On Fri, Jan 12, 2007 at 02:24:12PM +0000, Daniel P. Berrange wrote:
> On Fri, Jan 12, 2007 at 03:04:57PM +0100, Philippe Berthault wrote:
> > Daniel Veillard a écrit :
> > > I think Xen just returns -1 when the field is uninitialized, probably
> > >meaning 'all physical memory'. Once you use "xm mem-max 0 ..." then it
> > >consider the Dom0 domain constrained, but not before. And since there
> > >are machine where it's possible to hot plug new memory this is a way to
> > >not poll the current physical memory, a bit weird but that can be
> > >understood.
> > >
> > The signed value -1, "casted" in C/C++ in unsigned integer, gives the
> > value 4294967295. The Max memory value returned by virsh is 4294967292.
> > There is a small difference (3), so the value returned by Xen can't be -1.
>
> You're forgetting though, that xen measures this in terms of pages. Libvirt
> converts that to bytes. The virsh converts it to KB when printing it.
>
> $cat > demo.c <<EOF
>
> #include <stdio.h>
>
> int main() {
> unsigned int max_pages = ~0U;
> unsigned long long mem = (unsigned long long)max_pages * 4096ull;
> unsigned long memkb = mem / 1024;
>
> printf("%u %llu %lu\n",
> max_pages,
> mem,
> memkb);
> }
>
> EOF
> $ gcc -o demo demo.c
> $ ./demo
> 4294967295 17592186040320 4294967292
Oh wait, a minute, this is bogus - the last figure should be *4 the first
figure - which it clearly isn't. Well, it works on 64-bit platforms, but
on 32-bit platforms the 'unsigned long memkb = mem/1024' bit is overflowing.
It should be reporting 17179869180 kB. Still, this is just a bug in virsh
when printing out the data - the raw data from virDomainGetInfo() is the
correct value.
Dan.
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