Re: [Qemu-devel] [patch 2/2] target-i386: block migration and savevm if invariant tsc is exposed

[Eduardo Habkost]

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.

Now we can't do both at the same time[1].

(1) is important for management software;
(2) works only if you are lucky.

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.

-- 
Eduardo