Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state | Date | Sat, 20 Mar 2021 23:13:29 +0100 |
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On Sun, Feb 21 2021 at 10:56, Chang S. Bae wrote: > + > +/* Update MSR IA32_XFD with xfirstuse_not_detected() if needed. */ > +static inline void xdisable_switch(struct fpu *prev, struct fpu *next) > +{ > + if (!static_cpu_has(X86_FEATURE_XFD) || !xfirstuse_enabled()) > + return; > + > + if (unlikely(prev->state_mask != next->state_mask)) > + xdisable_setbits(xfirstuse_not_detected(next)); > +}
So this is invoked on context switch. Toggling bit 18 of MSR_IA32_XFD when it does not match. The spec document says:
"System software may disable use of Intel AMX by clearing XCR0[18:17], by clearing CR4.OSXSAVE, or by setting IA32_XFD[18]. It is recommended that system software initialize AMX state (e.g., by executing TILERELEASE) before doing so. This is because maintaining AMX state in a non-initialized state may have negative power and performance implications."
I'm not seeing anything related to this. Is this a recommendation which can be ignored or is that going to be duct taped into the code base once the first user complains about slowdowns of their non AMX workloads on that machine?
Thanks,
tglx
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