linux-kernel - Re: [PATCH] msi-x: let drivers retry when not enough vectors

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Message-Id: <200905071710.47204.sheng@linux.intel.com>
Date:	Thu, 7 May 2009 17:10:46 +0800
From:	Sheng Yang <sheng@...ux.intel.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	kvm@...r.kernel.org, jbarnes@...tuousgeek.org,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] msi-x: let drivers retry when not enough vectors

On Thursday 07 May 2009 17:05:06 Michael S. Tsirkin wrote:
> On Thu, May 07, 2009 at 04:51:24PM +0800, Sheng Yang wrote:
> > On Thursday 07 May 2009 16:28:41 Michael S. Tsirkin wrote:
> > > pci_enable_msix currently returns -EINVAL if you ask
> > > for more vectors than supported by the device, which would
> > > typically cause fallback to regular interrupts.
> > >
> > > It's better to return the table size, making the driver retry
> > > MSI-X with less vectors.
> >
> > Hi Michael
> >
> > I think driver should read from capability list to know how many vector
> > supported by this device before enable MSI-X for device, as
> > pci_msix_table_size() did...
>
> Drivers can do this, but it's more code. Since pci_enable_msix
> calls pci_msix_table_size already, let it do the work. Right?

If you don't know the vectors number before you enable MSI-X, how can you 
setup vectors? 

I don't think it's proper way to go.

-- 
regards
Yang, Sheng

>
> > --
> > regards
> > Yang, Sheng
> >
> > > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > > ---
> > >
> > > Hi Jesse,
> > > This came up when I was adding MSI-X support to virtio pci driver,
> > > which does not know the exact table size upfront.
> > > Could you consider this patch for 2.6.31 please?
> > >
> > >
> > >  drivers/pci/msi.c |    6 +++---
> > >  1 files changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> > > index 6f2e629..f5bd1c9 100644
> > > --- a/drivers/pci/msi.c
> > > +++ b/drivers/pci/msi.c
> > > @@ -687,8 +687,8 @@ int pci_msix_table_size(struct pci_dev *dev)
> > >   * indicates the successful configuration of MSI-X capability
> > > structure * with new allocated MSI-X irqs. A return of < 0 indicates a
> > > failure. * Or a return of > 0 indicates that driver request is
> > > exceeding the number - * of irqs available. Driver should use the
> > > returned value to re-send - * its request.
> > > + * of irqs or MSI-X vectors available. Driver should use the returned
> > > value to + * re-send its request.
> > >   **/
> > >  int pci_enable_msix(struct pci_dev* dev, struct msix_entry *entries,
> > > int nvec) {
> > > @@ -704,7 +704,7 @@ int pci_enable_msix(struct pci_dev* dev, struct
> > > msix_entry *entries, int nvec)
> > >
> > >  	nr_entries = pci_msix_table_size(dev);
> > >  	if (nvec > nr_entries)
> > > -		return -EINVAL;
> > > +		return nr_entries;
> > >
> > >  	/* Check for any invalid entries */
> > >  	for (i = 0; i < nvec; i++) {


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