On Mon, Jan 31, 2022 at 04:10:34PM -0300, Daniel Henrique Barboza wrote: > > > On 1/29/22 03:50, Nicholas Piggin wrote: > > The behaviour of the Address Translation Mode on Interrupt resource is > > not consistently supported by all CPU versions or all KVM versions. In > > particular KVM HV only supports mode 0 on POWER7 processors, and does > > not support mode 2 on any processors. KVM PR only supports mode 0. TCG > > can support all modes (0,2,3). > > > > This leads to inconsistencies in guest behaviour and could cause > > problems migrating guests. > > > > This was not too noticable for Linux guests for a long time because the > > kernel only used mode 0 or 3, and it used to consider AIL to be somewhat > > advisory (KVM would not always honor it either) and it kept both sets of > > interrupt vectors around. > > > > Recent Linux guests depend on the AIL mode working as defined by the ISA > > to support the SCV facility interrupt. If AIL mode 3 can not be provided, > > then Linux must be given an error so it can disable the SCV facility. > > Is this the scenario where migration failures can occur? I don't understand > what are the migration problems you cited that were possible to > happen.
The problem case (well, the main one) is migrating from qemu on a
recent KVM running with AIL==3 to qemu on an older KVM (or PR) where
AIL==3 doesn't work properly.
Theoretically, a qemu running with AIL==2 on TCG to a qemu running on
KVM is also a problem, though it's not going to arise in practice,
since AFAIK no guests we care about use AIL==2.
> > Add the ail-modes capability which is a bitmap of the supported values
> > for the H_SET_MODE Address Translation Mode on Interrupt resource. Add
> > a new KVM CAP that exports the same thing, and provide defaults for PR
> > and HV KVM that predate the cap.
>
> Why add a new machine cap in this case? Isn't something that the KVM
> capability
> should be able to handle by itself, where we always assume that we should have
> the best AIL value possible?
Because the "best AIL possible" might change across a migration.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
signature.asc
Description: PGP signature
