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

[Bae, Chang Seok]

On Jun 16, 2021, at 12:01, Dave Hansen <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.

My apologies -- I neglect not to check the log immediately. XGETBV(1) took
about 26 cycles on my test.

Thanks,
Chang