Messages in this thread | | | Subject | Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state | From | Dave Hansen <> | Date | Tue, 29 Jun 2021 10:54:06 -0700 |
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On 6/29/21 10:43 AM, Bae, Chang Seok wrote: > On Jun 16, 2021, at 12:01, Dave Hansen <dave.hansen@intel.com> 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.
How about we pay this 26-cycle cost, but only when XFD is in use? We could either look at the shadowed value of the XFD MSR, or flip a global variable the first time XFD gets used.
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