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List:       user-mode-linux-devel
Subject:    Re: [uml-devel] [PATCH 3/3] um: Implement a x86_64 vDSO
From:       Andrew Lutomirski <luto () mit ! edu>
Date:       2011-07-20 18:19:13
Message-ID: CAObL_7Gss0GcB8OtoCf-rUdsi8n4345gBESBub+5hu4g9fcssw () mail ! gmail ! com
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On Wed, Jul 20, 2011 at 2:09 PM, Richard Weinberger <richard@nod.at> wrote:
> Am Mittwoch 20 Juli 2011, 19:54:02 schrieb Andy Lutomirski:
>> On 07/18/2011 09:07 AM, Richard Weinberger wrote:
>> > Until now UML had no x86_64 vDSO.
>> > So glibc always used the vsyscall page for gettimeday()
>> > and friends.
>> > Calls to gettimeday() returned falsely the host time and
>> > confused some programs.
>> >
>> > This patch adds a vDSO which turns all __vdso_* calls into
>> > a system call so that UML can trap them.
>> >
>> > As glibc still uses the vsyscall page for static binaries this
>> > patch improves the situation only for dynamic binaries.
>> >
>> > Signed-off-by: Richard Weinberger<richard@nod.at>
>> >
>> > +   asm("syscall" : "=a" (ret) :
>> > +           "0" (__NR_clock_gettime), "D" (clock), "S" (ts) : "memory");
>> > +
>>
>> The x86-64 ABI and glibc think that rcx, and r11 are clobbered.  glibc
>> also thinks that cc is clobbered.  I personally think that rdx and
>> r8-r15 ought to be clobbered under the principle that syscall should
>> look like a function call.
>
> Hmm, I thought my implementation is fine because it follows
> arch/x86/vdso/vclock_gettime.c (Linus' tree).
>
> notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
> {
>  long ret;
>  asm("syscall" : "=a" (ret) :
>      "0" (__NR_clock_gettime),"D" (clock), "S" (ts) : "memory");
>  return ret;
> }
>
> Am I missing something or needs vdso_fallback_gettime() also some updates?

vdso_fallback_gettime is going away :)

I haven't read all of the sysret code paths, but it looks like at
least r11 is used in a comment.  That being said, your code is an
uninlinable function, and functions already clobber all those
registers, so the code shouldn't cause problems in real life.

--Andy

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