Il 24/04/2014 22:57, Eduardo Habkost ha scritto:
On Thu, Apr 24, 2014 at 04:42:33PM -0400, Paolo Bonzini wrote:
Il 22/04/2014 21:14, Eduardo Habkost ha scritto:
Not for "-cpu host". If somebody needs migration to work, they shouldn't
be using "-cpu host" anyway (I don't know if you have seen the other
comments in my message?).
I'm not entirely sure. If you have hosts with exactly identical
chipsets, "-cpu host" migration will in all likelihood work.
Marcelo's approach is safer.
If that didn't break other use cases, I would agree.
But "-cpu host" today covers two use cases: 1) enabling everything that
can be enabled, even if it breaks migration; 2) enabling all stuff that
can be safely enabled without breaking migration.
What does it enable *now* that breaks migration?
Now we can't do both at the same time[1].
(1) is important for management software;
(2) works only if you are lucky.
Or if you plan ahead. With additional logic even invariant TSC in
principle can be made to work across migration if the host clocks are
synchronized well enough (PTP accuracy is in the 100-1000 TSC ticks range).
Why would it make sense to break (1) to try make (2) work?
[1] I would even argue that we never did both at the same time."-cpu
host" depends on host hardware capabilities, host kernel capabilities,
and host QEMU version (we never took care of keeping guest ABI with
"-cpu host"). If migration did work, it was never supposed to.
I think this is where I disagree. Migration of the PMU is one thing
that obviously was done with "-cpu host" in mind.
Paolo
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