RE: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode

[Babu Moger]

> -----Original Message-----
> From: Igor Mammedov <imammedo@redhat.com>
> Sent: Thursday, August 27, 2020 4:19 PM
> To: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Cc: ehabkost@redhat.com; mst@redhat.com; qemu-devel@nongnu.org;
> Moger, Babu <Babu.Moger@amd.com>; pbonzini@redhat.com;
> rth@twiddle.net
> Subject: Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use
> generic decode
> 
> On Wed, 26 Aug 2020 15:10:46 +0100
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> 
> > * Igor Mammedov (imammedo@redhat.com) wrote:
> > > On Tue, 25 Aug 2020 16:25:21 +0100
> > > "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > >
> > > > * Igor Mammedov (imammedo@redhat.com) wrote:
> > > > > On Tue, 25 Aug 2020 09:15:04 +0100 "Dr. David Alan Gilbert"
> > > > > <dgilbert@redhat.com> wrote:
> > > > >
> > > > > > * Babu Moger (babu.moger@amd.com) wrote:
> > > > > > > Hi Dave,
> > > > > > >
> > > > > > > On 8/24/20 1:41 PM, Dr. David Alan Gilbert wrote:
> > > > > > > > * Babu Moger (babu.moger@amd.com) wrote:
> > > > > > > >> To support some of the complex topology, we introduced EPYC
> mode apicid decode.
> > > > > > > >> But, EPYC mode decode is running into problems. Also it
> > > > > > > >> can become quite a maintenance problem in the future. So,
> > > > > > > >> it was decided to remove that code and use the generic
> > > > > > > >> decode which works for majority of the topology. Most of
> > > > > > > >> the SPECed configuration would work just fine. With some
> non-SPECed user inputs, it will create some sub-optimal configuration.
> > > > > > > >> Here is the discussion thread.
> > > > > > > >> https://nam11.safelinks.protection.outlook.com/?url=https
> > > > > > > >> %3A%2F%2Flore.kernel.org%2Fqemu-devel%2Fc0bcc1a6-
> 1d84-a6e
> > > > > > > >> 7-e468-
> d5b437c1b254%40amd.com%2F&amp;data=02%7C01%7Cbabu.
> > > > > > > >>
> moger%40amd.com%7C9b15ee395daa4935640408d84acedf13%7C3dd8
> > > > > > > >>
> 961fe4884e608e11a82d994e183d%7C0%7C0%7C637341599663177545
> > > > > > > >>
> &amp;sdata=4okYGU%2F8QTYqEOZEd1EBC%2BEsIIrEV59HZrHzpbsR8s
> > > > > > > >> U%3D&amp;reserved=0
> > > > > > > >>
> > > > > > > >> This series removes all the EPYC mode specific apicid changes
> and use the generic
> > > > > > > >> apicid decode.
> > > > > > > >
> > > > > > > > Hi Babu,
> > > > > > > >   This does simplify things a lot!
> > > > > > > > One worry, what happens about a live migration of a VM from
> an old qemu
> > > > > > > > that was using the node-id to a qemu with this new scheme?
> > > > > > >
> > > > > > > The node_id which we introduced was only used internally. This
> wasn't
> > > > > > > exposed outside. I don't think live migration will be an issue.
> > > > > >
> > > > > > Didn't it become part of the APIC ID visible to the guest?
> > > > >
> > > > > Daniel asked similar question wrt hard error on start up, when
> > > > > CLI is not sufficient to create EPYC cpu.
> > > > >
> > > > >
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%
> > > > > 2Fwww.mail-archive.com%2Fqemu-
> devel%40nongnu.org%2Fmsg728536.htm
> > > > >
> l&amp;data=02%7C01%7Cbabu.moger%40amd.com%7C9b15ee395daa49356
> 404
> > > > >
> 08d84acedf13%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C63734
> 1
> > > > >
> 599663177545&amp;sdata=OnHz23W4F4TdYwlxPZwC%2B8YRY1K3qJ5U9Sfdo
> Oc
> > > > > GXtw%3D&amp;reserved=0
> > > > >
> > > > > Migration might fall into the same category.
> > > > > Also looking at the history, 5.0 commit
> > > > >   247b18c593ec29 target/i386: Enable new apic id encoding for
> > > > > EPYC based cpus models silently broke APIC ID (without versioning),
> for all EPYC models (that's were 1 new and 1 old one).
> > > > >
> > > > > (I'm not aware of somebody complaining about it)
> > > > >
> > > > > Another commit ed78467a21459, changed CPUID_8000_001E without
> versioning as well.
> > > > >
> > > > >
