Re: [PATCH 5/8] KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction

From: Maciej S. Szmigiero
Date: Mon Apr 04 2022 - 18:03:06 EST

On 4.04.2022 18:49, Sean Christopherson wrote:
On Mon, Apr 04, 2022, Maxim Levitsky wrote:
On Sat, 2022-04-02 at 01:09 +0000, Sean Christopherson wrote:
Re-inject INT3/INTO instead of retrying the instruction if the CPU
encountered an intercepted exception while vectoring the software
exception, e.g. if vectoring INT3 encounters a #PF and KVM is using
shadow paging. Retrying the instruction is architecturally wrong, e.g.
will result in a spurious #DB if there's a code breakpoint on the INT3/O,
and lack of re-injection also breaks nested virtualization, e.g. if L1
injects a software exception and vectoring the injected exception
encounters an exception that is intercepted by L0 but not L1.

Due to, ahem, deficiencies in the SVM architecture, acquiring the next
RIP may require flowing through the emulator even if NRIPS is supported,
as the CPU clears next_rip if the VM-Exit is due to an exception other
than "exceptions caused by the INT3, INTO, and BOUND instructions". To
deal with this, "skip" the instruction to calculate next_ript, and then
unwind the RIP write and any side effects (RFLAGS updates).


kvm_make_request(KVM_REQ_EVENT, vcpu);
vector = exitintinfo & SVM_EXITINTINFO_VEC_MASK;
@@ -3711,9 +3762,9 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
* hit a #NP in the guest, and the #NP encountered a #PF, the #NP will
* be the reported vectored event, but RIP still needs to be unwound.
- if (int3_injected && type == SVM_EXITINTINFO_TYPE_EXEPT &&
- kvm_is_linear_rip(vcpu, svm->int3_rip))
- kvm_rip_write(vcpu, kvm_rip_read(vcpu) - int3_injected);
+ if (soft_int_injected && type == SVM_EXITINTINFO_TYPE_EXEPT &&
+ kvm_is_linear_rip(vcpu, svm->soft_int_linear_rip))
+ kvm_rip_write(vcpu, kvm_rip_read(vcpu) - soft_int_injected);
switch (type) {
@@ -3726,14 +3777,6 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
if (vector == X86_TRAP_VC)
- /*
- * In case of software exceptions, do not reinject the vector,
- * but re-execute the instruction instead. Rewind RIP first
- * if we emulated INT3 before.
- */
- if (kvm_exception_is_soft(vector))
- break;
if (exitintinfo & SVM_EXITINTINFO_VALID_ERR) {
u32 err = svm->vmcb->control.exit_int_info_err;
kvm_requeue_exception_e(vcpu, vector, err);
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 47e7427d0395..a770a1c7ddd2 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -230,8 +230,8 @@ struct vcpu_svm {
bool nmi_singlestep;
u64 nmi_singlestep_guest_rflags;
- unsigned int3_injected;
- unsigned long int3_rip;
+ unsigned soft_int_injected;
+ unsigned long soft_int_linear_rip;
/* optional nested SVM features that are enabled for this guest */
bool nrips_enabled : 1;

I mostly agree with this patch, but think that it doesn't address the
original issue that Maciej wanted to address:

Suppose that there is *no* instruction in L2 code which caused the software
exception, but rather L1 set arbitrary next_rip, and set EVENTINJ to software
exception with some vector, and that injection got interrupted.

I don't think that this code will support this.

Argh, you're right. Maciej's selftest injects without an instruction, but it doesn't
configure the scenario where that injection fails due to an exception+VM-Exit that
isn't intercepted by L1 and is handled by L0. The event_inj test gets the coverage
for the latter, but always has a backing instruction.

I think that svm_complete_interrupts should store next_rip it in some field
like VMX does (vcpu->arch.event_exit_inst_len).

Yeah. The ugly part is that because next_rip is guaranteed to be cleared on exit
(the exit is gauranteed to be due to a fault-like exception), KVM has to snapshot
next_rip during the "original" injection and use the linear_rip matching heuristic
to detect this scenario.

That field also should be migrated, or we must prove that it works anyway.
E.g, what happens when we tried to inject event,
injection was interrupted by other exception, and then we migrate?

Ya, should Just Work if control.next_rip is used to cache the next rip.

Handling this doesn't seem to be too awful (haven't tested yet), it's largely the
same logic as the existing !nrips code.

In svm_update_soft_interrupt_rip(), snapshot all information regardless of whether
or not nrips is enabled:

svm->soft_int_injected = true;
svm->soft_int_csbase = svm->vmcb->save.cs.base;
svm->soft_int_old_rip = old_rip;
svm->soft_int_next_rip = rip;

if (nrips)
kvm_rip_write(vcpu, old_rip);

if (static_cpu_has(X86_FEATURE_NRIPS))
svm->vmcb->control.next_rip = rip;

and then in svm_complete_interrupts(), change the linear RIP matching code to look
for the old rip in the nrips case and stuff svm->vmcb->control.next_rip on match.

bool soft_int_injected = svm->soft_int_injected;
unsigned soft_int_rip;

svm->soft_int_injected = false;

if (soft_int_injected) {
if (nrips)
soft_int_rip = svm->soft_int_old_rip;
soft_int_rip = svm->soft_int_next_rip;


if soft_int_injected && type == SVM_EXITINTINFO_TYPE_EXEPT &&
kvm_is_linear_rip(vcpu, soft_int_rip + svm->soft_int_csbase)) {
if (nrips)
svm->vmcb->control.next_rip = svm->soft_int_next_rip;
kvm_rip_write(vcpu, svm->soft_int_old_rip);

Despite what the svm_update_soft_interrupt_rip() name might suggest this
handles only *soft exceptions*, not *soft interrupts*
(which are injected by svm_inject_irq() and also need proper next_rip

Also, I'm not sure that even the proposed updated code above will
actually restore the L1-requested next_rip correctly on L1 -> L2
re-injection (will review once the full version is available).