Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state

[Dave Hansen]

On 6/16/21 12:23 PM, Bae, Chang Seok wrote:
> On Jun 16, 2021, at 12:01, Hansen, Dave <dave.hansen@xxxxxxxxx> wrote:
>> On 6/16/21 11:47 AM, Bae, Chang Seok wrote:
>>> Reading XINUSE via XGETBV is cheap but not free. I don't know spending a
>>> hundred cycles for this WARN is big deal but this is one of the most
>>> performance-critical paths.
>> Is XGETBV(1) really a hundred cycles?  That seems absurdly high for a
>> non-serializing register read.
> This was checked to convince the benefit intended by PATCH25 --
> https://lore.kernel.org/lkml/20210523193259.26200-26-chang.seok.bae@xxxxxxxxx/

That's odd.  How is it possible that the performance of XGETBV(1)
informed the design of that patch without there being any mention of
XGETBV in the comments or changelog?