[AMD Official Use Only - Internal Distribution Only] > -----Original Message----- > From: Eduardo Habkost <ehabk...@redhat.com> > Sent: Wednesday, February 5, 2020 4:38 PM > To: Ani Sinha <ani.si...@nutanix.com> > Cc: Paolo Bonzini <pbonz...@redhat.com>; r...@twiddle.net; qemu- > de...@nongnu.org; Singh, Brijesh <brijesh.si...@amd.com>; Moger, Babu > <babu.mo...@amd.com> > Subject: Re: [PATCH] i386: pass CLZERO to guests with EPYC CPU model on > AMD ZEN platform > > Hi, > > Sorry for the delayed reply. I was away from work for the whole > month of January. > > On Mon, Jan 20, 2020 at 10:56:43AM +0000, Ani Sinha wrote: > > Sorry Eduardo, it took a little while for me to get to this thread again. > > > > > On Dec 18, 2019, at 8:41 PM, Eduardo Habkost <ehabk...@redhat.com> > wrote: > > > > > > On Wed, Dec 18, 2019 at 12:53:45PM +0100, Paolo Bonzini wrote: > > >> On 18/12/19 10:05, Ani Sinha wrote: > > >>> CLZERO CPUID should be passed on to the guests that use EPYC or > EPYC-IBPB CPU > > >>> model when the AMD ZEN based host supports it. This change makes it > recognize > > >>> this CPUID for guests which use EPYC or EPYC-IBPB CPU model. > > > > > > Can you clarify what's the intended use case here? Why the > > > "if host supports it" conditional? > > > > Looking at > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fww > w.amd.com%2Fsystem%2Ffiles%2FTechDocs%2F24594.pdf&data=02%7 > C01%7Cbabu.moger%40amd.com%7C4d00819020cb4892d50608d7aa8c016b% > 7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637165391335697450 > &sdata=3%2FFGZeeZoz387eeayAbjQJj9qGhemgw0xC0byr%2F0YJk%3D > &reserved=0 , it says : > > > > "The CLZERO instruction is supported if the feature flag CPUID > Fn8000_0008_EBX[CLZERO] is set.” > > > > This I interpreted to mean that not all AMD Zen architectures > > supports it. So when the host does support it, this CPUID > > should be passed on to the guest as well. > > This is not a supported use case of named CPU models. Named CPU > models should expose the same guest ABI on all hosts. This means > CPUID should be the same on all hosts if using the same CPU > model (and same machine type). > > If you need features to be automatically enabled/disabled > depending on host capabilities, I advise you to use "-cpu host" > or libvirt's mode=host-model. > > > > > > > > > > > If you need host-dependent CPU configuration, "-cpu host" (or the > > > libvirt "host-model" mode) is the most appropriate solution. > > > > Yes that is an option but we are going to use EPYC-IBPB model for now. > > > > > > > > > >>> > > >>> Signed-off-by: Ani Sinha <ani.si...@nutanix.com> > > >>> --- > > >>> target/i386/cpu.c | 2 ++ > > >>> 1 file changed, 2 insertions(+) > > >>> > > >>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c > > >>> index 69f518a..55f0691 100644 > > >>> --- a/target/i386/cpu.c > > >>> +++ b/target/i386/cpu.c > > >>> @@ -3813,6 +3813,8 @@ static X86CPUDefinition builtin_x86_defs[] = { > > >>> CPUID_EXT3_MISALIGNSSE | CPUID_EXT3_SSE4A | > CPUID_EXT3_ABM | > > >>> CPUID_EXT3_CR8LEG | CPUID_EXT3_SVM | > CPUID_EXT3_LAHF_LM | > > >>> CPUID_EXT3_TOPOEXT, > > >>> + .features[FEAT_8000_0008_EBX] = > > >>> + CPUID_8000_0008_EBX_CLZERO, > > >>> .features[FEAT_7_0_EBX] = > > >>> CPUID_7_0_EBX_FSGSBASE | CPUID_7_0_EBX_BMI1 | > CPUID_7_0_EBX_AVX2 | > > >>> CPUID_7_0_EBX_SMEP | CPUID_7_0_EBX_BMI2 | > CPUID_7_0_EBX_RDSEED | > > >>> > > >> > > >> This needs to be done only for newer machine type (or is it CPU model > > >> versions now? need Eduardo to respond). > > > > > > If we want to add it, it has to be done as a new CPU model version. > > > > I see what you mean. > > > > > > > > But I don't know yet if we want to add it. Do all EPYC CPUs have > > > CLZERO available? If not, it's probably not advisable to add it > > > to EPYC (even if it's just on EPYC-v3). > > > > Ok so I think we need to get this clarified from AMD if all > > their EPYC platforms supports this CPUID or not. Is there any > > contact point within AMD where we can get this information? > > I'm CCing Brijesh Singh and Babu Moger, who works on the EPYC CPU > model recently.
Ani, I am already working on it. Eduardo, I am still waiting for your feedback on this series. https://lore.kernel.org/qemu-devel/abd39b75-0a12-5198-5815-dd51a3d5c...@amd.com/ I have added all the missing feature bits for EPYC models(as EPYC-v3) and also added EPYC-Rome model. > > > > > For our use case, I just verified that even without this patch, > > if we pass CLZERO through libvirt CPU definition xml, like " > > <feature policy='require' name='clzero’/>”, it gets passed on > > to the guest. So this patch is not super critical for us. > > Note that this is done automatically by libvirt if using > mode=host-model. > > -- > Eduardo
