> -----Original Message----- > From: Igor Mammedov <imammedo@redhat.com> > Sent: Friday, August 28, 2020 3:43 AM > To: Moger, Babu <Babu.Moger@amd.com> > Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>; ehabkost@redhat.com; > mst@redhat.com; qemu-devel@nongnu.org; pbonzini@redhat.com; > rth@twiddle.net > Subject: Re: [PATCH v5 0/8] Remove EPYC mode apicid decode and use > generic decode > > On Thu, 27 Aug 2020 17:58:01 -0500 > Babu Moger <babu.moger@amd.com> wrote: > > > > -----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=h > > > > > > > > > >> ttps > > > > > > > > > >> %3A%2F%2Flore.kernel.org%2Fqemu-devel%2Fc0bcc1a6- > > > 1d84-a6e > > > > > > > > > >> 7-e468- > > > d5b437c1b254%40amd.com%2F&data=02%7C01%7Cbabu. > > > > > > > > > >> > > > moger%40amd.com%7C9b15ee395daa4935640408d84acedf13%7C3dd8 > > > > > > > > > >> > > > 961fe4884e608e11a82d994e183d%7C0%7C0%7C637341599663177545 > > > > > > > > > >> > > > &sdata=4okYGU%2F8QTYqEOZEd1EBC%2BEsIIrEV59HZrHzpbsR8s > > > > > > > > > >> U%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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%25 > > > > > > > 2Fwww.mail-archive.com%2Fqemu- > > > devel%40nongnu.org%2Fmsg728536.htm > > > > > > > > > > > l&data=02%7C01%7Cbabu.moger%40amd.com%7C9b15ee395daa49356 > > > 404 > > > > > > > > > > > 08d84acedf13%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C63734 > > > 1 > > > > > > > > > > > 599663177545&sdata=OnHz23W4F4TdYwlxPZwC%2B8YRY1K3qJ5U9Sfdo > > > Oc > > > > > > > GXtw%3D&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); > that will work when there is no -numa on CLI, when -numa is used, we > should use node id that user provided. > like you did in previous revision > "[PATCH v4 1/3] i386: Simplify CPUID_8000_001E for AMD" This might be a problem in the future with new BIOS options to change the NPS(Nodes per Socket). Nodes and dies may not match. Then we will end up with wrong CPUID_8000_001E encoding. That is why I wanted to keep both of them separate. Users have the option to configure the way it matches their bios config. > > > Still not clear if we need to add a warning when numa nodes != dies. > > Worried about adding that check and remove it again later. > Since there is objection wrt making it error and I'd go with warning for now, it > makes life of person who have to figure what's wrong a bit easier. > > > What about auto_enable_numa? Do we still need it? > > > > > > I can send the patches tomorrow if these things are clarified. > > Thanks > With auto_enable_numa it would be cleaner as you can reuse the same > numa code to set 0x8000001e.ecx vs hardcodding it as above. > > Maybe post series without auto_enable_numa so we fix migration > regression ASAP and then switch to auto_enable_numa on top. > > > > > > > > > > > > > > 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 > > > > > > > > > > > > > > > > > > > > > >
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