On Thu, 27 Aug 2020 15:07:52 -0400 Eduardo Habkost <ehabk...@redhat.com> wrote:
> 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/ in case of 1 die, -numa is not must have as it's one numa node only. Though having auto_enable_numa, will allow to reuse the CPU.node-id property to compose CPUID_Fn8000001E_ECX. i.e only code one path vs numa|non-numa variant. > > (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? die_id is per socket numa node index, so it's not numa node id in a sense we use it in qemu (that's where all the confusion started that led to current code) I understood that each die in EPYC chip is a numa node, which encodes NUMA node ID (system wide) in CPUID_Fn8000001E_ECX, that's why I wrote earlier that EPYC makes -numa non optional. In case of only one die we can either use auto_enable_numa to ensure that we have consistent code or special case it and just hardcode CPUID_Fn8000001E_ECX value which is hackish but will let us avoid enabling numa (explicitly or implictly). in case of multiple dies, CPUID_Fn8000001E_ECX (encodes number of nodes + systemwide numa node id looking at CPUID of real EPYC machine) shall match -numa mapping (otherwise it's a bug where CPUID and ACPI mismatch). Here we can go to ways: 1) ask user to provide sane config with -numa (I'd prefer that) and use that info to fill in CPUID_Fn8000001E_ECX 2) pretend that it's non numa machine, skip ACPI SRAT table but make up CPUID_Fn8000001E (i.e. another special case) (requires another code path and addition to -numa one) > > > > (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. > > >
