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[Qemu-devel] tsc=reliable in Linux guests

From: Richard W.M. Jones
Subject: [Qemu-devel] tsc=reliable in Linux guests
Date: Fri, 13 May 2016 11:15:21 +0100
User-agent: Mutt/1.5.20 (2009-12-10)

I was wondering why the Linux kernel was spending 21ms starting up
each secondary CPU when we boot it virtualized with qemu -smp >= 2.

It turns out the time was lost in check_tsc_sync_target().

This is easily avoided by using `tsc=reliable' on the command line,
saving 63ms when starting a guest with -smp 4 for example.

Is this option safe?  If so, why don't we enable it automatically in
the kernel when we detect that we are a KVM guest?


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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