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From: Julien Grall <julien@xen.org>
To: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Ian Jackson <ian.jackson@citrix.com>, Wei Liu <wl@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v3] evtchn/fifo: don't enforce higher than necessary alignment
Date: Thu, 29 Apr 2021 12:55:31 +0100	[thread overview]
Message-ID: <04ef688f-1eb4-c17f-02f0-2cb7e5cb5c68@xen.org> (raw)
In-Reply-To: <0377e61a-3813-0a9e-f724-418383c01050@suse.com>

Hi Jan,

On 22/04/2021 10:19, Jan Beulich wrote:
> On 21.04.2021 21:52, Julien Grall wrote:
>> Hi,
>> On 21/04/2021 15:36, Jan Beulich wrote:
>>> Neither the code nor the original commit provide any justification for
>>> the need to 8-byte align the struct in all cases. Enforce just as much
>>> alignment as the structure actually needs - 4 bytes - by using alignof()
>>> instead of a literal number.
>> I had another fresh look today at this patch. The 32-bit padding is
>> right after the field 'ready'.
>> I can't for sure tell how the second half is going to ever be used and how.
>> However, one possibility would be to extend the field 'ready' to 64-bit.
>> With the current code, we could easily make a single 64-bit access
>> without having to know whether the guest is able to interpret the top half.
> I don't think extending field sizes is generally to be considered ABI-
> compatible. I also don't think we can re-use the field at all, as I
> couldn't find any checking of it being zero (input) or it getting set
> to zero (output). 

That's would be fine so long we have a flag to control it. We can still 
write unconditionally because a guest can't rely on the pad...

> struct evtchn_init_control, which in principle could
> be a way to convey respective controlling flags, similarly has no room
> for extension, as its _pad[] also doesn't look to get checked anywhere.
Right, we would need to have a different way to convey. Yet, I am still 
unconvinced of the benefits change offer in this patch.

I am not going to Nack. So another maintainer in "THE REST" can express 
support for your patch and ack it.


Julien Grall

      reply	other threads:[~2021-04-29 11:55 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-22  8:13 [PATCH v2 0/2] common: XSA-327 follow-up Jan Beulich
2020-12-22  8:14 ` [PATCH v2 1/2] common: map_vcpu_info() cosmetics Jan Beulich
2021-04-01 16:02   ` Julien Grall
2020-12-22  8:15 ` [PATCH v2 2/2] evtchn/fifo: don't enforce higher than necessary alignment Jan Beulich
2021-04-21 14:36 ` [PATCH v3] " Jan Beulich
2021-04-21 19:52   ` Julien Grall
2021-04-22  9:19     ` Jan Beulich
2021-04-29 11:55       ` Julien Grall [this message]

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