linux-kernel - Re: [Xen-devel] [PATCH RFC] x86/smpboot: Set safer __max_logical

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Date:   Thu, 20 Apr 2017 13:09:43 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Prarit Bhargava <prarit@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [Xen-devel] [PATCH RFC] x86/smpboot: Set safer
 __max_logical_packages limit

On 04/20/2017 12:15 PM, Peter Zijlstra wrote:
> On Thu, Apr 20, 2017 at 05:40:37PM +0200, Vitaly Kuznetsov wrote:
>>> This is getting ludicrous. Xen is plain broken, and instead of fixing
>>> it, you propose to somehow deal with its obviously crack induced
>>> behaviour :-(
>> Totally agree and I don't like the solution I propose (and that's why
>> this is RFC)... The problem is that there are such Xen setups in the
>> wild and with the recent changes some guests will BUG() :-(
>>
>> Alternatively, we can just remove the BUG() and do something with CPUs
>> which have their pkg >= __max_logical_packages, e.g. assign them to the
>> last package. Far from ideal but will help to avoid the regression.
> So currently none of the stuff that uses this should appear in Xen. Its
> all drivers for hardware that isn't virtualized (afaik). So assigning to
> the last package 'works'.
>
> But the moment this ends up getting used that explodes, because we'll
> need different object instances for each piece of hardware.


This already gets used. I don't remember details but we had to fix
something due to RAPL code referencing topology info (and yes, there is
no reason for a guest to use RAPL, but we shouldn't crash neither)


>
> There just isn't a good solution; on the one hand the BIOS is prone to
> providing crap numbers, on the other hand virt (esp. Xen as it turns
> out) provides absolutely bonkers/inconsistent topology information.
>
> Very frustrating :-/
>

So we might need a way to bypass topology discovery and present some
sort of default topology (single package, or 1 CPU per package, for
example) and ignore APICID and all that.

-boris