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Message-ID: <20230821162337.imzjf3golstkrrgd@treble>
Date: Mon, 21 Aug 2023 09:23:37 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
Borislav Petkov <bp@...en8.de>,
Peter Zijlstra <peterz@...radead.org>,
Babu Moger <babu.moger@....com>,
Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <seanjc@...gle.com>, David.Kaplan@....com,
Nikolay Borisov <nik.borisov@...e.com>,
gregkh@...uxfoundation.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 03/22] KVM: x86: Support IBPB_BRTYPE and SBPB
On Mon, Aug 21, 2023 at 10:34:38AM +0100, Andrew Cooper wrote:
> On 21/08/2023 2:19 am, Josh Poimboeuf wrote:
> > The IBPB_BRTYPE and SBPB CPUID bits aren't set by HW.
>
> "Current hardware".
>
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index c381770bcbf1..dd7472121142 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3676,12 +3676,13 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > if (!msr_info->host_initiated && !guest_has_pred_cmd_msr(vcpu))
> > return 1;
> >
> > - if (!boot_cpu_has(X86_FEATURE_IBPB) || (data & ~PRED_CMD_IBPB))
> > + if (boot_cpu_has(X86_FEATURE_IBPB) && data == PRED_CMD_IBPB)
> > + wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
> > + else if (boot_cpu_has(X86_FEATURE_SBPB) && data == PRED_CMD_SBPB)
> > + wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_SBPB);
> > + else if (data)
> > return 1;
>
> SBPB | IBPB is an explicitly permitted combination, but will be rejected
> by this logic.
Ah yes, I see that now:
If software writes PRED_CMD with both bits 0 and 7 set to 1, the
processor performs an IBPB operation.
--
Josh
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