linux-kernel - Re: [PATCH 6/7] xen/evtch: use smp barriers for user event ring

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Date:   Mon, 8 Feb 2021 11:36:17 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 6/7] xen/evtch: use smp barriers for user event ring

On 08.02.2021 11:23, Andrew Cooper wrote:
> On 08/02/2021 09:50, Jan Beulich wrote:
>> On 08.02.2021 10:44, Andrew Cooper wrote:
>>> On 06/02/2021 10:49, Juergen Gross wrote:
>>>> The ring buffer for user events is used in the local system only, so
>>>> smp barriers are fine for ensuring consistency.
>>>>
>>>> Reported-by: Andrew Cooper <andrew.cooper3@...rix.com>
>>>> Signed-off-by: Juergen Gross <jgross@...e.com>
>>> These need to be virt_* to not break in UP builds (on non-x86).
>> Initially I though so, too, but isn't the sole vCPU of such a
>> VM getting re-scheduled to a different pCPU in the hypervisor
>> an implied barrier anyway?
> 
> Yes, but that isn't relevant to why UP builds break.
> 
> smp_*() degrade to compiler barriers in UP builds, and while that's
> mostly fine for x86 read/write, its not fine for ARM barriers.

Hmm, I may not know enough of Arm's memory model - are you saying
Arm CPUs aren't even self-coherent, i.e. later operations (e.g.
the consuming of ring contents) won't observe earlier ones (the
updating of ring contents) when only a single physical CPU is
involved in all of this? (I did mention the hypervisor level
context switch simply because that's the only way multiple CPUs
can get involved.)

Jan