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

[Juan Quintela <quintela@redhat.com>]

Thomas Gleixner <tglx@linutronix.de> wrote:

Hi Thomas

> On Tue, Dec 14 2021 at 20:07, Juan Quintela wrote:
>> Thomas Gleixner <tglx@linutronix.de> wrote:
>>> On Tue, Dec 14 2021 at 16:11, Wei W. Wang wrote:
>>>> 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.
>>
>> First of all, I claim ZERO knowledge about low level x86_64.
>
> Lucky you.

Well, that is true until I have to debug some bug, at that time I miss
the knowledge O:-)

>> Once told that, this don't matter for qemu migration, code is at
>
> Once, that was at the time where rubber boots were still made of wood,
> right? :)

I forgot to add: "famous last words".

>> 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);
>
> So this has already ordering requirements.
>
>>     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.
>
> From the migration data perspective that's correct, but I have the
> nagging feeling that this in not that simple.

Oh, I was not meaning that it was simple at all.

We have backward compatibility baggage on x86_64 that is grotesque at
this point.  We don't send a single msr (by that name) on the main
migration stream.  And then we send them based on "conditions".  So the
trick as somithing like:

- qemu reads the msrs from kvm at some order
- it stores them in a list of MSR's
- On migration pre_save we "mangle" that msr's and other cpu state to
  on the main (decades old) state
- then we send the main state
- then we send conditionally the variable state

on reception side:

- we receive everything that the sending side have sent
- we "demangle" it on pre_load
- then we create the list of MSR's that need to be transferred
- at that point we send them to kvm in another random order

So yes, I fully agree that it is not _that_ simple O:-)


>> I guess that Paolo will put some light here.
>
> I fear shining light on that will unearth quite a few skeletons :)

It is quite probable.

When a bugzilla start with: We found a bug while we were trying to
migrate during (BIOS) boot, I just ran for the hills O:-)

Later, Juan.