Re: [PATCH AUTOSEL 5.16 07/28] x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0

[Paolo Bonzini]

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.

Paolo