From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758119AbZEGKfW (ORCPT ); Thu, 7 May 2009 06:35:22 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753722AbZEGKfA (ORCPT ); Thu, 7 May 2009 06:35:00 -0400 Received: from mx2.redhat.com ([66.187.237.31]:44243 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751521AbZEGKe7 (ORCPT ); Thu, 7 May 2009 06:34:59 -0400 Date: Thu, 7 May 2009 13:32:31 +0300 From: "Michael S. Tsirkin" To: Sheng Yang Cc: Matthew Wilcox , kvm@vger.kernel.org, jbarnes@virtuousgeek.org, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, Matthew Wilcox , virtualization@lists.linux-foundation.org Subject: Re: [PATCH] msi-x: let drivers retry when not enough vectors Message-ID: <20090507103231.GG32039@redhat.com> References: <20090507082841.GA31751@redhat.com> <200905071740.16190.sheng@linux.intel.com> <20090507095302.GI8112@parisc-linux.org> <200905071819.54426.sheng@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200905071819.54426.sheng@linux.intel.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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