> -----Original Message----- > From: Eduardo Habkost [mailto:ehabkost@xxxxxxxxxx] > Sent: Friday, June 8, 2018 2:23 PM > To: Moger, Babu <Babu.Moger@xxxxxxx> > Cc: mst@xxxxxxxxxx; marcel.apfelbaum@xxxxxxxxx; pbonzini@xxxxxxxxxx; > rth@xxxxxxxxxxx; mtosatti@xxxxxxxxxx; qemu-devel@xxxxxxxxxx; > kvm@xxxxxxxxxxxxxxx; kash@xxxxxxxxxxxxxx; geoff@xxxxxxxxxxxxxxx; Juan > Quintela <quintela@xxxxxxxxxx>; xiaoguangrong@xxxxxxxxxxx > Subject: Re: [PATCH v12 3/4] i386: Enable TOPOEXT feature on AMD EPYC > CPU > > On Fri, Jun 08, 2018 at 06:40:16PM +0000, Moger, Babu wrote: > > Hi Eduardo, > > Sorry for the late response. Got pulled into something else. > > > > > -----Original Message----- > > > From: Eduardo Habkost [mailto:ehabkost@xxxxxxxxxx] > > > Sent: Wednesday, June 6, 2018 5:40 PM > > > To: Moger, Babu <Babu.Moger@xxxxxxx> > > > Cc: mst@xxxxxxxxxx; marcel.apfelbaum@xxxxxxxxx; > pbonzini@xxxxxxxxxx; > > > rth@xxxxxxxxxxx; mtosatti@xxxxxxxxxx; qemu-devel@xxxxxxxxxx; > > > kvm@xxxxxxxxxxxxxxx; kash@xxxxxxxxxxxxxx; geoff@xxxxxxxxxxxxxxx > > > Subject: Re: [PATCH v12 3/4] i386: Enable TOPOEXT feature on AMD EPYC > > > CPU > > > > > > On Wed, Jun 06, 2018 at 10:36:45AM -0400, Babu Moger wrote: > > > > Enable TOPOEXT feature on EPYC CPU. This is required to support > > > > hyperthreading on VM guests. Also extend xlevel to 0x8000001E. > > > > > > > > Disable TOPOEXT feature for legacy machines. > > > > > > > > Signed-off-by: Babu Moger <babu.moger@xxxxxxx> > > > > > > Now, I just noticed we have a problem here: > > > > > > "-machine pc -cpu EPYC -smp 64" works today > > > > > > This patch makes it stop working, but it shouldn't. > > > > No. It works fine. I have tested it. > > This doesn't sound right. The code in this series will error out > of TOPOEXT is enabled and you have more than 64 VCPUs. > > But I just noticed we have a bug introduced by: Oh.. Ok.. Let me retry again with the new patch. > > commit f548222c24342ca74689de7794f9006b43f86a54 > Author: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxx> > Date: Thu May 3 16:06:11 2018 +0800 > > migration: introduce decompress-error-check > > QEMU 3.0 enables strict check for compression & decompression to > make the migration more robust, that depends on the source to fix > the internal design which triggers the unexpected error conditions > > To make it work for migrating old version QEMU to 2.13 QEMU, we > introduce this parameter to disable the error check on the > destination which is the default behavior of the machine type > which is older than 2.13, alternately, the strict check can be > enabled explicitly as followings: > -M pc-q35-2.11 -global migration.decompress-error-check=true > > Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxx> > Reviewed-by: Juan Quintela <quintela@xxxxxxxxxx> > Signed-off-by: Juan Quintela <quintela@xxxxxxxxxx> > > This commits added PC_COMPAT_2_12 to the 3.0 machine-types. > Because of this bug, TOPOEXT is being unconditionally disabled on > all machine-types, unless I apply the fix below: > > > diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c > index 3d81136065..b4c5b03274 100644 > --- a/hw/i386/pc_piix.c > +++ b/hw/i386/pc_piix.c > @@ -430,7 +430,6 @@ static void > pc_i440fx_3_0_machine_options(MachineClass *m) > pc_i440fx_machine_options(m); > m->alias = "pc"; > m->is_default = 1; > - SET_MACHINE_COMPAT(m, PC_COMPAT_2_12); > } > > DEFINE_I440FX_MACHINE(v3_0, "pc-i440fx-3.0", NULL, > diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c > index b60cbb9266..83d6d75efa 100644 > --- a/hw/i386/pc_q35.c > +++ b/hw/i386/pc_q35.c > @@ -312,7 +312,6 @@ static void > pc_q35_3_0_machine_options(MachineClass *m) > { > pc_q35_machine_options(m); > m->alias = "q35"; > - SET_MACHINE_COMPAT(m, PC_COMPAT_2_12); > } > > DEFINE_Q35_MACHINE(v3_0, "pc-q35-3.0", NULL, > > > > > > > > > On the other hand, I believe you expect: > > > * "-machine pc -cpu EPYC -smp 8" to automatically enable topoext. > > Yes. Only on new machines-types > > > * "-machine pc -cpu Opteron_G1 -smp 8" to not enable topoext. > > Yes. > > > * What about "-machine -cpu Opteron_G1 -smp 8,threads=2"? > > No. This should not enable topoext. Topoext is not supported by > Opteron_G1. > > This should warn about hyperthreading and continue. > > OK, makes sense to me. > > > > > > > > > > We also have other requirements, I will try to enumerate all of > > > them below: > > > > > > 0) "-topoext" explicitly configured (any machine-type): > > > * Must never enable topoext. > > Yes. > > > > > > 1) "+topoext" explicitly configured (any machine-type): > > > * Must validate topology and refuse to start if unsupported. > > > > Yes. > > > > > > > > 2) Older machine-types: > > > * Must never enable topoext automatically, even if using "EPYC" > > > or "threads=2" > > > > > Yes. > > > > > 3) "EPYC" CPU model (on new machine-types): > > > * Should enable topoext automatically, but only if topology is > > > supported. > > > * Must not error out if topology is not supported. > > In new machine types we will enable topoext for "EPYC" CPU model. > > Right now(old machine type) we can disable for all the CPU models. > > So, we don't need two bits(topoext and auto-topoext) > > Right, so you agree that in this case we must _not_ error out if > topology is unsupported, correct? Otherwise we will break this > existing use case: > "-machine pc -cpu EPYC -smp 64". Ok. I will test this with new fix patch. > > > > > I thought we should error out if topology cannot be supported. But we can > warn(disable topoext) and continue that is another option. > > > > > * Should this enable topoext automatically even if threads=1? > > > > Yes. We should enable even with threads=1. > > > > > > > > 4) Other AMD CPU models with "threads=2" (on new machine-types): > > > * We might want to make this enable topoext automatically, too. > > > What do you think? > > > > No. We should not enable topoext here. We should depend on CPU > model table here. > > > > > > > > Is the above description accurate? Do you agree with these > > > requirements? > > > > With these requirements in mind, I will send that patches. We can start our > discussion. > > We don't need one more bits. That is my opinion. > > Thanks for confirming the requirements above. > > But it doesn't seem to be possible represent these requirements > with just one bit. Otherwise you can't differentiate explicit > "+topoext" (1 above) from topoext being implicitly enabled by > "-cpu EPYC" (3 above). > > Another problem is query-cpu-model-expansion QMP command: this > patch makes "topoext" appear on the output of > "query-cpu-model-expansion model=EPYC", meaning that management > software will assume everybody using the "EPYC" CPU model will > require +topoext. A separate "auto-topoext" property would avoid > this issue. Sure. Will work on it. Yes, I know all these combinations make it very tricky. > > (Yeah, this is tricky. I want to eventually encode these subtle > rules in automated test cases, so these issues could be detected > by software instead of code inspection.) > > -- > Eduardo
