there's a human interactively debugging the guest (or at least, trying to debug the guest). But I would say explicitly call out the ioctl(), "guest_debug" probably won't mean anything to a random user. And I vote to not print the value, that implies that the specific value is unsupported, not that debug in general is disallowed. Something like this (if we have to)? pr_warn_ratelimited("Suppressing KVM_SET_GUEST_DEBUG for SEV-ES guest\n" [*] https://lore.kernel.org/all/7edcf2c3-005f-04bd-7ec6-80baee236f40@amd.com > + > return 0; > } > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > index dc12de325cca..179952a31d3b 100644 > --- a/arch/x86/kvm/svm/svm.c > +++ b/arch/x86/kvm/svm/svm.c > @@ -1980,7 +1980,7 @@ static void svm_sync_dirty_debug_regs(struct kvm_vcpu *vcpu) > { > struct vcpu_svm *svm = to_svm(vcpu); > > - if (vcpu->arch.guest_state_protected) > + if (WARN_ON_ONCE(sev_es_guest(vcpu->kvm))) > return; > > get_debugreg(vcpu->arch.db[0], 0); > @@ -2698,6 +2698,14 @@ static int dr_interception(struct kvm_vcpu *vcpu) > unsigned long val; > int err = 0; > > + /* > + * SEV-ES intercepts DR7 only to disable guest debugging > + * and the guest issues a VMGEXIT for DR7 write only. KVM cannot Wrapping is a bit aggressive (wrap at 80, not earlier). > + * change DR7 (always swapped as type 'A') so return early. > + */ > + if (sev_es_guest(vcpu->kvm)) > + return 1; > + > if (vcpu->guest_debug == 0) { > /* > * No more DR vmexits; force a reload of the debug registers > -- > 2.39.1 >
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c index 0f4761a57d86..b4365622222b 100644 --- a/arch/x86/kvm/svm/sev.c +++ b/arch/x86/kvm/svm/sev.c @@ -639,6 +639,12 @@ static int __sev_launch_update_vmsa(struct kvm *kvm, struct kvm_vcpu *vcpu, return ret; vcpu->arch.guest_state_protected = true; + + if (vcpu->guest_debug) + pr_warn_ratelimited("guest_debug (%lx) not supported for SEV-ES", + vcpu->guest_debug); + vcpu->guest_debug = 0; + return 0; } diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index dc12de325cca..179952a31d3b 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -1980,7 +1980,7 @@ static void svm_sync_dirty_debug_regs(struct kvm_vcpu *vcpu) { struct vcpu_svm *svm = to_svm(vcpu); - if (vcpu->arch.guest_state_protected) + if (WARN_ON_ONCE(sev_es_guest(vcpu->kvm))) return; get_debugreg(vcpu->arch.db[0], 0); @@ -2698,6 +2698,14 @@ static int dr_interception(struct kvm_vcpu *vcpu) unsigned long val; int err = 0; + /* + * SEV-ES intercepts DR7 only to disable guest debugging + * and the guest issues a VMGEXIT for DR7 write only. KVM cannot + * change DR7 (always swapped as type 'A') so return early. + */ + if (sev_es_guest(vcpu->kvm)) + return 1; + if (vcpu->guest_debug == 0) { /* * No more DR vmexits; force a reload of the debug registers