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From: Paolo Bonzini <pbonzini@redhat.com>
Date: Thu, 11 Apr 2024 15:22:12 +0200
Message-ID: <CABgObfai1TCs6pNAP4i0x99qAjXTczJ4uLHiivNV7QGoah1pVg@mail.gmail.com>
Subject: Re: [PATCH] KVM: x86: Set BHI_NO in guest when host is not affected
 by BHI
To: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>, x86@kernel.org, kvm@vger.kernel.org, 
	linux-kernel@vger.kernel.org, daniel.sneddon@linux.intel.com, 
	pawan.kumar.gupta@linux.intel.com, tglx@linutronix.de, konrad.wilk@oracle.com, 
	peterz@infradead.org, gregkh@linuxfoundation.org, seanjc@google.com, 
	dave.hansen@linux.intel.com, nik.borisov@suse.com, kpsingh@kernel.org, 
	longman@redhat.com, bp@alien8.de
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On Thu, Apr 11, 2024 at 11:34=E2=80=AFAM Alexandre Chartre
<alexandre.chartre@oracle.com> wrote:
>
> So you mean we can't set ARCH_CAP_BHI_NO for the guest because we don't k=
now
> if the guest will run the (other) existing mitigations which are believed=
 to
> suffice to mitigate BHI?
>
> The problem is that we can end up with a guest running extra BHI mitigati=
ons
> while this is not needed. Could we inform the guest that eIBRS is not ava=
ilable
> on the system so a Linux guest doesn't run with extra BHI mitigations?

The (Linux or otherwise) guest will make its own determinations as to
whether BHI mitigations are necessary. If the guest uses eIBRS, it
will run with mitigations. If you hide bit 1 of
MSR_IA32_ARCH_CAPABILITIES from the guest, it may decide to disable
it. But if the guest decides to use eIBRS, I think it should use
mitigations even if the host doesn't.

It's a different story if the host isn't susceptible altogether. The
ARCH_CAP_BHI_NO bit *can* be set if the processor doesn't have the bug
at all, which would be true if cpu_matches(cpu_vuln_whitelist,
NO_BHI). I would apply a patch to do that.

Paolo