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From: Denys Vlasenko <vda.linux@googlemail.com>
Date: Wed, 25 Feb 2015 16:40:49 +0100
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Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Wed, Feb 25, 2015 at 3:43 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Wed, 25 Feb 2015 10:20:43 +0100 Ingo Molnar <mingo@kernel.org> wrote:
>> But, if we do that, we can do even better, and also do an
>> optimization of the 64-bit entry path as well: we could
>> simply mask RAX with 0x3ff and not do a compare. Pad the
>> syscall table up to 0x400 (1024) entries and fill in the
>> table with sys_ni syscall entries.
>>
>> This is valid on 64-bit and 32-bit kernels as well, and it
>> allows the removal of a compare from the syscall entry
>> path, at the cost of a couple of kilobytes of unused
>> syscall table.
>>
>> The downside would be that if we ever grow past 1024
>> syscall entries we'll be in trouble if new userspace calls
>> syscall 513 on an old kernel and gets syscall 1.
>
> What if we test against ~0x3ff and jump to sys_ni if anything is set.
> This would still work with new userspace calls on older kernels.

That would require a branch insn. The whole idea of masking
was merely to avoid that branch. If you need a branch,
then you can as well just retain current code.
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