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Re: [Qemu-devel] [PATCH 3/5] x86: fill high bits of mtrr mask

From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH 3/5] x86: fill high bits of mtrr mask
Date: Fri, 17 Jun 2016 12:27:37 -0300
User-agent: Mutt/1.6.1 (2016-04-27)

On Fri, Jun 17, 2016 at 04:25:57PM +0200, Paolo Bonzini wrote:
> On 17/06/2016 15:41, Eduardo Habkost wrote:
> > In theory, we should never initialize anything on the machine
> > based on the host we are running. In practice we sometimes do
> > that, and we know it's unsafe. Sending the value on the migration
> > stream is a solution to detect when this breaks something. We did
> > that before, for TSC frequency.
> > 
> > I prefer to add a little extra code, than to waste time debugging
> > when that stuff breaks.
> Fair enough (but still let's add and strip the 1s in pre_save and
> post_load).  It is a good justification for sending configuration over
> the wire, that we initialize things based on the host we're running.
> But we shouldn't initialize more things based on the host (e.g. new
> booleans should be "enforce"-style).

Strongly agreed.

> Also I don't really like
> introducing sanity checks for those that aren't based on the host (as in
> the LMCE thread).  This code will never trigger in practice, so it's
> just extra cost for no benefit.

Our experience with guest ABI and compatibility code tells me
otherwise. Keeping guest ABI is hard, so I would always support
adding extra code to help us catch mistakes (either in QEMU,
management software, or user mistakes).


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