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Re: [Xen-devel] Using SYSCALL/SYSRET with a minios kernel

To: Goswin von Brederlow <goswin-v-b@xxxxxx>
Subject: Re: [Xen-devel] Using SYSCALL/SYSRET with a minios kernel
From: Daniel Stodden <stodden@xxxxxxxxxx>
Date: Mon, 25 Feb 2008 03:26:21 +0100
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On Mon, 2008-02-25 at 02:55 +0100, Goswin von Brederlow wrote:
> Daniel Stodden <stodden@xxxxxxxxxx> writes:
> > Hi.
> >
> > On Mon, 2008-02-25 at 00:22 +0100, Goswin von Brederlow wrote: 
> >> Hi,
> >> 
> >> I'm trying to use the SYSCALL/SYSRET opcodes with a minios kernel
> >> without much success.
> >...
> > The PV interface simply does not support STAR/LSTAR. It's that
> > simple. :) I suppose you want to implement system calls? Check the
> > HYPERVISOR_set_callbacks() call. The syscall_address parameter presently
> > remains entirely unused in mini-os. But as far as I could tell
> > immediately from the source, syscall/sysret appears to be supported by
> > the general callback mechanism the same way sysenter/sysleave presently
> > is.
> >
> > regards,
> > daniel
> Ok, here is what I did for the callbacks:
> --- x86_64.S ---
> ENTRY(syscall_callback)
>       int $80
>       zeroentry do_syscall
> --- kernel.c ---
>   HYPERVISOR_set_callbacks((unsigned long)hypervisor_callback,
>                          (unsigned long)failsafe_callback,
>                          (unsigned long)syscall_callback);
>   __asm__ __volatile__("syscall");
> If I understood you right that should set the RIP to syscall_callback
> and execute from there.

Mööp! Only when calling in from virtual user mode. Otherwise, you're
triggering a hypercall service routine, and one might suspect you're
presently just generating an error condition with that. :)

BTW: I found building Xen with 'debug=y' generates a helpful comment on
the console every now and xen.

> But still, the syscall opcode does nothing.
> In case you wonder. The "int $80" is there to crash the domain and
> tell me it reached that point.


Daniel Stodden
LRR     -      Lehrstuhl für Rechnertechnik und Rechnerorganisation
Institut für Informatik der TU München             D-85748 Garching
http://www.lrr.in.tum.de/~stodden         mailto:stodden@xxxxxxxxxx
PGP Fingerprint: F5A4 1575 4C56 E26A 0B33  3D80 457E 82AE B0D8 735B

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