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Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode |
Date: |
Wed, 26 Aug 2020 15:10:46 +0100 |
User-agent: |
Mutt/1.14.6 (2020-07-11) |
* 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-a6e7-e468-d5b437c1b254%40amd.com%2F&data=02%7C01%7Cbabu.moger%40amd.com%7C74d90724af9c4adcc75008d8485d4d16%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637338912853492167&sdata=GTsMKcpeYXAA0CvpLTirPHKdNSdlJE3RuPjCtSyWtGQ%3D&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://www.mail-archive.com/qemu-devel@nongnu.org/msg728536.html
> > >
> > > 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?
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
> > > >
> > >
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
- [PATCH v5 3/8] Revert "target/i386: Enable new apic id encoding for EPYC based cpus models", (continued)
- [PATCH v5 3/8] Revert "target/i386: Enable new apic id encoding for EPYC based cpus models", Babu Moger, 2020/08/21
- [PATCH v5 7/8] Revert "hw/386: Add EPYC mode topology decoding functions", Babu Moger, 2020/08/21
- [PATCH v5 8/8] i386: Simplify CPUID_8000_001E for AMD, Babu Moger, 2020/08/21
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Dr. David Alan Gilbert, 2020/08/24
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Babu Moger, 2020/08/24
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Dr. David Alan Gilbert, 2020/08/25
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Igor Mammedov, 2020/08/25
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Dr. David Alan Gilbert, 2020/08/25
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Igor Mammedov, 2020/08/26
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode,
Dr. David Alan Gilbert <=
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Igor Mammedov, 2020/08/27
- RE: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Babu Moger, 2020/08/27
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Igor Mammedov, 2020/08/28
- RE: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Babu Moger, 2020/08/28
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Dr. David Alan Gilbert, 2020/08/28
- Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Igor Mammedov, 2020/08/28
Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use generic decode, Igor Mammedov, 2020/08/26