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

From: Nikolay Borisov <nik.borisov@suse.com>
To: Josh Poimboeuf <jpoimboe@kernel.org>, x86@kernel.org
Cc: linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Daniel Sneddon <daniel.sneddon@linux.intel.com>,
	Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Alexandre Chartre <alexandre.chartre@oracle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Sean Christopherson <seanjc@google.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	KP Singh <kpsingh@kernel.org>, Waiman Long <longman@redhat.com>,
	Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH v2 1/3] x86/bugs: Only harden syscalls when needed
Date: Mon, 15 Apr 2024 10:32:01 +0300	[thread overview]
Message-ID: <8aee08a9-0d7a-4b5f-aee1-c70bad9433ca@suse.com> (raw)
In-Reply-To: <079f24edefb5b47e922c2804f199463e04ccf0c8.1712944776.git.jpoimboe@kernel.org>

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@kernel.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?