From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: [PATCH 12/22] virtio_pci: allow duplicate capabilities. Date: Thu, 21 Mar 2013 09:04:48 -0700 Message-ID: <514B2FA0.9000204@zytor.com> References: <1363854584-25795-1-git-send-email-rusty@rustcorp.com.au> <1363854584-25795-13-git-send-email-rusty@rustcorp.com.au> <20130321102814.GC30493@redhat.com> <514B188A.3030502@zytor.com> <20130321144330.GA1454@redhat.com> <514B1D25.9090206@zytor.com> <20130321151937.GB1454@redhat.com> <514B26BC.6010700@zytor.com> <20130321155810.GA1925@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130321155810.GA1925@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: "Michael S. Tsirkin" Cc: virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org On 03/21/2013 08:58 AM, Michael S. Tsirkin wrote: >> >> Most of them do really ugly hacks in hardware (like putting in a "back >> door" in config space) to make that possible. > > config space register that let us access > registers within BAR actually sounds pretty reasonable. > Way better than an I/O BAR. > It is really, really, nasty, not to mention slow. >>> >>> Problem is, BIOS and OS normally assume failure to allocate >>> any resources means card won't function and disable it. >>> So it does not seem to be worth it to have such a >>> device specific failover ability. >>> >> >> That is a violation of the PCIe spec; the PCIe spec specifically states >> that failure to allocate an I/O BAR should still allow the device to >> function. > > Where does it say this? In PCI Express 1.1 base, it is section 1.3.2.2, third bullet. -hpa