Re: [Xen-devel] Re: [Patch] update cpumask handling for cpu pools in libxc and python

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

Please ignore previous mail, I hit the send-button too early...

On 09/17/10 11:44, Ian Campbell wrote:
> On Fri, 2010-09-17 at 10:20 +0100, Juergen Gross wrote:
> > On 09/17/10 11:00, Ian Campbell wrote:
> > > local_size has already been rounded up in get_cpumap_size. Do we really
> > > need to do it again?
> > I've made it more clear that this is rounding to uint64.
> Wouldn't that be "(.. + 63) / 8" then?
No, local_size is already bytes...

> > > > +    size = sizeof(xc_cpupoolinfo_t) + cpumap_size * 8 + local_size;
> > > Why do we need both "cpumap_size * 8" and local_size additional bytes
> > > here? Both contain the number of bytes necessary to contain a cpumap
> > > bitmask and in fact I suspect they are both equal at this point (see
> > > point about rounding above).
> > The hypervisor returns a cpumask based on bytes, the tools use uint64-based
> > cpumasks.
> Oh, I see, as well as xc_cpupool_info_t and the cpumap which it contains
> being allocated together in a single buffer you are also including a
> third buffer which is for local use in this function only but which is
> included in the memory region returned to the caller (who doesn't know
> about it). Seems a bit odd to me, why not just allocate it locally then
> free it (or use alloca)?
>
> Actually, when I complete my hypercall buffer patch this memory will
> need to be separately allocated any way since it needs to come from a
> special pool. I'd prefer it if you just used explicit separate
> allocation for this buffer from the start.
Okay.

> >   In practice this is equivalent as long as multiple of 8 bytes are
> > written by the hypervisor and the system is running little endian.
> > I prefer a clean interface mapping here.
> Using a single uint64 when there was a limit of 64 cpus made sense but
> now that it is variable length why not just use bytes everywhere? It
> would avoid a lot of confusion about what various size units are at
> various points etc. You would avoid needing to translate between the
> hypervisor and tools representations too, wouldn't you?
This would suggest changing xc_vcpu_setaffinity() and xc_vcpu_getaffinity(),
too.

--
Juergen Gross                 Principal Developer Operating Systems
TSP ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
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cpupool-tools-cpumask.patch
Description: Text Data

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