Date: Wed, 25 Feb 2015 09:43:12 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: "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>
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns
Message-ID: <20150225094312.2cfff453@gandalf.local.home>
In-Reply-To: <20150225092043.GB16165@gmail.com>
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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.

-- Steve

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