Re: re-writing on powerpc

[Avi Kivity <avi@redhat.com>]

On 12/13/2010 10:42 AM, Alexander Graf wrote:
> On 13.12.2010, at 09:35, Avi Kivity wrote:
>
> >  On 12/13/2010 06:45 AM, Yoder Stuart-B08248 wrote:
> >>  Avi/Hollis,
> >>
> >>  Exchanged some emails with Alex on the topic of rewriting on
> >>  powerpc KVM-- the current approach taken by Alex's PV patch is
> >>  to have a guest Linux paravirt  itself, by re-writing certain
> >>  instructions.
> >>
> >>  The downside to this approach (guest side patching) is that every OS
> >>  to be run on KVM has to be modified or dynamically patched.
> >>
> >>  What were the reasons for not going down the path of doing the
> >>  re-writing in the hypervisor?  (Alex couldn't remember the
> >>  specifics).    What about doing it from Qemu?
> >>
> >
> >  Rewriting is dangerous if the guest is unaware of it.  As soon as it is made aware of it, it might as well actually do it in the best way that suits it.
>
> Yeah, let me rephrase my exact memory on this:
>
> If the HV just rewrites instructions in the guest, it behaves different from real hw which is bad. It could potentially break checksumming inside the guest.
>
> If, however, the guest sends a hypercall to the HV saying "please patch me" or there's a flag on creation time to enable patching, I have a hard time finding a reason to do it inside the guest context.
>
> Back when I implemented this, we did however have discussions on exactly that distinction between patching in host or guest space and for some reason I remember that you and Hollis figured that guest patching is superior. I just really can't remember why and couldn't find traces of this in my inbox either :).

The interface is a lot simpler.  The guest decides what to patch and 
where to jump.  A "please patch me" flag needs a ton of documentation on 
what patch means and what the constraints on the guest environment are.

-- 
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