linux-kernel - Re: [PATCH 7/7] KVM: VMX: Introduce test mode related to EPT violation VE

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Date: Sat, 18 May 2024 00:05:01 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH 7/7] KVM: VMX: Introduce test mode related to EPT
 violation VE

On Fri, May 17, 2024 at 8:18 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > 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.
>
> 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

"depends on KVM_PROVE_MMU" is wrong, I think.  I'd like to keep it
enabled without slowing down too much the VMs, for example.

On the other hand "default DEBUG_KERNEL" is definitely too heavy
with these CPU issues.

Paolo