Re: [PATCH v5 07/34] KVM: SVM: Add required changes to support intercepts under SEV-ES

From: Paolo Bonzini
Date: Mon Dec 14 2020 - 10:36:09 EST

On 10/12/20 18:09, Tom Lendacky wrote:
@@ -2797,7 +2838,27 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
static int wrmsr_interception(struct vcpu_svm *svm)
{
- return kvm_emulate_wrmsr(&svm->vcpu);
+ u32 ecx;
+ u64 data;
+
+ if (!sev_es_guest(svm->vcpu.kvm))
+ return kvm_emulate_wrmsr(&svm->vcpu);
+
+ ecx = kvm_rcx_read(&svm->vcpu);
+ data = kvm_read_edx_eax(&svm->vcpu);
+ if (kvm_set_msr(&svm->vcpu, ecx, data)) {
+ trace_kvm_msr_write_ex(ecx, data);
+ ghcb_set_sw_exit_info_1(svm->ghcb, 1);
+ ghcb_set_sw_exit_info_2(svm->ghcb,
+ X86_TRAP_GP |
+ SVM_EVTINJ_TYPE_EXEPT |
+ SVM_EVTINJ_VALID);
+ return 1;
+ }
+
+ trace_kvm_msr_write(ecx, data);
+
+ return kvm_skip_emulated_instruction(&svm->vcpu);
}
static int msr_interception(struct vcpu_svm *svm)

This code duplication is ugly, and does not work with userspace MSR filters too.

But we can instead trap the completion of the MSR read/write to use ghcb_set_sw_exit_info_1 instead of kvm_inject_gp, with a callback like

static int svm_complete_emulated_msr(struct kvm_vcpu *vcpu, int err)
{
if (!sev_es_guest(svm->vcpu.kvm) || !err)
return kvm_complete_insn_gp(&svm->vcpu, err);

ghcb_set_sw_exit_info_1(svm->ghcb, 1);
ghcb_set_sw_exit_info_2(svm->ghcb,
X86_TRAP_GP |
SVM_EVTINJ_TYPE_EXEPT |
SVM_EVTINJ_VALID);
return 1;
}


...
.complete_emulated_msr = svm_complete_emulated_msr,

@@ -2827,7 +2888,14 @@ static int interrupt_window_interception(struct vcpu_svm *svm)
static int pause_interception(struct vcpu_svm *svm)
{
struct kvm_vcpu *vcpu = &svm->vcpu;
- bool in_kernel = (svm_get_cpl(vcpu) == 0);
+ bool in_kernel;
+
+ /*
+ * CPL is not made available for an SEV-ES guest, so just set in_kernel
+ * to true.
+ */
+ in_kernel = (sev_es_guest(svm->vcpu.kvm)) ? true
+ : (svm_get_cpl(vcpu) == 0);
if (!kvm_pause_in_guest(vcpu->kvm))
grow_ple_window(vcpu);

See below.

@@ -3273,6 +3351,13 @@ bool svm_interrupt_blocked(struct kvm_vcpu *vcpu)
struct vcpu_svm *svm = to_svm(vcpu);
struct vmcb *vmcb = svm->vmcb;
+ /*
+ * SEV-ES guests to not expose RFLAGS. Use the VMCB interrupt mask
+ * bit to determine the state of the IF flag.
+ */
+ if (sev_es_guest(svm->vcpu.kvm))
+ return !(vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK);

This seems wrong, you have to take into account SVM_INTERRUPT_SHADOW_MASK as well. Also, even though GIF is not really used by SEV-ES guests, I think it's nicer to put this check afterwards.

That is:

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 4372e45c8f06..2dd9c9698480 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3247,7 +3247,14 @@ bool svm_interrupt_blocked(struct kvm_vcpu *vcpu)
if (!gif_set(svm))
return true;

- if (is_guest_mode(vcpu)) {
+ if (sev_es_guest(svm->vcpu.kvm)) {
+ /*
+ * SEV-ES guests to not expose RFLAGS. Use the VMCB interrupt mask
+ * bit to determine the state of the IF flag.
+ */
+ if (!(vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK))
+ return true;
+ } else if (is_guest_mode(vcpu)) {
/* As long as interrupts are being delivered... */
if ((svm->nested.ctl.int_ctl & V_INTR_MASKING_MASK)
? !(svm->nested.hsave->save.rflags & X86_EFLAGS_IF)



if (!gif_set(svm))
return true;
@@ -3458,6 +3543,12 @@ static void svm_complete_interrupts(struct vcpu_svm *svm)
svm->vcpu.arch.nmi_injected = true;
break;
case SVM_EXITINTINFO_TYPE_EXEPT:
+ /*
+ * Never re-inject a #VC exception.
+ */
+ if (vector == X86_TRAP_VC)
+ break;
+
/*
* In case of software exceptions, do not reinject the vector,
* but re-execute the instruction instead. Rewind RIP first
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a3fdc16cfd6f..b6809a2851d2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4018,7 +4018,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
{
int idx;
- if (vcpu->preempted)
+ if (vcpu->preempted && !vcpu->arch.guest_state_protected)
vcpu->arch.preempted_in_kernel = !kvm_x86_ops.get_cpl(vcpu);

This has to be true, otherwise no directed yield will be done at all:

if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
!kvm_arch_vcpu_in_kernel(vcpu))
continue;

Or more easily, just use in_kernel == false in pause_interception, like

+ /*
+ * CPL is not made available for an SEV-ES guest, therefore
+ * vcpu->arch.preempted_in_kernel can never be true. Just
+ * set in_kernel to false as well.
+ */
+ in_kernel = !sev_es_guest(svm->vcpu.kvm) && svm_get_cpl(vcpu) == 0;

/*
@@ -8161,7 +8161,9 @@ static void post_kvm_run_save(struct kvm_vcpu *vcpu)
{
struct kvm_run *kvm_run = vcpu->run;
- kvm_run->if_flag = (kvm_get_rflags(vcpu) & X86_EFLAGS_IF) != 0;
+ kvm_run->if_flag = (vcpu->arch.guest_state_protected)
+ ? kvm_arch_interrupt_allowed(vcpu)
+ : (kvm_get_rflags(vcpu) & X86_EFLAGS_IF) != 0;

Here indeed you only want the interrupt allowed bit, not the interrupt window. But we can just be bold and always set it to true.

- for userspace irqchip, kvm_run->ready_for_interrupt_injection is set just below and it will always be false if kvm_arch_interrupt_allowed is false

- for in-kernel APIC, if_flag is documented to be invalid (though it actually is valid). For split irqchip, they can just use kvm_run->ready_for_interrupt_injection; for entirely in-kernel interrupt handling, userspace does not need if_flag at all.

Paolo

kvm_run->flags = is_smm(vcpu) ? KVM_RUN_X86_SMM : 0;
kvm_run->cr8 = kvm_get_cr8(vcpu);
kvm_run->apic_base = kvm_get_apic_base(vcpu);