From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754022AbcJUDUM (ORCPT ); Thu, 20 Oct 2016 23:20:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50294 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752956AbcJUDUJ (ORCPT ); Thu, 20 Oct 2016 23:20:09 -0400 Date: Thu, 20 Oct 2016 21:20:07 -0600 From: Alex Williamson To: Kirti Wankhede Cc: , , , , , , , , Subject: Re: [PATCH v9 10/12] vfio: Add function to get device_api string from vfio_device_info.flags Message-ID: <20161020212007.3e2b4d11@t450s.home> In-Reply-To: <3a51ce1c-1396-676f-3a9b-9faa8390c632@nvidia.com> References: <1476739332-4911-1-git-send-email-kwankhede@nvidia.com> <1476739332-4911-11-git-send-email-kwankhede@nvidia.com> <20161020133403.08828f47@t450s.home> <20161020150501.39f28c7e@t450s.home> <6687c068-33f0-c437-3106-bbd1d22f4d81@nvidia.com> <20161020152220.57f6ee06@t450s.home> <3a51ce1c-1396-676f-3a9b-9faa8390c632@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Fri, 21 Oct 2016 03:20:09 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 21 Oct 2016 08:30:53 +0530 Kirti Wankhede wrote: > On 10/21/2016 2:52 AM, Alex Williamson wrote: > > On Fri, 21 Oct 2016 02:44:37 +0530 > > Kirti Wankhede wrote: > > > ... > > >>>>>> > >>>>>> +extern const char *vfio_device_api_string(u32 flags); > >>>>>> + > >>>>>> struct pci_dev; > >>>>>> #ifdef CONFIG_EEH > >>>>>> extern void vfio_spapr_pci_eeh_open(struct pci_dev *pdev); > >>>>> > >>>>> Couldn't this simply be a #define in the uapi header? > >>>>> > >>>>> #define VFIO_DEVICE_PCI_API_STRING "vfio-pci" > >>>>> > >>>>> I don't really see why we need a lookup function. > >>>>> > >>>> > >>>> String is tightly coupled with the FLAG, right? > >>>> Instead user need to take care of making sure to return proper string, > >>>> and don't mis-match the string, I think having function is easier. > >>> > >>> That's exactly why I proposed putting the #define string in the uapi, > >>> by that I mean the vfio uapi header. That keeps the tight coupling to > >>> the flag, they're both defined in the same place, plus it gives > >>> userspace a reference so they're not just inventing a string to compare > >>> against. IOW, the vendor driver simply does an sprintf of > >>> VFIO_DEVICE_PCI_API_STRING and userspace (ie. libvirt) can do a strcmp > >>> with VFIO_DEVICE_PCI_API_STRING from the same header and everybody > >>> arrives at the same result. > >>> > >>>> Vendor driver should decide the type of device they want to expose and > >>>> set the flag, using this function vendor driver would return string > >>>> which is based on flag they set. > >>> > >>> Being a function adds no intrinsic value and being in a uapi header does > >>> add value to userspace. Thanks, > >>> > >> > >> Ok. The strings should be in uapi, but having function (like below) to > >> return proper string based on flag would be good to have for vendor driver. > >> > >> +const char *vfio_device_api_string(u32 flags) > >> +{ > >> + if (flags & VFIO_DEVICE_FLAGS_PCI) > >> + return VFIO_DEVICE_API_PCI_STRING; > >> + > >> + if (flags & VFIO_DEVICE_FLAGS_PLATFORM) > >> + return VFIO_DEVICE_API_PLATFORM_STRING; > >> + > >> + if (flags & VFIO_DEVICE_FLAGS_AMBA) > >> + return VFIO_DEVICE_API_AMBA_STRING; > >> + > >> + return ""; > >> +} > >> +EXPORT_SYMBOL(vfio_device_api_string); > > > > I disagree, it's pointless maintenance overhead. It's yet another > > function that we need to care about for kABI and it offers almost no > > value. Thanks, > > > > If any vendor driver sets VFIO_DEVICE_FLAGS_PLATFORM flag but sets > VFIO_DEVICE_API_PCI_STRING, we don't have a way to verify this in kernel > driver. Is that acceptable? a) The function doesn't solve that problem, as seen in the mtty sample driver there's no guarantee that the vendor driver passes device_info.flags vs the #define for the flag itself. So we're already expecting them to get it right in two separate places. b) It's not going to take them very long to figure this out if they care about userspace tools that make use of this field to determine the device API. This is a very obvious and simple bug. c) We can check and correct how open source vendor drivers work. Thanks, Alex From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54145) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bxQNS-0003DU-Jt for qemu-devel@nongnu.org; Thu, 20 Oct 2016 23:20:15 