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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 01:01:54 +0100
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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.
> Going by the manuals (and linux sources) I first have to setup the
> STAR and LSTAR registers to define the segment and instruction pointer
> to be used for SYSCALL:
>       /* 
>          * LSTAR and STAR live in a bit strange symbiosis.
>          * They both write to the same internal register. STAR allows to set
>          * CS/DS but only a 32bit target. LSTAR sets the 64bit rip.
>          */ 
>         wrmsrl(MSR_STAR,  ((uint64_t)__USER_CS)<<48  | 
> ((uint64_t)__KERNEL_CS)<<32); 
>         wrmsrl(MSR_LSTAR, system_call); 
> But all that does is give a message on the xen console:
> (XEN) traps.c:1467:d137 Domain attempted WRMSR 00000000c0000081 from 
> e023e010:00000000 to e033e033:00000000.
> (XEN) traps.c:1467:d137 Domain attempted WRMSR 00000000c0000082 from 
> ffff8300:001e0800 to 00000000:001054c0.

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


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
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