[pve-devel] [PATCH pve-docs] improve spectre/meltdown cpu flags documentation

[Alexandre Derumier]

-add new cpuflags for spectre v4.
-split cpuflags between amd && intel processors
---
 qm.adoc | 109 ++++++++++++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 86 insertions(+), 23 deletions(-)

diff --git a/qm.adoc b/qm.adoc
index 28d2a38..32e4fa6 100644
--- a/qm.adoc
+++ b/qm.adoc
@@ -307,56 +307,119 @@ theory this will give your guests maximum performance.
 Meltdown / Spectre related CPU flags
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-There are two CPU flags related to the Meltdown and Spectre vulnerabilities
+There are CPU flags related to the Meltdown and Spectre vulnerabilities
 footnote:[Meltdown Attack https://meltdownattack.com/] which need to be set
 manually unless the selected CPU type of your VM already enables them by default.
 
-The first, called 'pcid', helps to reduce the performance impact of the Meltdown
-mitigation called 'Kernel Page-Table Isolation (KPTI)', which effectively hides
-the Kernel memory from the user space. Without PCID, KPTI is quite an expensive
-mechanism footnote:[PCID is now a critical performance/security feature on x86
-https://groups.google.com/forum/m/#!topic/mechanical-sympathy/L9mHTbeQLNU].
-
-The second CPU flag is called 'spec-ctrl', which allows an operating system to
-selectively disable or restrict speculative execution in order to limit the
-ability of attackers to exploit the Spectre vulnerability.
-
-There are two requirements that need to be fulfilled in order to use these two
+There are two requirements that need to be fulfilled in order to use these
 CPU flags:
 
 * The host CPU(s) must support the feature and propagate it to the guest's virtual CPU(s)
 * The guest operating system must be updated to a version which mitigates the
   attacks and is able to utilize the CPU feature
 
-In order to use 'spec-ctrl', your CPU or system vendor also needs to provide a
+Otherwise you need to set the desired CPU flag of the virtual CPU, either by
+editing the CPU options in the WebUI, or by setting the 'flags' property of the
+'cpu' option in the VM configuration file.
+
+For Spectre v1,v2,v4 fixes, your CPU or system vendor also needs to provide a
 so-called ``microcode update'' footnote:[You can use `intel-microcode' /
 `amd-microcode' from Debian non-free if your vendor does not provide such an
 update. Note that not all affected CPUs can be updated to support spec-ctrl.]
 for your CPU.
 
-To check if the {pve} host supports PCID, execute the following command as root:
 
+Intel processors
+^^^^^^^^^^^^^^^^
+
+* 'pcid'
++
+helps to reduce the performance impact of the Meltdown (CVE-2017-5754) mitigation 
+called 'Kernel Page-Table Isolation (KPTI)', which effectively hides
+the Kernel memory from the user space. Without PCID, KPTI is quite an expensive
+mechanism footnote:[PCID is now a critical performance/security feature on x86
+https://groups.google.com/forum/m/#!topic/mechanical-sympathy/L9mHTbeQLNU].
++
+To check if the {pve} host supports PCID, execute the following command as root:
++
 ----
 # grep ' pcid ' /proc/cpuinfo
 ----
-
++
 If this does not return empty your host's CPU has support for 'pcid'.
 
+* 'spec-ctrl'
++
+Required to enable the Spectre v1 (CVE-2017-5753) and Spectre v2 (CVE-2017-5715) fix, 
+in cases where retpolines are not sufficient. 
+Included by default in Intel CPU models with -IBRS suffix. 
+Must be explicitly turned on for Intel CPU models without -IBRS suffix. 
+Requires the host CPU microcode (intel-microcode >= 20180425).
++
 To check if the {pve} host supports spec-ctrl, execute the following command as root:
++
+----
+# grep ' spectre_v1 ' /proc/cpuinfo
+# grep ' spectre_v2 ' /proc/cpuinfo
+----
++
+If this does not return empty your host's CPU has support for 'spec-ctrl'.
+
 
+* 'ssbd'
++
+Required to enable the Spectre V4 (CVE-2018-3639) fix. Not included by default in any Intel CPU model. 
+Must be explicitly turned on for all Intel CPU models. 
+Requires the host CPU microcode(intel-microcode >= 20180703).
++
+To check if the {pve} host supports spec-ctrl, execute the following command as root:
++
 ----
-# grep ' spec_ctrl ' /proc/cpuinfo
+# grep ' spec_store_bypass ' /proc/cpuinfo
 ----
++
+If this does not return empty your host's CPU has support for 'ssbd'.
 
-If this does not return empty your host's CPU has support for 'spec-ctrl'.
 
-If you use `host' or another CPU type which enables the desired flags by
-default, and you updated your guest OS to make use of the associated CPU
-features, you're already set.
+AMD processors
+^^^^^^^^^^^^^^
+
+* 'ibpb'
++
+Required to enable the Spectre v1 (CVE-2017-5753) and Spectre v2 (CVE-2017-5715) fix, 
+in cases where retpolines are not sufficient. 
+Included by default in AMD CPU models with -IBPB suffix. 
+Must be explicitly turned on for AMD CPU models without -IBPB suffix. 
+Requires the host CPU microcode to support this feature before it can be used for guest CPUs.
+
+
+
+* 'virt-ssbd'
++
+Required to enable the Spectre v4 (CVE-2018-3639) fix.
+Not included by default in any AMD CPU model. 
+Must be explicitly turned on for all AMD CPU models. 
+This should be provided to guests, even if amd-ssbd is also provided, for maximum guest compatibility. 
+Note for some QEMU / libvirt versions, this must be force enabled when when using "Host model", 
+because this is a virtual feature that doesn’t exist in the physical host CPUs.
+
+
+* 'amd-ssbd'
++
+Required to enable the Spectre v4 (CVE-2018-3639) fix. 
+Not included by default in any AMD CPU model. Must be explicitly turned on for all AMD CPU models. 
+This provides higher performance than virt-ssbd so should be exposed to guests whenever available in the host. 
+virt-ssbd should none the less also be exposed for maximum guest compatibility as some kernels only know about virt-ssbd.
+
+
+* 'amd-no-ssb'
++
+Recommended to indicate the host is not vulnerable to Spectre V4 (CVE-2018-3639).
+Not included by default in any AMD CPU model. 
+Future hardware generations of CPU will not be vulnerable to CVE-2018-3639, 
+and thus the guest should be told not to enable its mitigations, by exposing amd-no-ssb. 
+This is mutually exclusive with virt-ssbd and amd-ssbd.
 
-Otherwise you need to set the desired CPU flag of the virtual CPU, either by
-editing the CPU options in the WebUI, or by setting the 'flags' property of the
-'cpu' option in the VM configuration file.
 
 NUMA
 ^^^^
-- 
2.11.0