linux-kernel - Re: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get

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Message-ID: <20241029032735.pw3bg64bpneqnfhk@treble.attlocal.net>
Date: Mon, 28 Oct 2024 20:27:35 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Waiman Long <longman@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	linuxppc-dev@...ts.ozlabs.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Mark Rutland <mark.rutland@....com>,
	"Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit
 __get_user()

On Mon, Oct 28, 2024 at 06:56:15PM -0700, Josh Poimboeuf wrote:
> The barrier_nospec() in 64-bit __get_user() is slow.  Instead use
> pointer masking to force the user pointer to all 1's if a previous
> access_ok() mispredicted true for an invalid address.

Linus pointed out that __get_user() may be used by some code to access
both kernel and user space and in fact I found one such usage in
vc_read_mem()....

So I self-NAK this patch for now.

Still, it would be great if patch 1 could get merged as that gives a
significant performance boost.

-- 
Josh