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

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Date:	Sat, 12 Mar 2016 10:05:05 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs

On Sat, Mar 12, 2016 at 9:53 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> 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.

How carefully did you check the latter bit?  In my experience, if KVM
is used, your cpu will report as your native CPU's manufacturer
regardless of who actually makes the emulated CPU.  -machine accel=tcg
turns that off.

-- 
Andy Lutomirski
AMA Capital Management, LLC