On Thu, Aug 27, 2020 at 07:03:14PM +0200, Igor Mammedov wrote: > On Wed, 26 Aug 2020 16:03:40 +0100 > Daniel P. Berrangé <berra...@redhat.com> wrote: > > > On Wed, Aug 26, 2020 at 04:02:58PM +0200, Igor Mammedov wrote: > > > On Wed, 26 Aug 2020 14:36:38 +0100 > > > Daniel P. Berrangé <berra...@redhat.com> wrote: > > > > > > > On Wed, Aug 26, 2020 at 03:30:34PM +0200, Igor Mammedov wrote: > > > > > On Wed, 26 Aug 2020 13:50:59 +0100 > > > > > Daniel P. Berrangé <berra...@redhat.com> wrote: > > > > > > > > > > > On Wed, Aug 26, 2020 at 02:38:49PM +0200, Igor Mammedov wrote: > > > > > > > On Fri, 21 Aug 2020 17:12:19 -0500 > > > > > > > Babu Moger <babu.mo...@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://lore.kernel.org/qemu-devel/c0bcc1a6-1d84-a6e7-e468-d5b437c1b...@amd.com/ > > > > > > > > > > > > > > > > This series removes all the EPYC mode specific apicid changes > > > > > > > > and use the generic > > > > > > > > apicid decode. > > > > > > > > > > > > > > the main difference between EPYC and all other CPUs is that > > > > > > > it requires numa configuration (it's not optional) > > > > > > > so we need an extra patch on top of this series to enfoce that, > > > > > > > i.e: > > > > > > > > > > > > > > if (epyc && !numa) > > > > > > > error("EPYC cpu requires numa to be configured") > > > > > > > > > > > > Please no. This will break 90% of current usage of the EPYC CPU in > > > > > > real world QEMU deployments. That is way too user hostile to > > > > > > introduce > > > > > > as a requirement. > > > > > > > > > > > > Why do we need to force this ? People have been successfuly using > > > > > > EPYC CPUs without NUMA in QEMU for years now. > > > > > > > > > > > > It might not match behaviour of bare metal silicon, but that hasn't > > > > > > obviously caused the world to come crashing down. > > > > > So far it produces warning in linux kernel (RHBZ1728166), > > > > > (resulting performance might be suboptimal), but I haven't seen > > > > > anyone reporting crashes yet. > > > > > > > > > > > > > > > What other options do we have? > > > > > Perhaps we can turn on strict check for new machine types only, > > > > > so old configs can keep broken topology (CPUID), > > > > > while new ones would require -numa and produce correct topology. > > > > > > > > No, tieing this to machine types is not viable either. That is still > > > > going to break essentially every single management application that > > > > exists today using QEMU. > > > for that we have deprecation process, so users could switch to new CLI > > > that would be required. > > > > We could, but I don't find the cost/benefit tradeoff is compelling. > > > > There are so many places where we diverge from what bare metal would > > do, that I don't see a good reason to introduce this breakage, even > > if we notify users via a deprecation message. > I find (3) and (4) good enough reasons to use deprecation. > > > If QEMU wants to require NUMA for EPYC, then QEMU could internally > > create a single NUMA node if none was specified for new machine > > types, such that there is no visible change or breakage to any > > mgmt apps. > > (1) for configs that started without -numa &&|| without -smp dies>1, > QEMU can do just that (enable auto_enable_numa).
Why exactly do we need auto_enable_numa with dies=1? If I understand correctly, Babu said earlier in this thread[1] that we don't need auto_enable_numa. [1] https://lore.kernel.org/qemu-devel/11489e5f-2285-ddb4-9c35-c9f522d60...@amd.com/ > > (2) As for configs that are out of spec, I do not care much (junk in - junk > out) > (though not having to spend time on bug reports and debug issues, just to say > it's not supported in the end, makes deprecation sound like a reasonable > choice) > > (3) However if config matches bare metal i.e. CPU has more than 1 die and > within > dies limits (spec wise), QEMU has to produce valid CPUs. > In this case QEMU can't make up multiple numa nodes and mappings of RAM/CPUs > on user's behalf. That's where we have to error out and ask for explicit > numa configuration. > > For such configs, current code (since 5.0), will produce in the best case > performance issues due to mismatching data in APICID, CPUID and ACPI tables, > in the worst case issues might be related to invalid APIC ID if running on > EPYC host > and HW takes in account subfields of APIC ID (according to Babu real CPU uses > die_id(aka node_id) internally). > I'd rather error out on nonsense configs earlier than debug such issues > and than error out anyways later (upsetting more users). > The requirements are not clear to me. Is this just about making CPU die_id match the NUMA node ID, or are there additional constraints? > (4) > If I were non hobby user, I'd hate if QEMU allowed me to start invalid config, > that I'd have to spend time on debugging issues (including performance ones), > instead of clearly telling me what's wrong and how config should be corrected. > I'd probably jump to another hypervisor that does the job right, > instead of digging into QEMU codebase and CPU specs to figure out how > to hack and configure it. > -- Eduardo
