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




This interesting thread is temporarilu suspended because Michel has left for some (long) vacation until August 15th.
I think the last proposal is a good one and deserves to be implemented and tested.
I am confident that Michel will be willing to update its patch accordingly when he comes back.
Is it OK for you ?.

Side question: is there a defined "sign-off" procedure to follow when submitting patches ?

Cordialement.
Jean-Paul



Daniel Veillard <veillard redhat com>
Envoyé par : libvir-list-bounces redhat com

17/07/2006 15:08
Veuillez répondre à veillard

       
        Pour :        "Daniel P. Berrange" <berrange redhat com>
        cc :        libvir-list redhat com, michel ponceau bull net
        Objet :        Re: [Libvir] Re: Proposal : add 3 functions to Libvirt API,        for virtual CPUs


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

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