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Re: [Qemu-devel] [PATCH 17/38] ivshmem: Clean up MSI-X conditions
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH 17/38] ivshmem: Clean up MSI-X conditions |
Date: |
Wed, 02 Mar 2016 11:25:13 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) |
Marc-André Lureau <address@hidden> writes:
> On Mon, Feb 29, 2016 at 7:40 PM, Markus Armbruster <address@hidden> 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.