[MODERATED] Re: [PATCH 2/3] V5 more sampling fun 2

[mark gross <mgross@linux.intel.com>]

On Wed, Apr 08, 2020 at 03:14:19PM -0700, speck for mark gross wrote:
> On Mon, Apr 06, 2020 at 05:07:14PM -0500, speck for Josh Poimboeuf wrote:
> > On Thu, Jan 16, 2020 at 02:16:07PM -0800, speck for mark gross wrote:
> > > From: mark gross <mgross@linux.intel.com>
> > > Subject: [PATCH 2/3] x86/speculation: Special Register Buffer Data Sampling
> > >  (SRBDS) mitigation control.
> > > +enum srbds_mitigations {
> > > +	SRBDS_MITIGATION_OFF,
> > > +	SRBDS_MITIGATION_UCODE_NEEDED,
> > > +	SRBDS_MITIGATION_FULL,
> > > +	SRBDS_NOT_AFFECTED_TSX_OFF,
> > > +	SRBDS_HYPERVISOR,
> > 
> > These should all be prefixed with "SRBDS_MITIGATION_".
> > 
> > > +};
> > > +
> > > +enum srbds_mitigations srbds_mitigation __ro_after_init = SRBDS_MITIGATION_FULL;
> > 
> > This can be static.
> > 
> > > +static const char * const srbds_strings[] = {
> > > +	[SRBDS_MITIGATION_OFF]		= "Vulnerable",
> > > +	[SRBDS_MITIGATION_UCODE_NEEDED]	= "Vulnerable: no microcode",
> > 
> > "no microcode" should be capitalized:
> > 
> >   "Vulnerable: No microcode"
> > 
> > > +	[SRBDS_MITIGATION_FULL]		= "Mitigated",
> > 
> > All the other mitigations say "Mitigation: <description of mitigation>".
> > 
> > So to be consistent, and to not break dumb scripts which might rely on
> > that, how about:
> > 
> >   "Mitigation: Microcode"
> > 
> > > +	[SRBDS_NOT_AFFECTED_TSX_OFF]	= "Not affected (TSX disabled)",
> > 
> > Is this actually two distinct states?
> > 
> > 1) MDS_NO parts which support TSX, where the user has disabled TSX.  In
> >    which case it should say:
> > 
> >    "Mitigation: TSX disabled"
> > 
> > 2) MDS_NO CPUs which *don't* support TSX, which should say:
> > 
> >   "Not affected"
> > 
> >   (I presume the bug bit doesn't need to be set in this case)
> I've been working addressing this issue.  It is common for Intel to fuse off
> features on low end SKU's so this could happen.  After looking into how to
> disambiguate a CFL part (Family 6, Model 158 stepping 13) with TSX fused off
> from a CLF part where TSX is disabled using TSX_CNTRL MSR.  I'd like to side
> step the problem.
> 
> The solution is code for common.c that I have a hard time parsing even though I
> wrote it.  It makes my eyes bleed.  I wonder if it would be better to simply
> reword the string used for sysfs in the SRBDS_NOT_AFFECTED_TSX_OFF case to
> report simply "Not affected" i.e. drop the "(TSX disbled)" part to hide the
> ambiguous case?
> 
> FWIW this is what I'm thinking of adding to common.c if we choose to not hide
> this ambiguity.  We are checking with the uCode folks to confirm but
> ARCH_CAP_TSX_CTRL_MSR will not be set if the part has TSX fused off and we
> think we can use that to disambiguate:
> 
> 1166        if (cpu_matches(SRBDS, cpu_vuln_blacklist)) {
> 1167               /*
> 1168                * Some low-end SKUs on the affected list do not support
> 1169                * RDRAND or RDSEED. Make sure they show as "Not affected".
> 1170                */
> 1171               if (cpu_has(c, X86_FEATURE_RDRAND) ||
> 1172                   cpu_has(c, X86_FEATURE_RDSEED)) {
> 1173                      /* Some low end SKU's of parts with F/M/S in the
> 1174                       * blacklist that normally are only affected if TSX is
> 1175                       * enabled could have TSX fused off.  To avoid reporting 
> 1176                       * "Not affected (TSX disabled)" check !MDS_NO or available
> 1177                       * TSX_CTRL_MSR as a check for parts not fused
> 1178                       * off.
> 1179                       */
> 1180                      if (!(ia32_cap & ARCH_CAP_MDS_NO) ||.
> 1181                          (ia32_cap & ARCH_CAP_TSX_CTRL_MSR))
> 1182                             setup_force_cpu_bug(X86_BUG_SRBDS);
> 1183 
> 1184               }
> 1185        }
> 
> Another option is to add back the SRBDS_MITIGATION_NOT_AFFECTED and do this TSX
> fused off check of ARCH_CAP_TSX_CTRL_MSR in bugs.c when checking for
> X86_FEATURE_RTM.
> 
> What do you or others think?
Never mind.  Tony gave me some ideas to make it easier to read.  I'll post the
updated patchset Late Thursday after some testing.

thanks,

--mark