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From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>
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, 22 Apr 2021 11:19:22 +0200	[thread overview]
Message-ID: <0377e61a-3813-0a9e-f724-418383c01050@suse.com> (raw)
In-Reply-To: <69766ecb-d234-eebb-9b31-1533389a502e@xen.org>

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). 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.

Jan

> With your approach, we may need to have different path depending on the 
> padding and ensure the new extension cannot be enabled if the padding is 
> 4-byte. Otherwise, the atomicity would be broken.
> 
>> While relaxation of the requirements is intended here, the primary goal
>> is to simply get rid of the hard coded number as well its lack of
>> connection to the structure that is is meant to apply to.
> 
> Based on what I wrote above, I think the relaxation should not be done 
> to give us more flexibility about possible extension to the structure.
> 
> Although, I would be worth documenting the reasoning in the code.
> 
> Cheers,
> 



  reply	other threads:[~2021-04-22  9:19 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 [this message]
2021-04-29 11:55       ` Julien Grall

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