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
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