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

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Date:   Mon, 21 Aug 2023 10:34:38 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Cc:     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 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.

FWIW, my patch to Xen went something like:

---8<---
         if ( !cp->feat.ibrsb && !cp->extd.ibpb )
             goto gp_fault; /* MSR available? */
 
-        if ( val & ~PRED_CMD_IBPB )
+        rsvd = ~(PRED_CMD_IBPB |
+                 (cp->extd.sbpb ? PRED_CMD_SBPB : 0));
+
+        if ( val & rsvd )
             goto gp_fault; /* Rsvd bit set? */
 
         if ( v == curr )
---8<---

~Andrew