Xen devel - Nov 2005 - Privilege levels in xenolinux

The following assembly code is used to set the Descriptors in xenolinux.
Theoretically, for xenolinux we have Descriptor privilege level(DPL) = 1
for all its descriptors. But from the instruction,

        "movb %4,5(%2)\n\t"  (with the parameter, type =0x82 and 0x89)
it seems that the DPL is set to 0.

How is this possible. Dont we have xenolinux running at privilege level 1?
Am I going wrong somewhere?


#define _set_tssldt_desc(n,addr,limit,type) \
__asm__ __volatile__ ("movw %w3,0(%2)\n\t" \
        "movw %%ax,2(%2)\n\t" \
        "rorl $16,%%eax\n\t" \
        "movb %%al,4(%2)\n\t" \
        "movb %4,5(%2)\n\t" \
        "movb $0,6(%2)\n\t" \
        "movb %%ah,7(%2)\n\t" \
        "rorl $16,%%eax" \
        : "=m"(*(n)) : "a" (addr), "r"(n),
"ir"(limit), "i"(type))


The above inline assembly is called as :

_set_tssldt_desc(&per_cpu(cpu_gdt_table, cpu)[entry], (int)addr,
                offsetof(struct tss_struct, __cacheline_filler) - 1, 0x89);

_set_tssldt_desc(&per_cpu(cpu_gdt_table, cpu)[GDT_ENTRY_LDT], (int)addr,  
((size << 3)-1), 0x82);


Thanks for any replies,
Purav

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

On 8 Nov 2005, at 05:23, Purav Saraiya wrote:
> The following assembly code is used to set the Descriptors in 
> xenolinux.
> Theoretically, for xenolinux we have Descriptor privilege level(DPL) = 
> 1
> for all its descriptors. But from the instruction,
>
>         "movb %4,5(%2)\n\t"  (with the parameter, type =0x82 and
0x89)
> it seems that the DPL is set to 0.
>
> How is this possible. Dont we have xenolinux running at privilege 
> level 1?
> Am I going wrong somewhere?
That''s dead code in Xen Linux. We set up LDTs via an explicit 
hypercall, not via LLDT.

  -- Keir


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel