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

[Nikolay Borisov]

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@xxxxxxxxxx>
> ---
>   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?