Re: How should we handle illegal task FPU state?

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 01/10/2020 21:58, Sean Christopherson wrote:
> On Thu, Oct 01, 2020 at 01:32:04PM -0700, Yu, Yu-cheng wrote:
>> On 10/1/2020 10:43 AM, Andy Lutomirski wrote:
>>> The question is: what do we do about it?  We have two basic choices, I think.
>>>
>>> a) Decide that the saved FPU for a task *must* be valid at all times.
>>> If there's a failure to restore state, kill the task.
>>>
>>> b) Improve our failed restoration handling and maybe even
>>> intentionally make it possible to create illegal state to allow
>>> testing.
>>>
>>> (a) sounds like a nice concept, but I'm not convinced it's practical.
>>> For example, I'm not even convinced that the set of valid SSP values
>>> is documented.
> Eh, crappy SDM writing isn't a good reason to make our lives harder.  The
> SSP MSRs are canonical MSRs and follow the same rules as the SYSCALL,
> FS/GS BASE, etc... MSRs.  I'll file an SDM bug.

Don't forget the added constraint of being 4 byte aligned. ;)

But the SDM is fine in this regard, at least as far as Vol4 goes, even
if does have an excessively verbose way of expressing itself.

~Andrew