Re: [PATCH v3] evtchn/fifo: don't enforce higher than necessary alignment

[Jan Beulich <jbeulich@xxxxxxxx>]

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,
>