Re: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

Juan Quintela
2021-12-14T19:07:09Z

Thomas Gleixner <tglx@linutronix.de> wrote:
> Wei,
>
> On Tue, Dec 14 2021 at 16:11, Wei W. Wang wrote:
>> On Tuesday, December 14, 2021 11:40 PM, Thomas Gleixner wrote:
>>> On Tue, Dec 14 2021 at 15:09, Wei W. Wang wrote:
>>> > On Tuesday, December 14, 2021 10:50 AM, Thomas Gleixner wrote:
>>> >> + * Return: 0 on success, error code otherwise */ int
>>> >> +__fpu_update_guest_features(struct fpu_guest *guest_fpu, u64 xcr0,
>>> >> +u64
>>> >> +xfd) {
>>> >
>>> > I think there would be one issue for the "host write on restore" case.
>>> > The current QEMU based host restore uses the following sequence:
>>> > 1) restore xsave
>>> > 2) restore xcr0
>>> > 3) restore XFD MSR
>>>
>>> This needs to be fixed. Ordering clearly needs to be:
>>>
>>> XFD, XCR0, XSTATE
>>
>> Sorry, just to clarify that the ordering in QEMU isn't made by us
>> for this specific XFD enabling.
>> It has been there for long time for the general restoring of all the
>> XCRs and MSRs.
>> (if you are interested..FYI:
>> https://github.com/qemu/qemu/blob/master/target/i386/kvm/kvm.c#L4168).
>> - kvm_put_xsave()
>> - kvm_put_xcrs()
>> - kvm_put_msrs()
>>
>> We need to check with the QEMU migration maintainer (Dave and Juan CC-ed)
>> if changing that ordering would be OK.
>> (In general, I think there are no hard rules documented for this ordering)
>
> There haven't been ordering requirements so far, but with dynamic
> feature enablement there are.
>
> I really want to avoid going to the point to deduce it from the
> xstate:xfeatures bitmap, which is just backwards and Qemu has all the
> required information already.

Hi

First of all, I claim ZERO knowledge about low level x86_64.

Once told that, this don't matter for qemu migration, code is at

target/i386/kvm/kvm.c:kvm_arch_put_registers()

ret = kvm_put_xsave(x86_cpu);
if (ret < 0) {
return ret;
}
ret = kvm_put_xcrs(x86_cpu);
if (ret < 0) {
return ret;
}
/* must be before kvm_put_msrs */
ret = kvm_inject_mce_oldstyle(x86_cpu);
if (ret < 0) {
return ret;
}
ret = kvm_put_msrs(x86_cpu, level);
if (ret < 0) {
return ret;
}

If it needs to be done in any other order, it is completely independent
of whatever is inside the migration stream.

I guess that Paolo will put some light here.

Later, Juan.