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