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 :). Alex -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
