linux-kernel - Re: [PATCH 03/22] KVM: x86: Support IBPB

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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