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

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Date:   Wed, 12 Jun 2019 16:59:12 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 12.06.19 13:48, Peter Zijlstra wrote:
> 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.

Yes, that's the theory.

I agree completely, but this is ongoing work which will need some more
time. It probably would have been ready for some time now, but some
recent processor bugs required a shift in priorities what to do first.

In order to run a new kernel on existing Xen we need that patch to avoid
disabling random cpus.


Juergen