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

[Kashyap Chamarthy <kchamart@redhat.com>]

- Add the -noTSX variants for CascadeLake and SkyLake.

- Add a note aboute the 'mds-no' MSR.  Two confusing things about this:

  (1) The 'mds-no' will _not_ show up in the guest's /proc/cpuinfo.
      Rather it is used to fill in the guest's sysfs:

        sys/devices/system/cpu/vulnerabilities/mds:Not affected

      Paolo confirmed on IRC as such.

  (2) There are _three_ variants[+] of CascadeLake CPUs, with different
      stepping levels: 5, 6, and 7.  To quite wikichip.org[*]:

        "note that while steppings 6 & 7 are fully mitigated, earlier
        stepping 5 is not protected against MSBDS, MLPDS, nor MDSUM"

      The above is also indicated in the Intel's manual itself[+], as
      indicated by "No" under the three columns of MFBDS, MSBDS, and
      MLPDS.

      [+] https://software.intel.com/security-software-guidance/insights/processors-affected-microarchitectural-data-sampling
      [*] https://en.wikichip.org/wiki/intel/microarchitectures/cascade_lake#Key_changes_from_Skylake

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
---
TODO:
 - I think I also need to add a note about 'tsx-ctrl' bit.  Here too,
   same question as above -- does it show up in /proc/cpuinfo/?
---
 docs/qemu-cpu-models.texi | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/docs/qemu-cpu-models.texi b/docs/qemu-cpu-models.texi
index f88a1def0d042cc25213259172a648f0a9c514dc..e6c4058e764a81988d6bc97457c668cb0ad2ea37 100644
--- a/docs/qemu-cpu-models.texi
+++ b/docs/qemu-cpu-models.texi
@@ -72,14 +72,30 @@ between machines, if live migration compatibility is required, use the newest
 CPU model that is compatible across all desired hosts.
 
 @table @option
+
+@item @code{Cascadelake-Server-noTSX}
+
+Intell Xeon Processor (Cascade Lake, 2019-2020), with "stepping" levels
+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}
+
+
 @item @code{Skylake-Server}
 @item @code{Skylake-Server-IBRS}
+@item @code{Skylake-Server-noTSX-IBRS}
 
 Intel Xeon Processor (Skylake, 2016)
 
 
 @item @code{Skylake-Client}
 @item @code{Skylake-Client-IBRS}
+@item @code{Skylake-Client-noTSX-IBRS}
 
 Intel Core Processor (Skylake, 2015)
 
@@ -214,9 +230,28 @@ Must be explicitly turned on for all Intel CPU models.
 
 Requires the host CPU microcode to support this feature before it
 can be used for guest CPUs.
+
+@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.
+
 @end table
 
-
 @node preferred_cpu_models_amd_x86
 @subsubsection Preferred CPU models for AMD x86 hosts
 
-- 
2.21.0