Re: [Xen-devel] [PATCH RFC] x86: Don't increase ApicIdCoreSize past 7

From: Wei Liu <wl@xen.org>
To: George Dunlap <george.dunlap@citrix.com>
Cc: Juergen Gross <jgross@suse.com>, Steven Haigh <netwiz@crc.id.au>,
	Wei Liu <wl@xen.org>, Andrew Cooper <andrew.cooper3@citrix.com>,
	Andreas Kinzler <hfp@posteo.de>, Jan Beulich <jbeulich@suse.com>,
	Ian Jackson <ian.jackson@citrix.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [Xen-devel] [PATCH RFC] x86: Don't increase ApicIdCoreSize past 7
Date: Mon, 25 Nov 2019 12:57:51 +0000	[thread overview]
Message-ID: <20191125125751.bgriplkxu6loqttb@debian> (raw)
In-Reply-To: <20191125123923.2000028-1-george.dunlap@citrix.com>

On Mon, Nov 25, 2019 at 12:39:23PM +0000, George Dunlap wrote:
> Changeset ca2eee92df44 ("x86, hvm: Expose host core/HT topology to HVM
> guests") attempted to "fake up" a topology which would induce guest
> operating systems to not treat vcpus as sibling hyperthreads.  This
> involved actually reporting hyperthreading as available, but giving
> vcpus every other ApicId; which in turn led to doubling the ApicIds
> per core by bumping the ApicIdCoreSize by one.  In particular, Ryzen
> 3xxx series processors, and reportedly EPYC "Rome" cpus -- have an
> ApicIdCoreSize of 7; the "fake" topology increases this to 8.
> 
> Unfortunately, Windows running on modern AMD hardware -- including
> Ryzen 3xxx series processors, and reportedly EPYC "Rome" cpus --
> doesn't seem to cope with this value being higher than 7.  (Linux
> guests have so far continued to cope.)
> 
> A "proper" fix is complicated and it's too late to fix it either for
> 4.13, or to backport to supported branches.  As a short-term fix,
> limit this value to 7.
> 
> This does mean that a Linux guest, booted on such a system without
> this change, and then migrating to a system with this change, with
> more than 64 vcpus, would see an apparent topology change.  This is a
> low enough risk in practice that enabling this limit unilaterally, to
> allow other guests to boot without manual intervention, is worth it.
> 
> Reported-by: Steven Haigh <netwiz@crc.id.au>
> Reported-by: Andreas Kinzler <hfp@posteo.de>
> Signed-off-by: George Dunlap <george.dunlap@citrix.com>
> ---
> CC: Ian Jackson <ian.jackson@citrix.com>
> CC: Wei Liu <wl@xen.org>
> CC: Andrew Cooper <andrew.cooper3@citrix.com>
> CC: Jan Beulich <jbeulich@suse.com>
> CC: Juergen Gross <jgross@suse.com>
> ---
>  tools/libxc/xc_cpuid_x86.c | 7 ++++++-

I will defer this to x86 maintainers.

Seeing that you already have an Ack from Jan, feel free to add mine if
necessary.

Wei.

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