On Tue, Jun 07, 2022, Paolo Bonzini wrote: > On 6/6/22 23:27, Peter Xu wrote: > > On Mon, Jun 06, 2022 at 06:18:12PM +0200, Paolo Bonzini wrote: > > > > However there seems to be something missing at least to me, on why it'll > > > > fail a migration from 5.15 (without this patch) to 5.18 (with this patch). > > > > In my test case, user_xfeatures will be 0x7 (FP|SSE|YMM) if without this > > > > patch, but 0x0 if with it. > > > > > > What CPU model are you using for the VM? > > > > I didn't specify it, assuming it's qemu64 with no extra parameters. > > Ok, so indeed it lacks AVX and this patch can have an effect. > > > > For example, if the source lacks this patch but the destination has it, > > > the source will transmit YMM registers, but the destination will fail to > > > set them if they are not available for the selected CPU model. > > > > > > See the commit message: "As a bonus, it will also fail if userspace tries to > > > set fpu features (with the KVM_SET_XSAVE ioctl) that are not compatible to > > > the guest configuration. Such features will never be returned by > > > KVM_GET_XSAVE or KVM_GET_XSAVE2." > > > > IIUC you meant we should have failed KVM_SET_XSAVE when they're not aligned > > (probably by failing validate_user_xstate_header when checking against the > > user_xfeatures on dest host). But that's probably not my case, because here > > KVM_SET_XSAVE succeeded, it's just that the guest gets a double fault after > > the precopy migration completes (or for postcopy when the switchover is > > done). > > Difficult to say what's happening without seeing at least the guest code > around the double fault (above you said "fail a migration" and I thought > that was a different scenario than the double fault), and possibly which was > the first exception that contributed to the double fault. Regardless of why the guest explodes in the way it does, is someone planning on bisecting this (if necessary?) and sending a backport to v5.15? There's another bug report that is more than likely hitting the same bug. https://lore.kernel.org/all/48353e0d-e771-8a97-21d4-c65ff3bc4192@xxxxxxxxxx