> > > > > With current EPYC apicid code, if all starts align (no numa or 1
> > > > > numa node only on CLI and no -smp dies=) it might produce a valid
> CPU (apicid+CPUID_8000_001E).
> > > > > No numa is gray area, since EPYC spec implies that it has to be numa
> machine in case of real EPYC cpus.
> > > > > Multi-node configs would be correct only if user assigns cpus to
> > > > > numa nodes by duplicating internal node_id algorithm that this series
> removes.
> > > > >
> > > > > There might be other broken cases that I don't recall anymore
> > > > > (should be mentioned in previous versions of this series)
> > > > >
> > > > >
> > > > > To summarize from migration pov (ignoring ed78467a21459 change):
> > > > >
> > > > >  1) old qemu pre-5.0 ==>  qemu 5.0, 5.1 - broken migration
> > > >
> > > > Oh ....
> > > >
> > > > >  2) with this series (lets call it qemu 5.2)
> > > > >      pre-5.0 ==> qemu 5.2 - should work as series basically rollbacks
> current code to pre-5.0
> > > > >      qemu 5.0, 5.1 ==> qemu 5.2 - broken
> > > > >
> > > > > It's all about picking which poison to choose, I'd preffer 2nd
> > > > > case as it lets drop a lot of complicated code that doesn't work
> > > > > as expected.
> > > >
> > > > I think that would make our lives easier for other reasons; so I'm
> > > > happy to go with that.
> > >
> > > to make things less painful for users, me wonders if there is a way
> > > to block migration if epyc and specific QEMU versions are used?
> >
> > We have no way to block based on version - and that's a pretty painful
> > thing to do; we can block based on machine type.
> >
> > But before we get there; can we understand in which combinations that
> > things break and why exactly - would it break on a 1 or 2 vCPU guest -
> > or would it only break when we get to the point the upper bits start
> > being used for example?  Why exaclty would it break - i.e. is it going
> > to change the name of sections in the migration stream - or are the
> > values we need actually going to migrate OK?
> 
> it's values of APIC ID, where 4.2 and 5.0 QEMU use different values if numa is
> enabled.
> I'd expect guest to be very confused in when this happens.
> 
> here is an example:
> qemu-4.2 -cpu EPYC -smp 8,sockets=1,cores=8 -numa node,cpus=0-3 -numa
> node,cpus=4-7
> 
> (QEMU) qom-get path=/machine/unattached/device[8] property=apic-id {
>     "return": 7
> }
> 
> vs
> 
> qemu-5.1 -cpu EPYC -smp 8,sockets=1,cores=8 -numa node,cpus=0-3 -numa
> node,cpus=4-7
> (QEMU) qom-get path=/machine/unattached/device[8] property=apic-id {
>     "return": 15
> }
> 
> we probably can't do anything based on machine type versions, as
> 4.2 and older versions on qemu-5.0 and newer use different algorithm to
> calculate apic-id.
> 
> Hence was suggestion to leave 5.0/5.1 with broken apic id and revert back to
> 4.2 algorithm, which should encode APIC ID correctly when '-smp dies' is
> used.

That is correct. When we revert all the node_id related changes, we will
go back to 4.2 algorithm. It will work fine with user passing "-smp
dies=n". It also keeps the code simple. That is why I kept the decoding of
0x8000001e like this below. This will also match apicid decoding.

*ecx = ((topo_info->dies_per_pkg - 1) << 8) |  ((cpu->apic_id >>
apicid_die_offset(topo_info)) & 0xFF);


Still not clear if we need to add a warning when numa nodes != dies.
Worried about adding that check and remove it again later.

What about auto_enable_numa? Do we still need it?

I can send the patches tomorrow if these things are clarified.
Thanks

> 
> 
> > Dave
> >
> >
> > > > > PS:
> > > > >  I didn't review it yet, but with this series we aren't  making
> > > > > up internal node_ids that should match user provided numa node ids
> somehow.
> > > > >  It seems series lost the patch that would enforce numa in case
> > > > > -smp dies>1,  but otherwise it heads in the right direction.
> > > >
> > > > Dave
> > > >
> > > > > >
> > > > > > Dave
> > > > > >
> > > > >
> > >