some typo fixes

[pve-docs.git] / qm.adoc

diff --git a/qm.adoc b/qm.adoc
index e0d789c0b3b249dda61600cc19802e532a5e6323..756178de527b4c4f9db7bbfaa7d6f2d186a67d01 100644 (file)
--- a/qm.adoc
+++ b/qm.adoc
@@ -245,8 +245,8 @@ Resource Limits
 In addition to the number of virtual cores, you can configure how much resources
 a VM can get in relation to the host CPU time and also in relation to other
 VMs.
-With the *cpulimit* (`Host CPU Time') option you can limit how much CPU time the
-whole VM can use on the host. It is a floating point value representing CPU
+With the *cpulimit* (``Host CPU Time'') option you can limit how much CPU time
+the whole VM can use on the host. It is a floating point value representing CPU
 time in percent, so `1.0` is equal to `100%`, `2.5` to `250%` and so on. If a
 single process would fully use one single core it would have `100%` CPU Time
 usage. If a VM with four cores utilizes all its cores fully it would
@@ -304,6 +304,34 @@ the kvm64 default. If you don’t care about live migration or have a homogeneou
 cluster where all nodes have the same CPU, set the CPU type to host, as in
 theory this will give your guests maximum performance.
 
+PCID Flag
+^^^^^^^^^
+
+The *PCID* CPU flag helps to improve performance of the Meltdown vulnerability
+footnote:[Meltdown Attack https://meltdownattack.com/] mitigation approach. In
+Linux the mitigation is called 'Kernel Page-Table Isolation (KPTI)', which
+effectively hides the Kernel memory from the user space, which, without PCID,
+is an expensive operation footnote:[PCID is now a critical performance/security
+feature on x86
+https://groups.google.com/forum/m/#!topic/mechanical-sympathy/L9mHTbeQLNU].
+
+There are two requirements to reduce the cost of the mitigation:
+
+* The host CPU must support PCID and propagate it to the guest's virtual CPU(s)
+* The guest Operating System must be updated to a version which mitigates the
+  attack and utilizes the PCID feature marked by its flag.
+
+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. If you use
+`host' as CPU type and the guest OS is able to use it, you're done.
+Otherwise you need to set the PCID CPU flag for the virtual CPU. This can be
+done by editing the CPU options through the WebUI.
+
 NUMA
 ^^^^
 You can also optionally emulate a *NUMA*