On Wed, 23 May 2018 17:25:32 +0300 "Michael S. Tsirkin" <m...@redhat.com> wrote:
> On Tue, May 22, 2018 at 10:28:56PM -0600, Alex Williamson wrote: > > On Wed, 23 May 2018 02:38:52 +0300 > > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > > > On Tue, May 22, 2018 at 03:47:41PM -0600, Alex Williamson wrote: > > > > On Wed, 23 May 2018 00:44:22 +0300 > > > > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > > > > > > > On Tue, May 22, 2018 at 03:36:59PM -0600, Alex Williamson wrote: > > > > > > On Tue, 22 May 2018 23:58:30 +0300 > > > > > > "Michael S. Tsirkin" <m...@redhat.com> wrote: > > > > > > > > > > > > > > It's not hard to think of a use-case where >256 devices > > > > > > > are helpful, for example a nested virt scenario where > > > > > > > each device is passed on to a different nested guest. > > > > > > > > > > > > > > But I think the main feature this is needed for is numa modeling. > > > > > > > Guests seem to assume a numa node per PCI root, ergo we need more > > > > > > > PCI > > > > > > > roots. > > > > > > > > > > > > But even if we have NUMA affinity per PCI host bridge, a PCI host > > > > > > bridge does not necessarily imply a new PCIe domain. > > > > > > > > > > What are you calling a PCIe domain? > > > > > > > > Domain/segment > > > > > > > > 0000:00:00.0 > > > > ^^^^ This > > > > > > Right. So we can thinkably have PCIe root complexes share an ACPI segment. > > > I don't see what this buys us by itself. > > > > The ability to define NUMA locality for a PCI sub-hierarchy while > > maintaining compatibility with non-segment aware OSes (and firmware). > > Fur sure, but NUMA is a kind of advanced topic, MCFG has been around for > longer than various NUMA tables. Are there really non-segment aware > guests that also know how to make use of NUMA? I can't answer that question, but I assume that multi-segment PCI support is perhaps not as pervasive as we may think considering hardware OEMs tend to avoid it for their default configurations with multiple host bridges. > > > > Isn't that the only reason we'd need a new MCFG section and the reason > > > > we're limited to 256 buses? Thanks, > > > > > > > > Alex > > > > > > I don't know whether a single MCFG section can describe multiple roots. > > > I think it would be certainly unusual. > > > > I'm not sure here if you're referring to the actual MCFG ACPI table or > > the MMCONFIG range, aka the ECAM. Neither of these describe PCI host > > bridges. The MCFG table can describe one or more ECAM ranges, which > > provides the ECAM base address, the PCI segment associated with that > > ECAM and the start and end bus numbers to know the offset and extent of > > the ECAM range. PCI host bridges would then theoretically be separate > > ACPI objects with _SEG and _BBN methods to associate them to the > > correct ECAM range by segment number and base bus number. So it seems > > that tooling exists that an ECAM/MMCONFIG range could be provided per > > PCI host bridge, even if they exist within the same domain, but in > > practice what I see on systems I have access to is a single MMCONFIG > > range supporting all of the host bridges. It also seems there are > > numerous ways to describe the MMCONFIG range and I haven't actually > > found an example that seems to use the MCFG table. Two have MCFG > > tables (that don't seem terribly complete) and the kernel claims to > > find the MMCONFIG via e820, another doesn't even have an MCFG table and > > the kernel claims to find MMCONFIG via an ACPI motherboard resource. > > I'm not sure if I can enable PCI segments on anything to see how the > > firmware changes. Thanks, > > > > Alex > > Let me clarify. So MCFG have base address allocation structures. > Each maps a segment and a range of bus numbers into memory. > This structure is what I meant. Ok, so this is the ECAM/MMCONFIG range through which we do config accesses, which is described by MCFG, among other options. > IIUC you are saying on your systems everything is within a single > segment, right? Multiple pci hosts map into a single segment? Yes, for instance a single MMCONFIG range handles bus number ranges 0x00-0x7f within segment 0x0 and the system has host bridges with base bus numbers of 0x00 and 0x40, each with different NUMA locality. > If you do this you can do NUMA, but do not gain > 256 devices. Correct, but let's also clarify that we're not limited to 256 devices, a segment is limited to 256 buses and each PCIe slot is a bus, so the limitation is number of hotpluggable slots. "Devices" implies that it includes multi-function, ARI, and SR-IOV devices as well, but we can have 256 of those per bus, we just don't have the desired hotplug granularity for those. > Are we are the same page then? Seems so. Thanks, Alex