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

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Date:   Wed, 12 Jun 2019 12:36:49 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Stefano Stabellini <sstabellini@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        tglx@...utronix.de, xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>, mingo@...hat.com,
        linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [Xen-devel] [PATCH] x86/xen: disable nosmt in Xen guests

On 12.06.19 12:19, Jan Beulich wrote:
>>>> On 12.06.19 at 12:12, <jgross@...e.com> 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.
> 
> I don't agree with the second sentence: It is in principle fine for the
> hypervisor to expose HT (i.e. not disable it as bug mitigation), and
> leave it to the guest kernels to protect themselves. We're just not
> at the point yet where Xen offers sufficient / reliable data to guest
> kernels to do so, so _for the time being_ what you say is correct.

Okay, I'll add something like:

This is true as long Xen doesn't support core scheduling together with
exposing the (then) correct sibling information to the guest and
indicating that case via a sutable interface.


Juergen