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

From: Kashyap Chamarthy
Subject: [PATCH] docs: qemu-cpu-models: Document '-noTSX' variants and 'mds-no'
Date: Thu, 16 Jan 2020 18:36:38 +0100

- 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


Signed-off-by: Kashyap Chamarthy <address@hidden>
 - 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
--- 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.)
+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
+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

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