Re: [Qemu-devel] [PATCH v2 2/5] i386: Add CPUID bit and feature words for IA32_ARCH_CAPABILITIES MSR

[Robert Hoo <robert.hu@linux.intel.com>]

On Thu, 2018-06-28 at 15:28 -0300, Eduardo Habkost wrote:
> On Wed, Jun 27, 2018 at 07:27:21PM +0800, Robert Hoo wrote:
> > Support of IA32_PRED_CMD MSR already be enumerated by same CPUID bit as
> > SPEC_CTRL.
> > 
> > Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
> 
> Based on kernel commit 1eaafe91, it looks like we must always set
> IA32_ARCH_CAPABILITIES.RSBA[bit 2] unless we're really sure the
> VM will not be migrated to a vulnerable processor.
> 
> Considering this, I'd like to make "+arch-capabilities" set
> IA32_ARCH_CAPABILITIES.RSBA by default, unless RSBA is explicitly
> disabled by management software.
> 
Agree. But this seems beyond Icelake CPU model scope. How about I think
about this carefully and compose another patch (set) for this?
And you'd like to set  IA32_ARCH_CAPABILITIES.RSBA by default in qemu or
kvm layer?
> > ---
> >  target/i386/cpu.c | 2 +-
> >  target/i386/cpu.h | 1 +
> >  2 files changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index e6c2f8a..953098c 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -1002,7 +1002,7 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
> >              NULL, NULL, NULL, NULL,
> >              NULL, NULL, NULL, NULL,
> >              NULL, NULL, "spec-ctrl", NULL,
> > -            NULL, NULL, NULL, "ssbd",
> > +            NULL, "arch-capabilities", NULL, "ssbd",
> >          },
> >          .cpuid_eax = 7,
> >          .cpuid_needs_ecx = true, .cpuid_ecx = 0,
> > diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> > index 734a73e..1ef2040 100644
> > --- a/target/i386/cpu.h
> > +++ b/target/i386/cpu.h
> > @@ -688,6 +688,7 @@ typedef uint32_t FeatureWordArray[FEATURE_WORDS];
> >  #define CPUID_7_0_EDX_AVX512_4VNNIW (1U << 2) /* AVX512 Neural Network Instructions */
> >  #define CPUID_7_0_EDX_AVX512_4FMAPS (1U << 3) /* AVX512 Multiply Accumulation Single Precision */
> >  #define CPUID_7_0_EDX_SPEC_CTRL     (1U << 26) /* Speculation Control */
> > +#define CPUID_7_0_EDX_ARCH_CAPABILITIES (1U << 29)  /*Arch Capabilities of RDCL_NO and IBRS_ALL*/
> >  #define CPUID_7_0_EDX_SPEC_CTRL_SSBD  (1U << 31) /* Speculative Store Bypass Disable */
> >  
> >  #define CPUID_8000_0008_EBX_IBPB    (1U << 12) /* Indirect Branch Prediction Barrier */
> > -- 
> > 1.8.3.1
> > 
> > 
>