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

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Date:	Sat, 12 Mar 2016 18:53:00 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
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

On 03/12/2016 04:38 PM, Ingo Molnar wrote:
> 
> * 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)

Yes.

test_syscall_vdso32 checks vdso syscall (if available)
and direct int80 syscall.
Booting two times, with different qemu flags:

	qemu-system-x86_64 -cpu Opteron_G4
	qemu-system-x86_64 -cpu SandyBridge

makes kernel choose either SYSCALL or SYSENTER vdso.
So it's all covered.