Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state

[Andy Lutomirski]

> On Mar 29, 2021, at 9:06 AM, Len Brown <lenb@xxxxxxxxxx> wrote:
> 
> On Mon, Mar 29, 2021 at 11:43 AM Len Brown <lenb@xxxxxxxxxx> wrote:
>> 
>> On Mon, Mar 29, 2021 at 9:33 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> 
>>>> I found the author of this passage, and he agreed to revise it to say this
>>>> was targeted primarily at VMMs.
>>> 
>>> Why would this only a problem for VMMs?
>> 
>> VMMs may have to emulate different hardware for different guest OS's,
>> and they would likely "context switch" XCR0 to achieve that.
>> 
>> As switching XCR0 at run-time would confuse the heck out of user-space,
>> it was not imagined that a bare-metal OS would do that.
> 
> to clarify...
> *switching* XCR0 on context switch is slow, but perfectly legal.

How slow is it?  And how slow is switching XFD?  XFD is definitely serializing?