Anthony Liguori <aliguori@xxxxxxxxxx> writes: > Gerd Hoffmann <kraxel@xxxxxxxxxx> writes: > >> Hi, >> >>>> So we could have for virtio something like this: >>>> >>>> Capabilities: [??] virtio-regs: >>>> legacy: BAR=0 offset=0 >>>> virtio-pci: BAR=1 offset=1000 >>>> virtio-cfg: BAR=1 offset=1800 >>> >>> This would be a vendor specific PCI capability so lspci wouldn't >>> automatically know how to parse it. >> >> Sure, would need a patch to actually parse+print the cap, >> /me was just trying to make my point clear in a simple way. >> >>>>>> 2) ISTR an argument about mapping the ISR register separately, for >>>>>> performance, but I can't find a reference to it. >>>>> >>>>> I think the rationale is that ISR really needs to be PIO but everything >>>>> else doesn't. PIO is much faster on x86 because it doesn't require >>>>> walking page tables or instruction emulation to handle the exit. >>>> >>>> Is this still a pressing issue? With MSI-X enabled ISR isn't needed, >>>> correct? Which would imply that pretty much only old guests without >>>> MSI-X support need this, and we don't need to worry that much when >>>> designing something new ... >>> >>> It wasn't that long ago that MSI-X wasn't supported.. I think we should >>> continue to keep ISR as PIO as it is a fast path. >> >> No problem if we allow to have both legacy layout and new layout at the >> same time. Guests can continue to use ISR @ BAR0 in PIO space for >> existing virtio devices, even in case they want use mmio for other >> registers -> all fine. >> >> New virtio devices can support MSI-X from day one and decide to not >> expose a legacy layout PIO bar. > > I think having BAR1 be an MMIO mirror of the registers + a BAR2 for > virtio configuration space is probably not that bad of a solution. Well, we also want to clean up the registers, so how about: BAR0: legacy, as is. If you access this, don't use the others. BAR1: new format virtio-pci layout. If you use this, don't use BAR0. BAR2: virtio-cfg. If you use this, don't use BAR0. BAR3: ISR. If you use this, don't use BAR0. I prefer the cases exclusive (ie. use one or the other) as a clear path to remove the legacy layout; and leaving the ISR in BAR0 leaves us with an ugly corner case in future (ISR is BAR0 + 19? WTF?). As to MMIO vs PIO, the BARs are self-describing, so we should explicitly endorse that and leave it to the devices. The detection is simple: if BAR1 has non-zero length, it's new-style, otherwise legacy. Thoughts? Rusty. -- 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
