Re: [PATCH] docs: qemu-cpu-models: Document '-noTSX' variants and 'mds-no'

[Kashyap Chamarthy]

On Tue, Jan 21, 2020 at 04:45:08PM +0000, Daniel P. Berrangé wrote:
> On Thu, Jan 16, 2020 at 06:36:38PM +0100, Kashyap Chamarthy wrote:

[...]

> >  @table @option
> > +
> > +@item @code{Cascadelake-Server-noTSX}
> 
> Also needs
> 
>    @item @code{Cascadelake-Server}

Will add.

> > +
> > +Intell Xeon Processor (Cascade Lake, 2019-2020), with "stepping" levels
> 
> s/Intell/Intel/
> 
> s/-2020//  as we only need the initial year of introduction IMHO.

Will do.  (I wasn't sure, hence I put both years :-))

> > +6 or 7 only.  (The Cascade Lake Xeon processor with @b{stepping 5 is
> > +vulnerable to MDS variants}; refer below.)
> > +
> > +@code{/proc/cpuinfo}.
> > +
> > +The @code{mds-no} bit does not show up under @code{/proc/cpuinfo}.
> > +Rather it shows up under the @code{sysfs}, as
> > +@code{/sys/devices/system/cpu/vulnerabilities/mds:Not affected}
> 
> We already talk about this later on we I thin kwe can trim the
> /proc/cpinfo bit

True, will remove this redundancy.

[...]

> > +@item @code{mds-no}
> > +
> > +This is an MSR (Model-Specific Register) used by QEMU to indicate that
> > +the host is @i{not} vulnerable to any of the MDS variants ([MFBDS]
> > +CVE-2018-12130, [MLPDS] CVE-2018-12127, [MSBDS] CVE-2018-12126).
> > +
> > +Note that there are @i{three} versions of Intel's Cascade Lake
> > +processor, as distinguished by their "stepping" levels 5, 6, and 7.  The
> > +CPU with stepping "5" is @b{vulnerable to MDS variants}; and the CPUs
> > +with steppings "6" and "7" are @b{not vulnerable} to the above mentioned
> > +MDS variants.  The processor "stepping" is reported in
> > +@code{/proc/cpuinfo}.
> > +
> > +Confusingly, the @code{mds-no} bit does not show up under
> > +@code{/proc/cpuinfo} inside the guest.  Rather the kernel uses it to
> > +fill in the @code{sysfs}, as
> > +@code{/sys/devices/system/cpu/vulnerabilities/mds: Not affected},
> > +assuming the processor is running with stepping 6 or 7.
> 
> I think we can simplify this a little - we don't need to talk
> about CPU steppings - the user simply needs to know whether the
> sysfs file reports vulnerable or not.
> 
> So perhaps this text:
> 
>   Recommended to inform the guest OS that the host is @i{not]
>   vulnerable to any of the MDS variants ([MFBDS]
>   CVE-2018-12130, [MLPDS] CVE-2018-12127, [MSBDS] CVE-2018-12126).
> 
>   This is a MSR feature rather than a CPUID feature, so will not
>   appear in the Linux @code{/proc/cpuinfo} in the host or guest.
> 
>   It should only be enabled for VMs if the host reports
>   @code{Not affected} in the
>   @code{/sys/devices/system/cpu/vulnerabilities/mds} file.

Your phrasing is indeed simpler and more to-the-point; will incorporate
it.

I'll also add similar sections about the other two MSRs: 'taa-no' and
'tsx-ctrl' (as mentioned here:
https://lists.nongnu.org/archive/html/qemu-devel/2020-01/msg03685.html).

Thanks for the careful review.

-- 
/kashyap