linux-kernel - Re: [RFC][PATCH 07/17] x86/cpu/kvm: Provide UNTRAIN_RET

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Date:   Mon, 14 Aug 2023 12:35:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, David.Kaplan@....com,
        Andrew.Cooper3@...rix.com, jpoimboe@...nel.org,
        gregkh@...uxfoundation.org
Subject: Re: [RFC][PATCH 07/17] x86/cpu/kvm: Provide UNTRAIN_RET_VM

On Sun, Aug 13, 2023 at 12:36:57PM +0200, Borislav Petkov wrote:
> On Wed, Aug 09, 2023 at 09:12:25AM +0200, Peter Zijlstra wrote:
> > With the difference being that UNTRAIN_RET_VM uses
> > X86_FEATURE_IBPB_ON_VMEXIT instead of X86_FEATURE_ENTRY_IBPB.
> > 
> > This cures VMEXIT doing potentially unret+IBPB or double IBPB.
> 
> Can't - I have a separate flag for that and I set it only when !IBPB:
> 
>         case SRSO_CMD_IBPB_ON_VMEXIT:
>                 if (IS_ENABLED(CONFIG_CPU_SRSO)) {
>                         if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
>                                 setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
> 

Of course you can, just also set it with regular IBPB :-)