Bug 961612

Summary:	there is no -redhat-disable-KSM introduction in man page
Product:	Red Hat Enterprise Linux 7	Reporter:	Sibiao Luo <sluo>
Component:	qemu-kvm	Assignee:	Andrea Arcangeli <aarcange>
Status:	CLOSED NOTABUG	QA Contact:	Yumei Huang <yuhuang>
Severity:	low	Docs Contact:
Priority:	low
Version:	7.0	CC:	aarcange, chayang, hhuang, jinzhao, juzhang, knoel, qzhang, virt-maint, xfu, yuhuang
Target Milestone:	rc
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Last Closed:	2019-12-06 13:21:30 UTC	Type:	Bug
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
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Description Sibiao Luo 2013-05-10 05:24:48 UTC

Description of problem:
We have fixed this issue in rhel6(bug 719269), but did not backport the code to rhel7.

Version-Release number of selected component (if applicable):
host info:
3.9.0-0.55.el7.x86_64
qemu-kvm-1.4.0-4.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.# man qemu-kvm and search redhat-disable-KSM.
or # /usr/libexec/qemu-kvm --help | grep -i ksm
2.
3.
  
Actual results:
no item about redhat-disable-KSM found

Expected results:
--redhat-disable-KSM item should be listed in the man page, like:
...
-redhat-disable-KSM
    Disable KSM on the virtual RAM.
...
# /usr/libexec/qemu-kvm --help | grep -i ksm
-redhat-disable-KSM  disable KSM on guest physical memory

Additional info:

Comment 1 Andrea Arcangeli 2014-02-25 18:48:11 UTC

In RHEL7 the --redhat-disable-KSM command line doesn't exist because it was obsoleted, it wasn't just the entry missing in the manpage.

Instead there is a mem-merge command line given to the machine type that you should find in the documentation too:

-machine mem=merge=on|off

           mem-merge=on|off
               Enables or disables memory merge support. This feature, when  upported by the host, de-
               duplicates identical memory pages among VMs instances (enabled by default).


So the manager software should start the VM with -machine mem-merge=off if the host is a RHEL7 and it wants to disable KSM.

Unfortunately specifying the KSM on or off option as part of the machine type doesn't make sense because KSM on or off cannot affect live migration and is totally invisible to the guest and independent of the actual machine topology lawyout emulated hardware or anything related to the machine type. But this is the upstream API so we need to live with it and embed this to the machine type. I learnt about this mistake once it was already committed upstream so it's not worth fixing now.