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bxQNO-0003iq-1H for qemu-devel@nongnu.org; Thu, 20 Oct 2016 23:20:14 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54460) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1bxQNN-0003iS-Qc for qemu-devel@nongnu.org; Thu, 20 Oct 2016 23:20:09 -0400 Date: Thu, 20 Oct 2016 21:20:07 -0600 From: Alex Williamson Message-ID: <20161020212007.3e2b4d11@t450s.home> In-Reply-To: <3a51ce1c-1396-676f-3a9b-9faa8390c632@nvidia.com> References: <1476739332-4911-1-git-send-email-kwankhede@nvidia.com> <1476739332-4911-11-git-send-email-kwankhede@nvidia.com> <20161020133403.08828f47@t450s.home> <20161020150501.39f28c7e@t450s.home> <6687c068-33f0-c437-3106-bbd1d22f4d81@nvidia.com> <20161020152220.57f6ee06@t450s.home> <3a51ce1c-1396-676f-3a9b-9faa8390c632@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v9 10/12] vfio: Add function to get device_api string from vfio_device_info.flags List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kirti Wankhede Cc: pbonzini@redhat.com, kraxel@redhat.com, cjia@nvidia.com, qemu-devel@nongnu.org, kvm@vger.kernel.org, kevin.tian@intel.com, jike.song@intel.com, bjsdjshi@linux.vnet.ibm.com, linux-kernel@vger.kernel.org On Fri, 21 Oct 2016 08:30:53 +0530 Kirti Wankhede wrote: > On 10/21/2016 2:52 AM, Alex Williamson wrote: > > On Fri, 21 Oct 2016 02:44:37 +0530 > > Kirti Wankhede wrote: > > > ... > > >>>>>> > >>>>>> +extern const char *vfio_device_api_string(u32 flags); > >>>>>> + > >>>>>> struct pci_dev; > >>>>>> #ifdef CONFIG_EEH > >>>>>> extern void vfio_spapr_pci_eeh_open(struct pci_dev *pdev); > >>>>> > >>>>> Couldn't this simply be a #define in the uapi header? > >>>>> > >>>>> #define VFIO_DEVICE_PCI_API_STRING "vfio-pci" > >>>>> > >>>>> I don't really see why we need a lookup function. > >>>>> > >>>> > >>>> String is tightly coupled with the FLAG, right? > >>>> Instead user need to take care of making sure to return proper string, > >>>> and don't mis-match the string, I think having function is easier. > >>> > >>> That's exactly why I proposed putting the #define string in the uapi, > >>> by that I mean the vfio uapi header. That keeps the tight coupling to > >>> the flag, they're both defined in the same place, plus it gives > >>> userspace a reference so they're not just inventing a string to compare > >>> against. IOW, the vendor driver simply does an sprintf of > >>> VFIO_DEVICE_PCI_API_STRING and userspace (ie. libvirt) can do a strcmp > >>> with VFIO_DEVICE_PCI_API_STRING from the same header and everybody > >>> arrives at the same result. > >>> > >>>> Vendor driver should decide the type of device they want to expose and > >>>> set the flag, using this function vendor driver would return string > >>>> which is based on flag they set. > >>> > >>> Being a function adds no intrinsic value and being in a uapi header does > >>> add value to userspace. Thanks, > >>> > >> > >> Ok. The strings should be in uapi, but having function (like below) to > >> return proper string based on flag would be good to have for vendor driver. > >> > >> +const char *vfio_device_api_string(u32 flags) > >> +{ > >> + if (flags & VFIO_DEVICE_FLAGS_PCI) > >> + return VFIO_DEVICE_API_PCI_STRING; > >> + > >> + if (flags & VFIO_DEVICE_FLAGS_PLATFORM) > >> + return VFIO_DEVICE_API_PLATFORM_STRING; > >> + > >> + if (flags & VFIO_DEVICE_FLAGS_AMBA) > >> + return VFIO_DEVICE_API_AMBA_STRING; > >> + > >> + return ""; > >> +} > >> +EXPORT_SYMBOL(vfio_device_api_string); > > > > I disagree, it's pointless maintenance overhead. It's yet another > > function that we need to care about for kABI and it offers almost no > > value. Thanks, > > > > If any vendor driver sets VFIO_DEVICE_FLAGS_PLATFORM flag but sets > VFIO_DEVICE_API_PCI_STRING, we don't have a way to verify this in kernel > driver. Is that acceptable? a) The function doesn't solve that problem, as seen in the mtty sample driver there's no guarantee that the vendor driver passes device_info.flags vs the #define for the flag itself. So we're already expecting them to get it right in two separate places. b) It's not going to take them very long to figure this out if they care about userspace tools that make use of this field to determine the device API. This is a very obvious and simple bug. c) We can check and correct how open source vendor drivers work. Thanks, Alex