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

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Message-ID: <20090507103231.GG32039@redhat.com>
Date:	Thu, 7 May 2009 13:32:31 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sheng Yang <sheng@...ux.intel.com>
Cc:	Matthew Wilcox <matthew@....cx>, 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 Thu, May 07, 2009 at 06:19:53PM +0800, Sheng Yang wrote:
> On Thursday 07 May 2009 17:53:02 Matthew Wilcox wrote:
> > On Thu, May 07, 2009 at 05:40:15PM +0800, Sheng Yang wrote:
> > > It's indeed weird. Why the semantic of pci_enable_msix can be changed to
> > > "enable msix, or tell me how many vector do you have"? You can simply
> > > call pci_msix_table_size() to get what you want, also without any more
> > > work, no? I can't understand...
> >
> > Here's a good example.  Let's suppose you have a driver which supports
> > two different models of cards, one has 16 MSI-X interrupts, the other
> > has 10.  You can call pci_enable_msix() asking for 16 vectors.  If your
> > card is model A, you get 16 interrupts.  If your card is model B, it says
> > "you can have 10".
> >
> > This is less work in the driver (since it must implement falling back to
> > a smaller number of interrupts *anyway*) than interrogating the card to
> > find out how many interrupts there are, then requesting the right number,
> > and still having the fallback path which is going to be less tested.
> 
> Yeah, partly understand now.
> 
> But the confusing of return value is not that pleasure compared to this 
> benefit. And even you have to fall back if return > 0 anyway, but in the past, 
> you just need fall back once at most; but now you may fall back twice.

I don't think that's right - you might not be able to get the
number of interrupts that pci_enable_msix reported.

> This 
> make thing more complex - you need either two ifs or a simple loop. And just 
> one "if" can deal with it before. All that required is one call for 
> pci_msix_table_size(), and I believe most driver would like to know how much 
> vector it have before it fill the vectors, so mostly no extra cost. But for 
> this ambiguous return meaning, you have to add more code for fall back - yes, 
> the driver may can assert that the positive return value always would be irq 
> numbers if it call pci_msix_table_size() before, but is it safe in logic?

If you know how many vectors the card has, then the only failure mode
is when you are out of irqs. No change there.

-- 
MST
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