Date: Tue, 24 Feb 2015 20:58:01 +0100
From: Borislav Petkov <bp@alien8.de>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, 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@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns
Message-ID: <20150224195801.GM3420@pd.tnic>
References: <1424803895-4420-1-git-send-email-dvlasenk@redhat.com>
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On Tue, Feb 24, 2015 at 07:51:32PM +0100, Denys Vlasenko wrote:
> In all three 32-bit entry points, %eax is zero-extended to %rax.
> It is safe to do 32-bit compare when checking that syscall#
> is not too large.
> 
> The last instance of "mysterious" SS+8 constant is replaced by SIZEOF_PTREGS.
> 
> The $AUDIT_ARCH_X86_64 parameter to syscall_trace_enter_phase1/2
> is a 32-bit constant, loading it with 64-bit MOV produces 10-byte insn
> instead of 5-byte one.
> 
> After TEST insn, JE anctually means "jump of zero",
> let's use JZ mnemonic instead.

Actually, JE == LZ as that's the same opcode for testing ZF=1.

And I have to object:

        testl $3,CS(%rsp)
	je retint_kernel

is much more understandable than

        testl $3,CS(%rsp)
	jz retint_kernel

It basically says are $3 and CS(%rsp) equal.

JZ, on the other hand, not so clear: the TEST ANDed the two operands and
set flags accordingly, so JZ is testing the ZF. This means, you actually
know what TEST does and you haven't forgotten.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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