Messages in this thread | | | Date | Thu, 1 Dec 2022 23:04:06 +0000 | From | Sean Christopherson <> | Subject | Re: [PATCH v4 2/4] x86/virt: Force GIF=1 prior to disabling SVM (for reboot flows) |
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On Thu, Dec 01, 2022, Andrew Cooper wrote: > On 30/11/2022 23:36, Sean Christopherson wrote: > > diff --git a/arch/x86/include/asm/virtext.h b/arch/x86/include/asm/virtext.h > > index 8757078d4442..0acb14806a74 100644 > > --- a/arch/x86/include/asm/virtext.h > > +++ b/arch/x86/include/asm/virtext.h > > @@ -126,7 +126,18 @@ static inline void cpu_svm_disable(void) > > > > wrmsrl(MSR_VM_HSAVE_PA, 0); > > rdmsrl(MSR_EFER, efer); > > - wrmsrl(MSR_EFER, efer & ~EFER_SVME); > > + if (efer & EFER_SVME) { > > + /* > > + * Force GIF=1 prior to disabling SVM, e.g. to ensure INIT and > > + * NMI aren't blocked. Eat faults on STGI, as it #UDs if SVM > > + * isn't enabled and SVM can be disabled by an NMI callback. > > I'd be tempted to tweak this for clarity. > > How about "We don't know the state of GIF, and if NMIs are enabled, > there is a race condition where EFER.SVME can be cleared behind our > back. Ignore #UD, and force GIF=1 in case INIT/NMI are currently > blocked." ? > > The STGI can't actually #UD on real hardware, because SKINIT and SVM > exist in identical sets of parts, but it can #UD in principle in a VM > which doesn't offer emulate SKINIT.
Ah, right, "may #UD", not "will #UD". And despite writing this, I also keep forgetting why forcing GIF is even necessary. How about?
/* * Force GIF=1 prior to disabling SVM to ensure INIT and NMI * aren't blocked, e.g. if a fatal error occurred between CLGI * and STGI. Note, STGI may #UD if SVM is disabled from NMI * context between reading EFER and executing STGI. In that * case, GIF must already be set, otherwise the NMI would have * been blocked, so just eat the fault. */
> Given that we are in cpu_svm_disable(), there's also > MSR_VM_CR.INIT_REDIRECTION to consider, but perhaps that's better left > to the series which adds SKINIT support. > > ~Andrew
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