[PATCH RFC 0/5] x86/KVM/svm: get rid of hardcoded instructions lengths

Vitaly Kuznetsov
2019-06-20T11:02:46Z

Jim rightfully complains that hardcoding instuctions lengths is not always
correct: additional (redundant) prefixes can be used. Luckily, the ugliness
is mostly harmless: modern AMD CPUs support NRIP_SAVE feature but I'd like
to clean things up and sacrifice speed in favor of correctness.

Early RFC. Unfortunately, I got distracted by some other problems so
sending it out half-baked.

TODO:
- Get rid of hardcoded '+ 3' in vmrun_interception().
- Test.

P.S. If you'd like to test the series you'll have to have a CPU without
NRIP_SAVE feature or forcefully disable it, something like:

index 8d4e50428b68..93c7eaad7915 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -922,6 +922,9 @@ static void init_amd(struct cpuinfo_x86 *c)
/* AMD CPUs don't reset SS attributes on SYSRET, Xen does. */
if (!cpu_has(c, X86_FEATURE_XENPV))
set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
+
+ /* No nrips */
+ clear_cpu_cap(c, X86_FEATURE_NRIPS);
}

#ifdef CONFIG_X86_32
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 5beca1030c9a..5b2ea34bc9f2 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -773,11 +773,11 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
{
struct vcpu_svm *svm = to_svm(vcpu);

- if (svm->vmcb->control.next_rip != 0) {
+/* if (svm->vmcb->control.next_rip != 0) {
WARN_ON_ONCE(!static_cpu_has(X86_FEATURE_NRIPS));
svm->next_rip = svm->vmcb->control.next_rip;
}
-
+*/
if (!svm->next_rip) {
if (kvm_emulate_instruction(vcpu, EMULTYPE_SKIP) !=
EMULATE_DONE)

Vitaly Kuznetsov (5):
x86: KVM: svm: don't pretend to advance RIP in case
wrmsr_interception() results in #GP
x86: KVM: svm: avoid flooding logs when skip_emulated_instruction()
fails
x86: KVM: svm: clear interrupt shadow on all paths in
skip_emulated_instruction()
x86: KVM: add xsetbv to the emulator
x86: KVM: svm: remove hardcoded instruction length from intercepts

arch/x86/include/asm/kvm_emulate.h | 1 +
arch/x86/kvm/emulate.c | 9 ++++++++-
arch/x86/kvm/svm.c | 19 ++++++-------------
3 files changed, 15 insertions(+), 14 deletions(-)

--
2.20.1