[Qemu-devel] tsc=reliable in Linux guests

[Richard W.M. Jones]

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?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
http://libguestfs.org/virt-builder.1.html