Re: [Qemu-devel] [PATCH] hw/acpi-build: build SRAT memory affinity structures for NVDIMM

[Igor Mammedov]

On Thu, 22 Feb 2018 09:40:00 +0800
Haozhong Zhang <address@hidden> wrote:

> On 02/21/18 14:55 +0100, Igor Mammedov wrote:
> > On Tue, 20 Feb 2018 17:17:58 -0800
> > Dan Williams <address@hidden> wrote:
> >   
> > > On Tue, Feb 20, 2018 at 6:10 AM, Igor Mammedov <address@hidden> wrote:  
> > > > On Sat, 17 Feb 2018 14:31:35 +0800
> > > > Haozhong Zhang <address@hidden> wrote:
> > > >    
> > > >> ACPI 6.2A Table 5-129 "SPA Range Structure" requires the proximity
> > > >> domain of a NVDIMM SPA range must match with corresponding entry in
> > > >> SRAT table.
> > > >>
> > > >> The address ranges of vNVDIMM in QEMU are allocated from the
> > > >> hot-pluggable address space, which is entirely covered by one SRAT
> > > >> memory affinity structure. However, users can set the vNVDIMM
> > > >> proximity domain in NFIT SPA range structure by the 'node' property of
> > > >> '-device nvdimm' to a value different than the one in the above SRAT
> > > >> memory affinity structure.
> > > >>
> > > >> In order to solve such proximity domain mismatch, this patch build one
> > > >> SRAT memory affinity structure for each NVDIMM device with the
> > > >> proximity domain used in NFIT. The remaining hot-pluggable address
> > > >> space is covered by one or multiple SRAT memory affinity structures
> > > >> with the proximity domain of the last node as before.
> > > >>
> > > >> Signed-off-by: Haozhong Zhang <address@hidden>    
> > > > If we consider hotpluggable system, correctly implemented OS should
> > > > be able pull proximity from Device::_PXM and override any value from 
> > > > SRAT.
> > > > Do we really have a problem here (anything that breaks if we would use 
> > > > _PXM)?
> > > > Maybe we should add _PXM object to nvdimm device nodes instead of 
> > > > massaging SRAT?    
> > > 
> > > Unfortunately _PXM is an awkward fit. Currently the proximity domain
> > > is attached to the SPA range structure. The SPA range may be
> > > associated with multiple DIMM devices and those individual NVDIMMs may
> > > have conflicting _PXM properties.  
> > There shouldn't be any conflict here as  NVDIMM device's _PXM method,
> > should override in runtime any proximity specified by parent scope.
> > (as parent scope I'd also count boot time NFIT/SRAT tables).
> > 
> > To make it more clear we could clear valid proximity domain flag in SPA
> > like this:
> > 
> > diff --git a/hw/acpi/nvdimm.c b/hw/acpi/nvdimm.c
> > index 59d6e42..131bca5 100644
> > --- a/hw/acpi/nvdimm.c
> > +++ b/hw/acpi/nvdimm.c
> > @@ -260,9 +260,7 @@ nvdimm_build_structure_spa(GArray *structures, 
> > DeviceState *dev)
> >       */
> >      nfit_spa->flags = cpu_to_le16(1 /* Control region is strictly for
> >                                         management during hot add/online
> > -                                       operation */ |
> > -                                  2 /* Data in Proximity Domain field is
> > -                                       valid*/);
> > +                                       operation */);
> >  
> >      /* NUMA node. */
> >      nfit_spa->proximity_domain = cpu_to_le32(node);
> >   
> > > Even if that was unified across
> > > DIMMs it is ambiguous whether a DIMM-device _PXM would relate to the
> > > device's control interface, or the assembled persistent memory SPA
> > > range.  
> > I'm not sure what you mean under 'device's control interface',
> > could you clarify where the ambiguity comes from?
> > 
> > I read spec as: _PXM applies to address range covered by NVDIMM
> > device it belongs to.
> > 
> > As for assembled SPA, I'd assume that it applies to interleaved set
> > and all NVDIMMs with it should be on the same node. It's somewhat
> > irrelevant question though as QEMU so far implements only
> >   1:1:1/SPA:Region Mapping:NVDIMM Device/
> > mapping.
> > 
> > My main concern with using static configuration tables for proximity
> > mapping, we'd miss on hotplug side of equation. However if we start
> > from dynamic side first, we could later complement it with static
> > tables if there really were need for it.  
> 
> This patch affects only the static tables and static-plugged NVDIMM.
> For hot-plugged NVDIMMs, guest OSPM still needs to evaluate _FIT to
> get the information of the new NVDIMMs including their proximity
> domains.
> 
> One intention of this patch is to simulate the bare metal as much as
> possible. I have been using this patch to develop and test NVDIMM
> enabling work on Xen, and think it might be useful for developers of
> other OS and hypervisors.
It's simpler on bare metal as systems usually statically partitioned
according to capacity slots are able to handle.

The patch is technically correct and might be useful,
especially in current case case flag /* Data in Proximity Domain field is 
valid*/
set, to conform to the spec. So just complement the patch with
test case as requested and it should be fine to merge.

PS:
while adding ranges for present NVDIMMs in SRAT it would be
better to generalize a bit and include present pc-dimms
there as well to be consistent with SRAT partitioning.

PS2:
As food for future discussion, what I'm trying to figure out if
we should use the valid flag or not, as its use creates multiple
sources for proximity domain SRAT / SPA / _PXM / HMAT that all must
match each other, which is rather fragile and easy to break.

On the other hand if we clear the flag, OSPM will have the single source
of information (_PXM) per nvdimm device, which has 'override' behavior over 
SRAT.



> Haozhong
>