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. I just tried two other options with: (1) -cpu host, and (2) -cpu Haswell (the choice of Haswell was really random..), with the same 5.15->5.18 migration scenario, both of them will not trigger the same guest kernel crash. Only qemu64 will. Both hosts have Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz. > 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). Thanks, -- Peter Xu
