linux-kernel - Re: [PATCH] x86/smp: Factor out parts of native_smp_prepare

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Date:   Tue, 9 Nov 2021 16:21:02 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        tglx@...utronix.de, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH] x86/smp: Factor out parts of native_smp_prepare_cpus()

On 09.11.21 16:10, Peter Zijlstra wrote:
> On Mon, Nov 08, 2021 at 12:20:26PM -0500, Boris Ostrovsky wrote:
>>
>> On 11/8/21 10:11 AM, Peter Zijlstra wrote:
>>> On Tue, Nov 02, 2021 at 07:36:36PM -0400, Boris Ostrovsky wrote:
>>>> Commit 66558b730f25 ("sched: Add cluster scheduler level for x86")
>>>> introduced cpu_l2c_shared_map mask which is expected to be initialized
>>>> by smp_op.smp_prepare_cpus(). That commit only updated
>>>> native_smp_prepare_cpus() version but not xen_pv_smp_prepare_cpus().
>>>> As result Xen PV guests crash in set_cpu_sibling_map().
>>>>
>>>> While the new mask can be allocated in xen_pv_smp_prepare_cpus() one can
>>>> see that both versions of smp_prepare_cpus ops share a number of common
>>>> operations that can be factored out. So do that instead.
>>>>
>>>> Fixes: 66558b730f25 ("sched: Add cluster scheduler level for x86")
>>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>> Thanks! I'll go stick that somewhere /urgent (I've had another report on
>>> that here:
>>>
>>>     https://lkml.kernel.org/r/20211105074139.GE174703@worktop.programming.kicks-ass.net
>>> )
>>
>>
>> Thank you. (I don't see this message btw)
> 
> Urgh, that thread never went to lkml :/
> 
>>> But looking at those functions; there seems to be more spurious
>>> differences. For example, the whole sched_topology thing.
>>
>>
>> I did look at that and thought this should be benign given that Xen PV
>> is not really topology-aware. I didn't see anything that would be a
>> cause for concern but perhaps you can point me to things I missed.
> 
> And me not being Xen aware... What does Xen-PV guests see of the CPUID
> topology fields? Does it fully sanitize the CPUID data, or is it a clean
> pass-through from whatever CPU the vCPU happens to run on at the time?

The latter. :-(


Juergen

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