linux-kernel - Re: [RFC 02/10] x86/kvm: Add IBPB support

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Date:   Sat, 20 Jan 2018 20:36:20 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Liran Alon <liran.alon@...cle.com>, <karahmed@...zon.de>
CC:     <labbott@...hat.com>, <luto@...nel.org>,
        <Janakarajan.Natarajan@....com>, <bp@...e.de>,
        <torvalds@...ux-foundation.org>, <asit.k.mallick@...el.com>,
        <rkrcmar@...hat.com>, <dave.hansen@...el.com>, <hpa@...or.com>,
        <mingo@...hat.com>, <jun.nakajima@...el.com>, <x86@...nel.org>,
        <ashok.raj@...el.com>, <arjan.van.de.ven@...el.com>,
        <tim.c.chen@...ux.intel.com>, <pbonzini@...hat.com>,
        <linux-kernel@...r.kernel.org>, <ak@...ux.intel.com>,
        <peterz@...radead.org>, <tglx@...utronix.de>,
        <gregkh@...uxfoundation.org>, <mhiramat@...nel.org>,
        <arjan@...ux.intel.com>, <thomas.lendacky@....com>,
        <dan.j.williams@...el.com>, <joro@...tes.org>,
        <kvm@...r.kernel.org>, <aarcange@...hat.com>
Subject: Re: [RFC 02/10] x86/kvm: Add IBPB support

On Sat, 2018-01-20 at 12:28 -0800, Liran Alon wrote:
> Isn't it cleaner to check for "boot_cpu_has(X86_FEATURE_IBPB)" both
> in svm_vcpu_init_msrpm() and hardware_setup()?

Strictly speaking that's a different check. That's checking if we're
*using* IBPB, not if it exists.

Now that's probably OK here, since we need it for retpoline *and* IBRS-
based mitigations. And we *might* argue that 'nospectre_v2' on the host
kernel command line should indeed stop us exposing the features to
guests. Maybe.

But next comes IBRS support, and we definitely *won't* want to make
exposing that to guests conditional on X86_FEATURE_IBRS, because in the
retpoline case that won't be set and we probably *will* still want to
expose it to guests based merely on the fact that it exists.

So I think Karim has it right here (modulo the change I already made).

If we want a separate control for "don't expose these to guests", we
should do that explicitly.
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