[Qemu-devel] FreeBSD clock_gettime()

[Qemu-devel] FreeBSD clock_gettime()

[Sai Prajeeth]

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Hi list,

I have done experiments and it seems that FreeBSD's libc function
clock_gettime() results in a system call when running on QEMU whereas on
hardware it does not. Does anybody know why? For those interested , you can
find the clock_gettime libc call in freebsd source here:
https://github.com/lattera/freebsd/blob/master/lib/libc/sys/clock_gettime.c

Thanks!

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Re: [Qemu-devel] FreeBSD clock_gettime()

[Stefan Hajnoczi]

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On Sun, May 31, 2015 at 07:11:25PM +0530, Sai Prajeeth wrote:
> I have done experiments and it seems that FreeBSD's libc function
> clock_gettime() results in a system call when running on QEMU whereas on
> hardware it does not. Does anybody know why? For those interested , you can
> find the clock_gettime libc call in freebsd source here:
> https://github.com/lattera/freebsd/blob/master/lib/libc/sys/clock_gettime.c

This is more of a FreeBSD kernel question than a QEMU question.  I don't
have time to dig into the FreeBSD sources but you should be able to find
the answer there.

Check out the FreeBSD kernel's vdso implementation.  Linux has something
similar, it's a library mapped into userspace processes that is provided
by the kernel.  It includes low-level utilities that depend on the
specific kernel.

Stefan

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Re: [Qemu-devel] FreeBSD clock_gettime()

[Peter Maydell]

On 2 June 2015 at 15:49, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Sun, May 31, 2015 at 07:11:25PM +0530, Sai Prajeeth wrote:
>> I have done experiments and it seems that FreeBSD's libc function
>> clock_gettime() results in a system call when running on QEMU whereas on
>> hardware it does not. Does anybody know why? For those interested , you can
>> find the clock_gettime libc call in freebsd source here:
>> https://github.com/lattera/freebsd/blob/master/lib/libc/sys/clock_gettime.c
>
> This is more of a FreeBSD kernel question than a QEMU question.  I don't
> have time to dig into the FreeBSD sources but you should be able to find
> the answer there.

The underlying answer is presumably "some difference between
the (emulated?) (KVM?) CPU QEMU exposes to the guest and the
CPU on real h/w makes FreeBSD decide it can't provide gettime
in the vdso". Exactly what that difference is you'd need to
look at the FreeBSD sources to find out, I agree.

-- PMM