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:
> > On Tue, Jul 11, 2006 at 04:14:12PM +0200, michel.ponceau@xxxxxxxx wrote:
> >
> > Hum, I could see compilers righteously complaining about
> > unsigned char cpumap[];
> > in a structure. Maybe we should default to 256 / 8 but allow at the API level
> > to grow over that value. What we could do is define a default of 256 available
> > in the structure and allow an extra parameter which could point to a larger
> > array like the following:
> > - restore VIR_MAX_CPUS as VIR_STD_MAX_CPUS 256
> > - virDomainGetVcpus(virDomainPtr domain, virVcpuInfoPtr info, int maxinfo,
> > char *lcpumap, int lmaplen);
> >
> > lcpumap and lmaplen being extra arguments for very large arrays
> > for most use case, we will fit in 256 processors, those will be respectively
> > NULL and 0, but assuming we have more than 256 processors:
> > + maxinfo > VIR_STD_MAX_CPUS
> > + lcpumap points to a array of bytes, they are interpreted as an
> > array of cpumap of ((maxinfo + 7) div 8) bytes each. So
> > if lmaplen != ((maxinfo + 7) div 8) * maxinfo then there is an
> > error.
> > in that case the cpumap structures of info are not filled on return.
> >
> > 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
>
> 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);
>
>
> 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.
Oh one other benefit of this, is if the application wishes to use C-99
they will not have to use malloc at all, instead making use of local
declarations & dynamically sized stack variables:
virDomainInfo info;
virDomainGetInfo(domain, &info);
virDomainVcpuInfo cpuInfo[info.nrVirtCpu];
int cpuMapLen = (info.nrVirtCpu + 7) / 8 ;
char cpuMap[cpuMapLen];
virDomainGetVCpus(domain, cpuInfo, info.nrVirtCpu, cpuMap, cpuMapLen);
Regards,
Dan.
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