[Libvir] Re: Proposal : add 3 functions to Libvirt API, for virtual CPUs

Daniel Veillard veillard at redhat.com
Mon Jul 17 13:08:59 UTC 2006

On Mon, Jul 17, 2006 at 01:50:25PM +0100, Daniel P. Berrange wrote:
> On Tue, Jul 11, 2006 at 10:50:53AM -0400, Daniel Veillard wrote:
> >   We still have a relatively simple API for the common case, and for special
> > cases we have an extension capability with relatively clear definitions. it's
> > a bitstrange but I think that should cover most case as best as possible
> I dont particularly like this as an API because I think it will be error
> prone for application developers. Most app developers will only ever have
> a handful of CPUs in their test machines, so they'll never the alternate
> codepath for > 256 cpu case.  Likewise I don't like the idea of a virVcpuInfo
> struct which has a variable size because it will totally confuse people who
> haven't read the API docs very carefully, again leading to obscure bugs.
> The root problem is that we have two conflicting goals here 
>   1. Want to have virVcpuInfo be a fixed size struct
>   2. We want a cpumap of arbitrary size

  Right this is confusing, I wanted to avoid another allocation in the general
case but this made things even more confusing.

> The obvious solution to this problem is to *remove*  the cpumap data from
> the virVcpuInfo structure completely, and always pass in a separately 
> malloc'd array of the correct size. So I'd suggest:
>    typedef struct _virVcpuInfo virVcpuInfo;
>    struct _virVcpuInfo {
>      unsigned int number;        /* virtual CPU number */
>      int state;                  /* value from virVcpuState */
>      unsigned long long cpuTime; /* CPU time used, in nanoseconds */
>      int cpu;                    /* real CPU number, or -1 if offline */
>    }
>   virDomainGetVcpus(virDomainPtr domain, virVcpuInfoPtr info, int maxinfo,
>                     char *cpumap, int maplen);

  Agreed, this is the most logical approach

> The client applications calling this API already have to malloc() the memory
> region for the 'info' parameter of a correct size, so having to also malloc
> the cpumap parameter is no extra trouble.
>   virDomainInfo info;
>   virDomainVpuInfoPtr cpuInfo;
>   int cpuMapLen;
>   char *cpuMap;
>   virDomainGetInfo(domain, &info);
>   cpuInfo = malloc(sizeof(virDomainVcpuInfo)*info.nrVirtCpu);
>   cpuMapLen = (info.nrVirtCpu + 7) / 8 ;
>   cpuMap = malloc(cpuMapLen);
>   virDomainGetVCpus(domain, cpuInfo, info.nrVirtCpu, cpuMap, cpuMapLen);
>   ... do stuff with the data ...
>   free(cpuInfo);
>   free(cpuMap);
> So you can see there is minimal extra work to always pass in cpuMap as 
> a separate parameter. If an application didn't care about the cpuMap
> data they could simply pass in NULL.

  Agree. We should keep an example of use (like above completed) in the source
tree to help developpers.


Daniel Veillard      | Red Hat http://redhat.com/
veillard at redhat.com  | libxml GNOME XML XSLT toolkit  http://xmlsoft.org/
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