Re: [PATCH 3/3] um: Implement a x86_64 vDSO
From: Andrew Lutomirski
Date: Wed Jul 20 2011 - 14:19:37 EST
On Wed, Jul 20, 2011 at 2:09 PM, Richard Weinberger <richard@xxxxxx> 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@xxxxxx>
>> > + 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.
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