Re: [PATCH RFC 4/4] x86/entry/pti: don't switch PGD on tasks holding flag TIF_NOPTI

[Dave Hansen <dave.hansen@intel.com>]

Please cc Andy on this stuff.  I can't imagine patching entry_64.S at
this point without cc'ing him.  *Surely* you didn't even bother to run
get_maintainer.pl on this.

> @@ -214,6 +215,11 @@
>  .macro SWITCH_TO_KERNEL_CR3 scratch_reg:req
>  	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
>  	mov	%cr3, \scratch_reg
> +
> +	/* if we're already on the kernel PGD, we don't switch */
> +	testq $(PTI_SWITCH_PGTABLES_MASK), \scratch_reg
> +	jz .Lend_\@
> +
>  	ADJUST_KERNEL_CR3 \scratch_reg
>  	mov	\scratch_reg, %cr3
>  .Lend_\@:

This is an optimization that we can do generally without your feature.
Actually, it would be a welcome bit of benchmarking if you could see if
just this hunk helps your workload.

You touched on it in the description, but this is a *very* clever way to
do what you need without needing to look at the task flag at
user->kernel entry which also happens to be a place you don't have
task_struct mapped.  It *greatly* simplifies what this would have to do
otherwise.

That needs calling out specifically though.