Hello Peter, On Tue, Jun 7, 2022 at 5:07 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > On Tue, Jun 07, 2022 at 02:17:54PM -0400, Peter Xu wrote: > > On Tue, Jun 07, 2022 at 03:04:27PM +0000, Sean Christopherson wrote: > > > 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. > > > > What's the bisection you mentioned? I actually did a bisection and I also > > checked reverting Leo's change can also fix this issue. Or do you mean > > something else? > > Ah, I forgot to mention on the "stable tree decisions": IIUC it also means > we should apply Leo's patch to all the stable trees if possible, then > migrations between them won't trigger the misterous faults anymore, > including when migrating to the latest Linux versions. > > However there's the delimma that other kernels (any kernel that does not > have Leo's patch) will start to fail migrations to the stable branches that > apply Leo's patch too.. IIUC, you commented before that the migration issue should be solved with a QEMU fix, is that correct? That would mean something like 'QEMU is relying on a kernel bug to work', and should be no blocker for fixing the kernel. If that's the case, I think we should apply the fix to every supported stable branch that have the fpku issue, and in parallel come with a qemu fix for that. What do you think about it? Best regards, Leo > So that's kind of a slight pity. It's just IIUC > the stable trees are more important, because it should have a broader > audience (most Linux distros)? > > > > > > > > > https://lore.kernel.org/all/48353e0d-e771-8a97-21d4-c65ff3bc4192@xxxxxxxxxx > > > > That is kvm64, and I agree it could be the same problem since both qemu64 > > and kvm64 models do not have any xsave feature bit declared in cpuid 0xd, > > so potentially we could be migrating some fpu states to it even with > > user_xfeatures==0 on dest host. > > > > So today I continued the investigation, and I think what's really missing > > is qemu seems to be ignoring the user_xfeatures check for KVM_SET_XSAVE and > > continues even if it returns -EINVAL. IOW, I'm wondering whether we should > > fail properly and start to check kvm_arch_put_registers() retcode. But > > that'll be a QEMU fix, and it'll at least not causing random faults > > (e.g. double faults) in guest but we should fail the migration gracefully. > > > > Sean: a side note is that I can also easily trigger one WARN_ON_ONCE() in > > your commit 98c25ead5eda5 in kvm_arch_vcpu_ioctl_run(): > > > > WARN_ON_ONCE(kvm_lapic_hv_timer_in_use(vcpu)); > > > > It'll be great if you'd like to check that up. > > > > Thanks, > > > > -- > > Peter Xu > > -- > Peter Xu >
