Re: [Qemu-devel] [PATCH v5 3/3] x86: define a new MSR based feature word -- FEATURE_WORDS_ARCH_CAPABILITIES

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Oct 26, 2018 at 11:01:25AM +0800, Robert Hoo wrote:
> On Wed, 2018-10-24 at 07:06 -0300, Eduardo Habkost wrote:
> > On Mon, Oct 15, 2018 at 12:47:25PM +0800, Robert Hoo wrote:
> > > Note RSBA is specially treated -- no matter host support it or not,
> > > qemu
> > > pretends it is supported.
> > > 
> > > Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
> > 
> > I am now wondering what else we need to be able to remove
> > CPUID_7_0_EDX_ARCH_CAPABILITIES from
> > feature_word_info[FEAT_7_0_EDX].unmigratable_flags.
> > 
> > This series is necessary for that, be I think we still can't let
> > the VM be migrated if arch-capabilities is enabled and we're
> > running on a host that doesn't have MSR_IA32_ARCH_CAPABILITIES on
> > kvm_feature_msrs.
> > 
> > Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
> > 
> > > ---
> > >  target/i386/cpu.c | 31 ++++++++++++++++++++++++++++++-
> > >  target/i386/cpu.h |  8 ++++++++
> > >  target/i386/kvm.c | 11 +++++++++++
> > >  3 files changed, 49 insertions(+), 1 deletion(-)
> > > 
> [...]
> > >  
> > >  typedef struct X86RegisterInfo32 {
> > > @@ -3696,7 +3717,15 @@ static uint32_t
> > > x86_cpu_get_supported_feature_word(FeatureWord w,
> > >                                                          wi-
> > > >cpuid.reg);
> > >              break;
> > >          case MSR_FEATURE_WORD:
> > > -            r = kvm_arch_get_supported_msr_feature(kvm_state, wi-
> > > >msr.index);
> > > +            /* Special case:
> > > +             * No matter host status, IA32_ARCH_CAPABILITIES.RSBA
> > > [bit 2]
> > > +             * is always supported in guest.
> > > +             */
> > > +            if (wi->msr.index == MSR_IA32_ARCH_CAPABILITIES) {
> > > +                r = MSR_ARCH_CAP_RSBA;
> > > +            }
> > > +            r |= kvm_arch_get_supported_msr_feature(kvm_state,
> > > +                        wi->msr.index);
> > >              break;
> After I add the filtering out MSR feature, whose CPUID dependency fails
> , in x86_cpu_filter_features(), 1 issue comes out here: 
> 
> If running on an old platform that doesn't have ARCH_CAPABILITIES MSR,
> but we still pretends it here, then qemu will always print out
> "warning: host doesn't support requested feature: MSR(10AH).rsba [bit
> 2]", with -cpu 'host', which does not look comfortable.
> How about remove this hunk for now? leave it to when we fully decide
> how to handle ARCH_CAPABILITIES live-migration safely. 

I will remove that hunk in x86-next, thanks for noting!

-- 
Eduardo