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Message-ID: <ZULN4vMwP1t_mKg7@google.com>
Date: Wed, 1 Nov 2023 15:14:58 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: Yang Weijiang <weijiang.yang@...el.com>, pbonzini@...hat.com,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
dave.hansen@...el.com, peterz@...radead.org, chao.gao@...el.com,
rick.p.edgecombe@...el.com, john.allen@....com
Subject: Re: [PATCH v6 23/25] KVM: x86: Enable CET virtualization for VMX and
advertise to userspace
On Tue, Oct 31, 2023, Maxim Levitsky wrote:
> On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> > @@ -685,6 +686,13 @@ void kvm_set_cpu_caps(void)
> > kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
> > if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
> > kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
> > + /*
> > + * The feature bit in boot_cpu_data.x86_capability could have been
> > + * cleared due to ibt=off cmdline option, then add it back if CPU
> > + * supports IBT.
> > + */
> > + if (cpuid_edx(7) & F(IBT))
> > + kvm_cpu_cap_set(X86_FEATURE_IBT);
>
> The usual policy is that when the host doesn't support a feature, then the guest
> should not support it either. On the other hand, for this particular feature,
> it is probably safe to use it. Just a point for a discussion.
Agreed, this needs extra justification. It's "safe" in theory, but if the admin
disabled IBT because of a ucode bug, then all bets are off.
I'm guessing this was added because of the virtualization hole? I.e. if KVM
allows CR4.CET=1 for shadow stacks, then KVM can't (easily?) prevent the guest
from also using IBT.
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