On 4/20/22 18:15, Sean Christopherson wrote:
Let's just require X86_FEATURE_NRIPS, either in general or just to
enable nested virtualiazation
👍
Hmm, so requiring NRIPS for nested doesn't actually buy us anything. KVM still
has to deal with userspace hiding NRIPS from L1, so unless I'm overlooking something,
the only change would be:
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index bdf8375a718b..7bed4e05aaea 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -686,7 +686,7 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
*/
if (svm->nrips_enabled)
vmcb02->control.next_rip = svm->nested.ctl.next_rip;
- else if (boot_cpu_has(X86_FEATURE_NRIPS))
+ else
vmcb02->control.next_rip = vmcb12_rip;
if (is_evtinj_soft(vmcb02->control.event_inj)) {
And sadly, because SVM doesn't provide the instruction length if an exit occurs
while vectoring a software interrupt/exception, making NRIPS mandatory doesn't buy
us much either.
I believe the below diff is the total savings (plus the above nested thing) against
this series if NRIPS is mandatory (ignoring the setup code, which is a wash). It
does eliminate the rewind in svm_complete_soft_interrupt() and the funky logic in
svm_update_soft_interrupt_rip(), but that's it AFAICT. The most obnoxious code of
having to unwind EMULTYPE_SKIP when retrieving the next RIP for software int/except
injection doesn't go away:-(
I'm not totally opposed to requiring NRIPS, but I'm not in favor of it either.
Yeah, you're right. However:
* the rewind might already be worth it;
* if we require NRIPS for nested, we can also assume that the SVM save
state data has a valid next_rip; even if !svm->nrips_enabled. There's
the pesky issue of restoring from an old system that did not have NRIPS,
but let's assume for now that NRIPS was set on the source as well.
Paolo
