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

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Message-ID: <e68f76a5-27ce-a6d8-88a3-1e8fa1c30659@citrix.com>
Date:   Mon, 8 Feb 2021 10:23:02 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Jan Beulich <jbeulich@...e.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 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.

virt_*() exist specifically to be smp_*() which don't degrade to broken
in UP builds.

~Andrew