@@ -61,10 +61,15 @@ mixture of host CPU models between machines, if live migration
compatibility is required, use the newest CPU model that is compatible
across all desired hosts.
-``Skylake-Server``, ``Skylake-Server-IBRS``
+``Cascadelake-Server``, ``Cascadelake-Server-noTSX``
+ Intel Xeon Processor (Cascade Lake, 2019), with "stepping" levels 6
+ or 7 only. (The Cascade Lake Xeon processor with *stepping 5 is
+ vulnerable to MDS variants*.)
+
+``Skylake-Server``, ``Skylake-Server-IBRS``, ``Skylake-Server-IBRS-noTSX``
Intel Xeon Processor (Skylake, 2016)
-``Skylake-Client``, ``Skylake-Client-IBRS``
+``Skylake-Client``, ``Skylake-Client-IBRS``, ``Skylake-Client-noTSX-IBRS}``
Intel Core Processor (Skylake, 2015)
``Broadwell``, ``Broadwell-IBRS``, ``Broadwell-noTSX``, ``Broadwell-noTSX-IBRS``
@@ -160,6 +165,54 @@ features are included if using "Host passthrough" or "Host model".
Requires the host CPU microcode to support this feature before it
can be used for guest CPUs.
+``mds-no``
+ Recommended to inform the guest OS that the host is *not* vulnerable
+ to any of the MDS variants ([MFBDS] CVE-2018-12130, [MLPDS]
+ CVE-2018-12127, [MSBDS] CVE-2018-12126).
+
+ This is an MSR (Model-Specific Register) feature rather than a CPUID feature,
+ so it will not appear in the Linux ``/proc/cpuinfo`` in the host or
+ guest. Instead, the host kernel uses it to populate the MDS
+ vulnerability file in ``sysfs``.
+
+ So it should only be enabled for VMs if the host reports @code{Not
+ affected} in the ``/sys/devices/system/cpu/vulnerabilities/mds`` file.
+
+``taa-no``
+ Recommended to inform that the guest that the host is ``not``
+ vulnerable to CVE-2019-11135, TSX Asynchronous Abort (TAA).
+
+ This too is an MSR feature, so it does not show up in the Linux
+ ``/proc/cpuinfo`` in the host or guest.
+
+ It should only be enabled for VMs if the host reports ``Not affected``
+ in the ``/sys/devices/system/cpu/vulnerabilities/tsx_async_abort``
+ file.
+
+``tsx-ctrl``
+ Recommended to inform the guest that it can disable the Intel TSX
+ (Transactional Synchronization Extensions) feature; or, if the
+ processor is vulnerable, use the Intel VERW instruction (a
+ processor-level instruction that performs checks on memory access) as
+ a mitigation for the TAA vulnerability. (For details, refer to
+ Intel's `deep dive into MDS
+ <https://software.intel.com/security-software-guidance/insights/deep-dive-intel-analysis-microarchitectural-data-sampling>`_.)
+
+ Expose this to the guest OS if and only if: (a) the host has TSX
+ enabled; *and* (b) the guest has ``rtm`` CPU flag enabled.
+
+ By disabling TSX, KVM-based guests can avoid paying the price of
+ mitigating TSX-based attacks.
+
+ Note that ``tsx-ctrl`` too is an MSR feature, so it does not show
+ up in the Linux ``/proc/cpuinfo`` in the host or guest.
+
+ To validate that Intel TSX is indeed disabled for the guest, there are
+ two ways: (a) check for the *absence* of ``rtm`` in the guest's
+ ``/proc/cpuinfo``; or (b) the
+ ``/sys/devices/system/cpu/vulnerabilities/tsx_async_abort`` file in
+ the guest should report ``Mitigation: TSX disabled``.
+
Preferred CPU models for AMD x86 hosts
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Add the '-noTSX' variants for CascadeLake and SkyLake. - Document the three MSR bits: 'mds-no', 'taa-no', and 'tsx-ctrl' Two confusing things about 'mds-no' (and the first point applies to the other two MSRs too): (1) The 'mds-no' bit 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 quote 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 document[+], as indicated by "No" under the three columns of MFBDS, MSBDS, and MLPDS. I've expressed this in the docs without belabouring the details. [+] 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> --- v4: - Fix two minor rST-related things [pbonzini] --- docs/system/qemu-cpu-models.rst | 57 +++++++++++++++++++++++++++++++-- 1 file changed, 55 insertions(+), 2 deletions(-)