From: Juergen Gross <jgross@suse.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: xen-devel@lists.xenproject.org, x86@kernel.org,
linux-kernel@vger.kernel.org,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
Stefano Stabellini <sstabellini@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] x86/xen: disable nosmt in Xen guests
Date: Wed, 12 Jun 2019 16:59:12 +0200 [thread overview]
Message-ID: <74c04cc4-4b05-2bca-d788-ea3605853fcc@suse.com> (raw)
In-Reply-To: <20190612114836.GI3436@hirez.programming.kicks-ass.net>
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
prev parent reply other threads:[~2019-06-12 14:59 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-12 10:12 Juergen Gross
2019-06-12 10:19 ` [Xen-devel] " Jan Beulich
2019-06-12 10:36 ` Juergen Gross
2019-06-12 11:48 ` Peter Zijlstra
2019-06-12 14:59 ` Juergen Gross [this message]
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