linux-kernel - Re: [PATCH v2 1/3] x86/bugs: Only harden syscalls when needed

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Date: Mon, 15 Apr 2024 10:32:01 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Alexandre Chartre <alexandre.chartre@...cle.com>,
 Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sean Christopherson <seanjc@...gle.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, KP Singh <kpsingh@...nel.org>,
 Waiman Long <longman@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v2 1/3] x86/bugs: Only harden syscalls when needed


On 12.04.24 г. 21:10 ч., Josh Poimboeuf wrote:
> Syscall hardening (i.e., converting the syscall indirect branch to a
> series of direct branches) may cause performance regressions in certain
> scenarios.  Only use the syscall hardening when indirect branches are
> considered unsafe.
>
> Fixes: 1e3ad78334a6 ("x86/syscall: Don't force use of indirect calls for system calls")
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>   arch/x86/entry/common.c            | 15 ++++++++++++---
>   arch/x86/entry/syscall_32.c        | 11 +----------
>   arch/x86/entry/syscall_64.c        |  6 ------
>   arch/x86/entry/syscall_x32.c       |  7 ++++++-
>   arch/x86/include/asm/cpufeatures.h |  1 +
>   arch/x86/include/asm/syscall.h     |  8 +++++++-
>   arch/x86/kernel/cpu/bugs.c         | 31 +++++++++++++++++++++++++++++-
>   7 files changed, 57 insertions(+), 22 deletions(-)
>

To ask again, what do we gain by having this syscall hardening at the 
same time as the always on BHB scrubbing sequence?