> -----Original Message----- > From: kvm-owner@xxxxxxxxxxxxxxx [mailto:kvm-owner@xxxxxxxxxxxxxxx] > On Behalf Of Babu Moger > Sent: Tuesday, June 12, 2018 2:47 PM > To: Eduardo Habkost <ehabkost@xxxxxxxxxx> > Cc: mst@xxxxxxxxxx; marcel.apfelbaum@xxxxxxxxx; pbonzini@xxxxxxxxxx; > rth@xxxxxxxxxxx; mtosatti@xxxxxxxxxx; qemu-devel@xxxxxxxxxx; > kvm@xxxxxxxxxxxxxxx; kash@xxxxxxxxxxxxxx; geoff@xxxxxxxxxxxxxxx; Jiri > Denemark <jdenemar@xxxxxxxxxx> > Subject: Re: [PATCH v13 3/5] i386: Enable TOPOEXT feature on AMD EPYC > CPU > > > > On 06/12/2018 02:05 PM, Eduardo Habkost wrote: > > On Tue, Jun 12, 2018 at 06:38:08PM +0000, Moger, Babu wrote: > > [...] > >>> I'm starting to think that enabling TOPOEXT automatically is > >>> adding too much complexity and compatibility problems, and it's > >>> better to leave this task to management software. > >>> > >>> The main problem here is: > >>> > >>> This works today with QEMU 2.12 + Linux <= 4.15: > >>> $ $QEMU -machine pc -cpu EPYC,enforce -smp > >>> 8,sockets=2,cores=2,threads=2" > >>> and must keep working with QEMU 3.0 and Linux <= 4.15. > >>> > >>> In addition to that, the results for: > >>> $ $QEMU -machine pc-q35-3.0 -cpu EPYC,enforce [...] > >>> must be deterministic and expose exactly the same CPUID data even > >>> if host hardware or software changes, as long as the QEMU > >>> command-line is the same. > >>> > >>> Do you see a way to fulfill those two constraints while making > >>> "-machine pc-q35-3.0 -cpu EPYC" enable TOPOEXT automatically? > >>> > >> Now(setting feature before x86_cpu_expand_features), enabling > >> TOPOEXT appears to work fine. > > What about the above constraints? Are you really fulfilling > > them? > > > > This one is tricky: > > > > ] This works today with QEMU 2.12 + Linux <= 4.15: > > ] $ $QEMU -machine pc -cpu EPYC,enforce -smp > 8,sockets=2,cores=2,threads=2" > > ] and must keep working with QEMU 3.0 and Linux <= 4.15. > This works fine on kernel <= 4.15 with some warnings(-smp > 8,sockets=2,cores=2,threads=2 -cpu EPYC). > > qemu-system-x86_64: warning: host doesn't support requested feature: > CPUID.80000001H:EDX.rdtscp [bit 27] > qemu-system-x86_64: warning: host doesn't support requested feature: > CPUID.80000001H:ECX.topoext [bit 22] > qemu-system-x86_64: This family of AMD CPU doesn't support > hyperthreading(2). Please configure -smp options properly or try > enabling topoext feature. > continues.. > > > > If we enable TOPOEXT unconditionally, the command-line won't work > > with Linux <= 4.15. > > > > If we enable TOPOEXT only if the kernel returns TOPOEXT on > > GET_SUPPORTED_CPUID, we break the second constraint: > > > > ] The results for: > > ] $ $QEMU -machine pc-q35-3.0 -cpu EPYC,enforce [...] > > ] must be deterministic and expose exactly the same CPUID data even > > ] if host hardware or software changes, as long as the QEMU > > ] command-line is the same. > > > This fails on kernel <= 4.15 with following messages((-smp > 8,sockets=2,cores=2,threads=2 -cpu EPYC,enforce). > qemu-system-x86_64: warning: host doesn't support requested feature: > CPUID.80000001H:EDX.rdtscp [bit 27] > qemu-system-x86_64: warning: host doesn't support requested feature: > CPUID.80000001H:ECX.topoext [bit 22] > qemu-system-x86_64: Host doesn't support requested features > exits.. > > What do you think our options are here? Should we drop automatic topoext completely and move forward? What are your thoughts?
