Re: [Xen-devel] [PATCH] add count-only option to DOM0_GETDOMAININFOLIST hypercall

[Anthony Liguori <aliguori@xxxxxxxxxx>]

There's a race condition here.  There's no way of knowing that domains 
haven't been destroyed (or created) in between these calls.
Unfortunately, the only way to avoid this is grab them one at a time.

Regards,

Anthony Liguori

Mike D. Day wrote:
> signed-off-by: Mike D. Day <ncmike@xxxxxxxxxx>
>
> When retreiving a list of domain info structs it would be helpful
> to know the current domain count prior to making the hcall. This 
> would make it simpler to allocate a return buffer of the 
> appropriate size. 
>
> Adding the six lines to the case for the GETDOMAININFOLIST dom0 op
> allows the caller to retrieve only the count of the domains. The 
> use model is as follows: 
>
>     int number_domains;
>     struct dom0_op info;
>     info.cmd = DOM0_GETDOMAININFOLIST;
>     info.u.getdomaininfolist.max_domains = 0;
>
>     HYPERCALL_dom0_op(&info);
>
>     number_domains = info.u.getdomaininfolist.max_domains;
>
> Having the number of domains, the caller can now allocate
> a buffer of the correct size to get the actual domain 
> information.    
>
> # HG changeset patch
> # User mdday@xxxxxxxxxxxxxxxxxxxx
> # Node ID cd6869742400411fdaf295cc4e0167361e967619
> # Parent  5d3c2cb42ec41984cb1e586d3e47a8692eb8b132
> modify domain infolist hypercall to optionally return only a count of domains
>
> diff -r 5d3c2cb42ec4 -r cd6869742400 xen/common/dom0_ops.c
> --- a/xen/common/dom0_ops.c     Wed Mar 22 19:05:50 2006 +0100
> +++ b/xen/common/dom0_ops.c     Wed Mar 22 19:30:28 2006 -0500
> @@ -416,6 +416,12 @@ long do_dom0_op(GUEST_HANDLE(dom0_op_t) 
>  
>          for_each_domain ( d )
>          {
> +            if (op->u.getdomaininfolist.max_domains == 0) 
> +            {
> +                num_domains++;
> +                continue;
> +            }
> +                
>              if ( d->domain_id < op->u.getdomaininfolist.first_domain )
>                  continue;
>              if ( num_domains == op->u.getdomaininfolist.max_domains )
>
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>   

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