Subject: Re: [PATCH RFC 4/4] x86/entry/pti: don't switch PGD on tasks holding
 flag TIF_NOPTI
To: Willy Tarreau <w@1wt.eu>, linux-kernel@vger.kernel.org,
        x86@kernel.org
References: <1515427939-10999-1-git-send-email-w@1wt.eu>
 <1515427939-10999-5-git-send-email-w@1wt.eu>
Cc: tglx@linutronix.de, gnomes@lxorguk.ukuu.org.uk,
        torvalds@linux-foundation.org, Andy Lutomirski <luto@kernel.org>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <9d6917dc-1a6c-a8d4-95fe-f767495f99d3@intel.com>
Date: Mon, 8 Jan 2018 09:20:08 -0800
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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.