Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH RFC 3/7] x86: kernel: cpu: bugs.c: modify static_cpu_has to boot_cpu_has | Date | Mon, 06 Jul 2020 16:46:33 +0200 |
| |
Mihai Carabas <mihai.carabas@oracle.com> writes:
> The usage of static_cpu_has in bugs.c file is counter-productive > since the code is executed once but there is extra effort to patch > it and keep alternatives in a special section --- so there is both > space and time cost. > > Quote from _static_cpu_has definition: > /* > * Static testing of CPU features. Used the same as boot_cpu_has(). It > * statically patches the target code for additional performance. Use > * static_cpu_has() only in fast paths, where every cycle counts. Which > * means that the boot_cpu_has() variant is already fast enough for the > * majority of cases and you should stick to using it as it is generally > * only two instructions: a RIP-relative MOV and a TEST. > */ > > There are two other places where static_cpu_has is used and might be > considered critical paths: __speculation_ctrl_update() and vmx_l1d_flush(). > > Given these facts, changing static_cpu_has to boot_cpu_has is done in > order to bypass alternative instructions which cannot be updated at runtime > for now.
Not going to happen. We are not adding 4 conditionals to context switch just to support that late loading horrors. There are better ways to do that.
Thanks,
tglx
| |