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SubjectRe: [PATCH 7/7] KVM: VMX: Introduce test mode related to EPT violation VE
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On Fri, May 17, 2024, Paolo Bonzini wrote:
> On 5/17/24 18:38, Sean Christopherson wrote:
> > > > I've hit this three times now when running KVM-Unit-Tests (I'm pretty sure it's
> > > > the EPT test, unsurprisingly). And unless I screwed up my testing, I verified it
> > > > still fires with Isaku's fix[*], though I'm suddenly having problems repro'ing.
> > > >
> > > > I'll update tomorrow as to whether I botched my testing of Isaku's fix, or if
> > > > there's another bug lurking.
> > > >
> > > > https://lore.kernel.org/all/20240515173209.GD168153@ls.amr.corp.intel.com
> > > I cannot reproduce it on a Skylake (Xeon Gold 5120), with or without Isaku's
> > > fix, with either ./runtests.sh or your reproducer line.
> > >
> > > However I can reproduce it only if eptad=0 and with the following line:
> > >
> > > ./x86/run x86/vmx.flat -smp 1 -cpu max,host-phys-bits,+vmx -m 2560 \
> > > -append 'ept_access_test_not_present ept_access_test_read_only'
> >
> > FWIW, I tried that on RPL, still no failure.
>
> Ok, so it does look like a CPU issue. Even with the fixes you identified, I
> don't see any other solution than adding scary text in Kconfig, defaulting
> it to "n", and adding an also-very-scary pr_err_once("...") the first time
> VMPTRLD is executed with CONFIG_KVM_INTEL_PROVE_VE.

I don't think we need to make it super scary, at least not yet. KVM just needs
to not kill the VM, which thanks to the BUSY flag is trivial: just resume the guest.
Then the failure is "just" a WARN, which won't be anywhere near as problematic for
KVM developers. I doubt syzbot will hit this, purely because syzbot runs almost
exclusively in VMs, i.e. won't have #VE support.

If we don't have a resolution by rc6 or so, then maybe consider doing something
more drastic?

I agree that it should be off by default though. And the help text should be
more clear that this intended only for developers and testing environments.

I have a handful of patches, including one to not kill the VM. I'll try to post
them later today, mostly just need to write changelogs.

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 75082c4a9ac4..5c22186671e9 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -98,15 +98,15 @@ config KVM_INTEL

config KVM_INTEL_PROVE_VE
bool "Check that guests do not receive #VE exceptions"
- default KVM_PROVE_MMU || DEBUG_KERNEL
- depends on KVM_INTEL
+ depends on KVM_INTEL && KVM_PROVE_MMU
help
-
Checks that KVM's page table management code will not incorrectly
let guests receive a virtualization exception. Virtualization
exceptions will be trapped by the hypervisor rather than injected
in the guest.

+ This should never be enabled in a production environment.
+
If unsure, say N.

config X86_SGX_KVM

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