linux-kernel - Re: [PATCH] x86/xen: disable nosmt in Xen guests

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Date:   Wed, 12 Jun 2019 13:48:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/xen: disable nosmt in Xen guests

On Wed, Jun 12, 2019 at 12:12:28PM +0200, Juergen Gross wrote:
> When running as a Xen guest selecting "nosmt" either via command line
> or implicitly via default settings makes no sense, as the guest has no
> clue about the real system topology it is running on. With Xen it is
> the hypervisor's job to ensure the proper bug mitigations are active
> regarding smt settings.
> 
> So when running as a Xen guest set cpu_smt_control to "not supported"
> in order to avoid disabling random vcpus.

If it doesn't make sense; then the topology should not expose SMT
threads and the knob will not have any effect.

If you expose SMT topology info to the guest, then you get what you ask
for.