Re: [Qemu-devel] [RFC 3/3] acpi-build: allocate mcfg for multiple host bridges

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, May 23, 2018 at 08:57:51AM -0600, Alex Williamson wrote:
> On Wed, 23 May 2018 17:25:32 +0300
> "Michael S. Tsirkin" <mst@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" <mst@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" <mst@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" <mst@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.

Right, I consider a group of PF and all its VFs a device,
and all functions in a multi-function device a single
device for this purpose.

> > Are we are the same page then?
> 
> Seems so.  Thanks,
> 
> Alex