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

[Liu, Jing2]

On 6/17/2021 3:28 AM, Dave Hansen wrote:
> 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?
Hi Chang,

I noticed the XGETBV(1) cycles you ran, however I calculated only ~16 
cycles
in the corresponding machine.

BRs,
Jing