linux-kernel - Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt

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Date:	Sat, 12 Mar 2016 16:38:08 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs


* Denys Vlasenko <dvlasenk@...hat.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