Re: [Qemu-devel] [PATCH 17/38] ivshmem: Clean up MSI-X conditions

[Markus Armbruster <armbru@redhat.com>]

Marc-André Lureau <marcandre.lureau@gmail.com> writes:

> On Mon, Feb 29, 2016 at 7:40 PM, Markus Armbruster <armbru@redhat.com> wrote:
>> There are three predicates related to MSI-X:
>>
>> * ivshmem_has_feature(s, IVSHMEM_MSI) is true unless the non-MSI-X
>>   variant of the device is selected with msi=off.
>>
>> * msix_present() is true when the device has the PCI capability MSI-X.
>>   It's initially false, and becomes true during successful realize of
>>   the MSI-X variant of the device.  Thus, it's the same as
>>   ivshmem_has_feature(s, IVSHMEM_MSI) for realized devices.
>>
>> * msix_enabled() is true when msix_present() is true and guest software
>>   has enabled MSI-X.
>>
>> Code that differs between the non-MSI-X and the MSI-X variant of the
>> device needs to be guarded by ivshmem_has_feature(s, IVSHMEM_MSI) or
>> by msix_present(), except the latter works only for realized devices.
>>
>> Code that depends on whether MSI-X is in use needs to be guarded with
>> msix_enabled().
>>
>> Code review led me to two minor messes:
>>
>> * ivshmem_vector_notify() calls msix_notify() even when
>>   !msix_enabled(), unlike most other MSI-X-capable devices.  As far as
>>   I can tell, msix_notify() does nothing when !msix_enabled().  Add
>>   the guard anyway.
>>
>
> sure, feel free to split in a seperate patch with my Review-by.
>
>> * Most callers of ivshmem_use_msix() guard it with
>>   ivshmem_has_feature(s, IVSHMEM_MSI).  Not necessary, because
>>   ivshmem_use_msix() does nothing when !msix_present().  That's
>>   ivshmem's only use of msix_present(), though.  Rename
>>   ivshmem_use_msix() to ivshmem_vector_use(), replace msix_present()
>>   by ivshmem_has_feature() there, and drop the redundant guards.
>
> I prefer that code related to msix remains within msix blocks if
> possible, improving readability imho.
>
> Furthermore, since the function is msix specific, I think it's worth
> keeping the "msix" in the name. Since ivshmem_msix_use() wasn't good
> enough for you, perhaps we need the full-blown
> ivshmem_msix_vectors_use() instead.

"Vectors" means actually two related, but distinct things with ivshmem:

* the communication channels to transmit interrupts among peers, and

* the MSI-X vectors.

You can have the former without the latter, with msi=off.

I guess there are two views of the function, both reasonable:

1. Prepare usage of "vectors", i.e. either kind.  Name the function
ivshmem_msix_vectors_use(), and call it unconditionally.  The fact that
it does only MSI-X stuff is implementation detail.

2. Prepare usage of MSI-X vectors.  Name the function
ivshmem_msix_vectors_use() or similar, and calls it only when
ivshmem_has_feature(s, IVSHMEM_MSI), for consistency with other MSI-X
functions.

You prefer 2, I prefer 1.  But it's not a deal-breaker for me; if you
feel strongly, I can do 2.