"Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: > On Wed, Jun 05, 2013 at 07:59:33AM -0500, Anthony Liguori wrote: >> "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: >> >> > On Tue, Jun 04, 2013 at 03:01:50PM +0930, Rusty Russell wrote: >> > You mean make BAR0 an MMIO BAR? >> > Yes, it would break current windows guests. >> > Further, as long as we use same address to notify all queues, >> > we would also need to decode the instruction on x86 and that's >> > measureably slower than PIO. >> > We could go back to discussing hypercall use for notifications, >> > but that has its own set of issues... >> >> So... does "violating the PCI-e" spec really matter? Is it preventing >> any guest from working properly? > > Yes, absolutely, this wording in spec is not there without reason. > > Existing guests allocate io space for PCI express ports in > multiples on 4K. > > Since each express device is behind such a port, this means > at most 15 such devices can use IO ports in a system. > > That's why to make a pci express virtio device, > we must allow MMIO and/or some other communication > mechanism as the spec requires. This is precisely why this is an ABI breaker. If you disable IO bars in the BIOS, than the interface that the OS sees will *not have an IO bar*. This *breaks existing guests*. Any time the programming interfaces changes on a PCI device, the revision ID and/or device ID must change. The spec is very clear about this. We cannot disable the IO BAR without changing revision ID/device ID. > That's on x86. > > Besides x86, there are achitectures where IO is unavailable or very slow. > >> I don't think we should rush an ABI breakage if the only benefit is >> claiming spec compliance. >> >> Regards, >> >> Anthony Liguori > > Why do you bring this up? No one advocates any ABI breakage, > I only suggest extensions. It's an ABI breakage. You're claiming that the guests you tested handle the breakage reasonably but it is unquestionably an ABI breakage. Regards, Anthony Liguori > > >> > >> > -- >> > MST >> > -- >> > To unsubscribe from this list: send the line "unsubscribe kvm" in >> > the body of a message to majordomo@xxxxxxxxxxxxxxx >> > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
