From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54175) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WX9gF-0006Zf-9I for qemu-devel@nongnu.org; Mon, 07 Apr 2014 09:33:49 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WX9g9-0001s7-4f for qemu-devel@nongnu.org; Mon, 07 Apr 2014 09:33:43 -0400 Received: from mx1.redhat.com ([209.132.183.28]:6102) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WX9g8-0001rx-RR for qemu-devel@nongnu.org; Mon, 07 Apr 2014 09:33:37 -0400 Date: Mon, 7 Apr 2014 16:34:15 +0300 From: "Michael S. Tsirkin" Message-ID: <20140407133415.GB16541@redhat.com> References: <1396868342-12005-1-git-send-email-marcel.a@redhat.com> <20140407121508.GD16369@redhat.com> <1396874646.5001.76.camel@nilsson.home.kraxel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1396874646.5001.76.camel@nilsson.home.kraxel.org> Subject: Re: [Qemu-devel] [SeaBIOS] [PATCH] hw/pci: reserve IO and mem for pci-2-pci bridges with no devices attached List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gerd Hoffmann Cc: kevin@koconnor.net, seabios@seabios.org, qemu-devel@nongnu.org, Marcel Apfelbaum On Mon, Apr 07, 2014 at 02:44:06PM +0200, Gerd Hoffmann wrote: > Hi, > > > > + u8 shpc_cap = pci_find_capability(s->bus_dev, PCI_CAP_ID_SHPC); > > > One thing I'd do is maybe check that the relevant memory type is > > enabled in the bridge (probably just by writing fff to base and reading > > it back). > > > This will give hypervisors an option to avoid wasting resources: > > e.g. it's uncommon for express devices to claim IO. > > I don't think we'll need that for the SHPC bridge. Why not? I'm referring to this text in the bridge specification: The I/O Base and I/O Limit registers are optional and define an address range that is used by the bridge to determine when to forward I/O transactions from one interface to the other. If a bridge does not implement an I/O address range, then both the I/O Base and I/O Limit registers must be implemented as read-only registers that return zero when read. If a bridge supports an I/O address range, then these registers must be initialized by configuration software so default states are not specified. So we should probe bridge for I/O support before wasting I/O resources on it. The spec does not provide a way to detect this, but we can do it like this: - write value ffffffff to I/O base register - read back value value 0 means bridge does not support I/O. A similar trick should work for other optional resources. > For express it indeed makes sense to avoid claiming IO address space. > I'd try to find something more automatic though, where you don't need > some kind of "disable io for this express port" config option. Won't same trick as above work? > For express ports which can only have a single device underneath we can > check whenever we have a device and if one is present already don't > bother claiming extra resources for hotplug. > > > > + for (cap = pci_config_readb(pci->bdf, PCI_CAPABILITY_LIST); cap; > > > + cap = pci_config_readb(pci->bdf, cap + PCI_CAP_LIST_NEXT)) > > > + if (pci_config_readb(pci->bdf, cap + PCI_CAP_LIST_ID) == cap_id) > > > + return cap; > > > > I would also limit this to 256 iterations, to make sure > > we dont' get into an infinite loop with a broken device. > > Good point. > > cheers, > Gerd >