[17/38] ivshmem: Clean up MSI-X conditions

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.

* 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.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
---
 hw/misc/ivshmem.c | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

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.

>
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  hw/misc/ivshmem.c | 22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)
>
> diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
> index 7191914..cfea151 100644
> --- a/hw/misc/ivshmem.c
> +++ b/hw/misc/ivshmem.c
> @@ -274,7 +274,9 @@ static void ivshmem_vector_notify(void *opaque)
>
>      IVSHMEM_DPRINTF("interrupt on vector %p %d\n", pdev, vector);
>      if (ivshmem_has_feature(s, IVSHMEM_MSI)) {
> -        msix_notify(pdev, vector);
> +        if (msix_enabled(pdev)) {
> +            msix_notify(pdev, vector);
> +        }
>      } else {
>          ivshmem_IntrStatus_write(s, 1);
>      }
> @@ -712,13 +714,12 @@ static void ivshmem_check_version(void *opaque, const uint8_t * buf, int size)
>  /* Select the MSI-X vectors used by device.
>   * ivshmem maps events to vectors statically, so
>   * we just enable all vectors on init and after reset. */
> -static void ivshmem_use_msix(IVShmemState * s)
> +static void ivshmem_vector_use(IVShmemState *s)
>  {
>      PCIDevice *d = PCI_DEVICE(s);
>      int i;
>
> -    IVSHMEM_DPRINTF("%s, msix present: %d\n", __func__, msix_present(d));
> -    if (!msix_present(d)) {
> +    if (!ivshmem_has_feature(s, IVSHMEM_MSI)) {
>          return;
>      }
>
> @@ -733,7 +734,7 @@ static void ivshmem_reset(DeviceState *d)
>
>      s->intrstatus = 0;
>      s->intrmask = 0;
> -    ivshmem_use_msix(s);
> +    ivshmem_vector_use(s);
>  }
>
>  static int ivshmem_setup_interrupts(IVShmemState *s)
> @@ -747,9 +748,9 @@ static int ivshmem_setup_interrupts(IVShmemState *s)
>          }
>
>          IVSHMEM_DPRINTF("msix initialized (%d vectors)\n", s->vectors);
> -        ivshmem_use_msix(s);
>      }
>
> +    ivshmem_vector_use(s);
>      return 0;
>  }
>
> @@ -1034,12 +1035,7 @@ static int ivshmem_pre_load(void *opaque)
>
>  static int ivshmem_post_load(void *opaque, int version_id)
>  {
> -    IVShmemState *s = opaque;
> -
> -    if (ivshmem_has_feature(s, IVSHMEM_MSI)) {
> -        ivshmem_use_msix(s);
> -    }
> -
> +    ivshmem_vector_use(opaque);
>      return 0;
>  }
>
> @@ -1067,11 +1063,11 @@ static int ivshmem_load_old(QEMUFile *f, void *opaque, int version_id)
>
>      if (ivshmem_has_feature(s, IVSHMEM_MSI)) {
>          msix_load(pdev, f);
> -        ivshmem_use_msix(s);
>      } else {
>          s->intrstatus = qemu_get_be32(f);
>          s->intrmask = qemu_get_be32(f);
>      }
> +    ivshmem_vector_use(s);
>
>      return 0;
>  }
> --
> 2.4.3
>
>

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.