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Re: [PATCH 6/7] xen/evtch: use smp barriers for user event ring
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 8 Feb 2021 10:23:02 +0000
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- Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, <linux-kernel@xxxxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
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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@xxxxxxxxxx>
>>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>> 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.
virt_*() exist specifically to be smp_*() which don't degrade to broken
in UP builds.
~Andrew
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