From: Denys Vlasenko <dvlasenk@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
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
Date: Sat, 12 Mar 2016 18:53:00 +0100 [thread overview]
Message-ID: <56E4577C.1030107@redhat.com> (raw)
In-Reply-To: <20160312153808.GC17873@gmail.com>
On 03/12/2016 04:38 PM, Ingo Molnar wrote:
>
> * 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)
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.
next prev parent reply other threads:[~2016-03-12 17:53 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-11 16:53 [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs Denys Vlasenko
2016-03-12 15:38 ` Ingo Molnar
2016-03-12 17:53 ` Denys Vlasenko [this message]
2016-03-12 18:05 ` Andy Lutomirski
2016-03-12 21:41 ` Denys Vlasenko
2016-03-12 15:45 ` Ingo Molnar
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