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Date: Sun, 18 Oct 2009 13:23:30 +0200
Message-ID: <761ea48b0910180423n4da24b29i65f67d972992ff16@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH] User mode: Handle x86_64 vsyscall
From: Laurent Desnogues <laurent.desnogues@gmail.com>
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To: Jamie Lokier <jamie@shareable.org>
Cc: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>, qemu-devel@nongnu.org

On Sun, Oct 18, 2009 at 4:47 AM, Jamie Lokier <jamie@shareable.org> wrote:
> Laurent Desnogues wrote:
>> A recent compiler (gcc 4.4.0) produces this code for a statically
>> compiled program:
>>
>> 00000000005779e0 <time>:
>> =A0 5779e0: =A0 =A0 48 83 ec 08 =A0 =A0 =A0 =A0 =A0 =A0 sub =A0 =A0$0x8,=
%rsp
>> =A0 5779e4: =A0 =A0 48 c7 c0 00 04 60 ff =A0 =A0mov =A0 =A0$0xffffffffff=
600400,%rax
>> =A0 5779eb: =A0 =A0 ff d0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 callq =A0*=
%rax
>> =A0 5779ed: =A0 =A0 48 83 c4 08 =A0 =A0 =A0 =A0 =A0 =A0 add =A0 =A0$0x8,=
%rsp
>> =A0 5779f1: =A0 =A0 c3 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0retq
>
> Yes. =A0It's a fixed address. =A0See the kernel at
> linux/arch/x86/kernel/vsyscall_64.c. =A0There are only 3 vsyscall
> functions defined: vgettimeofday, vtime and vgetcpu.

My proposed patch already implements vgettimeofday and
vtime. vgetcpu is TBD.

> Even though it's a statically linked program, I'm not sure if the
> above code will work on really old kernels.

All I can say is that it's what my toolchain generates.

Interestingly, dynamically shared programs don't run with
QEMU on my machine.  It looks like the PC is completely
wrong.

> The vsyscall page is different from the vdso, which has variable
> address, and the address is supplied to Glibc. =A0vdso provides nearly
> the same functions in a different way.

I don't understand how vdso and vsyscall interact (or don't
interact):  as I showed above, a statically linked program will
use vsyscall.  For dynamically linked program, I can't say,
given what I said above;  there might be a loader issue here.


Laurent