On Wed, Aug 26, 2020 at 04:03:40PM +0100, Daniel P. Berrangé 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. > > 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.
Is anything expected to break if we just set auto_enable_numa=true unconditionally on pc-*-5.2 and newer? -- Eduardo
