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Date: Sat, 12 Mar 2016 16:38:08 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
        x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs
Message-ID: <20160312153808.GC17873@gmail.com>
References: <1457715214-7432-1-git-send-email-dvlasenk@redhat.com>
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* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> Use of a temporary R8 register here seems to be unnecessary.
> 
> "push %r8" is a two-byte insn (it needs REX prefix to specify R8),
> "push $0" is two-byte too. It seems just using the latter would be
> no worse.
> 
> Thus, code had an unnecessary "xorq %r8,%r8" insn.

Neat!

> It probably costs nothing in execution time here since we are probably
> limited by store bandwidth at this point, but still.
> 
> Run-tested under QEMU: 32-bit calls still work:
> 
> / # ./test_syscall_vdso32

Did you manage to test all 3 compat variants:

> @@ -72,24 +72,23 @@ ENTRY(entry_SYSENTER_compat)
> @@ -205,17 +204,16 @@ ENTRY(entry_SYSCALL_compat)
> @@ -316,11 +314,10 @@ ENTRY(entry_INT80_compat)

?

Thanks,

	Ingo