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Re: [Libvir] big-endian support for libvirt - introduce GUEST_HANDLE infrastructure ?



On Fri, Jul 06, 2007 at 11:06:54AM +0200, Christian Ehrhardt wrote:
> Hello everyone,
> we currently try to use/port libvirt to run on xenppc. While doing that 
> I found a endianess issue.
> 
> First a simple example:
> 
>        Sample code:
>                #include <stdio.h>
>                
>                union u {
>                        int i;
>                        long long l;
>                };
>                
>                int main(void)
>                {
>                        union u u;
>                        u.l = 0;
>                        u.i = ~0;
>                        printf("%016llx\n", u.l);
>                
>                        return 0;
>                }
>        
>        Little-endian output: 00000000ffffffff
>        Big-endian output:    ffffffff00000000
>        
>        Now look at the padding in e.g. xen_v2s3_getdomaininfolistop: it
>        doesn't work on big-endian systems. This problem is why the
>        GUEST_HANDLE infrastructure is present in Xen. To work on
>        big-endian systems, libvirt will need this or a similar mechanism.
> 
> 
> 
> --- Current Questions ---
> We could now add several ugly ifdefs to libvirt code to differentiate 
> powerpc from the others and solve the issue described above, but I think 
> thats not a good solution.
> The GUEST_HANDLE mechanism would provide an architecture abstraction and 
> is already implemented/working in libxc.a

libxc is GPL, while libvirt is LGPL.

> The question is, shouldn't libvirt actually use the GUEST_HANDLE 
> mechanism like libxc does (at least for the _v2d5_ structures) or are 
> there big arguments against it?

We can't use the structures directly because libvirt needs to compile in
support for *all*  hypervisor ABIs so it can automatically use the correct
ABI at runtime. We don't want a build of libvirt to be tied to the HV it
was built against because that creates serious pain in upgrades.

> More or less everything in libvirt that would use GUEST_HANDLE in libxc:
> I saw the effect on a bad pointer passed down by the structure I use for 
> my example below, but in the end it may affect all the stuff that uses 
> the *GUEST_HANDLE* stuff in libxc which are e.g. uchar,ulong,... here an 
> example:
> libxc:
>   uint64_aligned_t max_pages;  // <- arch dependent mapping of 
> uint64_aligned_t
> libvirt:
>   #define ALIGN_64 __attribute__((aligned(8)))
>   uint64_t max_pages ALIGN_64; // <- this align is fix on all 
> architectures

Would it be sufficient to add  #ifndef PPC  or some kind of conditional
around the  '#define ALIGN_64'  definitions ?

> XEN_GUEST_HANDLE usage in libvirt:
> - In my example where XEN_GUEST_HANDLE is in libxc libvirt currently 
> uses a  union based structure like the following:
> struct xen_v2s3_getdomaininfolistop {
>   domid_t   first_domain;
>   uint32_t  max_domains;
>   union {
>       struct xen_v2d5_getdomaininfo *v;
>       uint64_t pad ALIGN_64;
>   } buffer;
>   uint32_t  num_domains;
> };
> typedef struct xen_v2s3_getdomaininfolistop xen_v2s3_getdomaininfolistop;
> 
> The union used here to add the 64 bit padding does not work on big 
> endian powerpc because reading this two half words in 64bit receiver 
> gives 0xFFFFFFFF00000000 (The two half words are stored in the wrong 
> order) while  on ia64 it is read as the the valid 0x00000000FFFFFFFF.

I'm fine with any suggestions for changes to the structs above to make it
work correctly on big-endian, provided  they don't involve copying code 
from libxc

Regards,
Dan.
-- 
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