Re: [PATCH 3/3] libxl: update vcpus bitmap in retrieved guest config

[Wei Liu <wei.liu2@citrix.com>]

On Tue, Jun 07, 2016 at 05:36:05PM +0100, Anthony PERARD wrote:
> On Tue, Jun 07, 2016 at 05:03:07PM +0100, Wei Liu wrote:
> > On Tue, Jun 07, 2016 at 03:45:29PM +0100, Anthony PERARD wrote:
> > > On Tue, Jun 07, 2016 at 12:24:02PM +0100, Wei Liu wrote:
> > > > ... because the available vcpu bitmap can change during domain life time
> > > > due to cpu hotplug and unplug.
> > > > 
> > > > For QEMU upstream, we interrogate QEMU for the number of vcpus. For
> > > > others, we look directly into xenstore for information.
> > > > 
> > > > Reported-by: Jan Beulich <jbeulich@suse.com>
> > > > Signed-off-by: Wei Liu <wei.liu2@citrix.com>
> > > > ---
> > > >  tools/libxl/libxl.c | 91 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 91 insertions(+)
> > > > 
> > > > diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
> > > > index 006b83f..4f8b663 100644
> > > > --- a/tools/libxl/libxl.c
> > > > +++ b/tools/libxl/libxl.c
> > > > @@ -7222,6 +7222,57 @@ void libxl_mac_copy(libxl_ctx *ctx, libxl_mac *dst, libxl_mac *src)
> > > >          (*dst)[i] = (*src)[i];
> > > >  }
> > > >  
> > > > +static int libxl__update_avail_vcpus_qmp(libxl__gc *gc, uint32_t domid,
> > > > +                                         unsigned int max_vcpus,
> > > > +                                         libxl_bitmap *map)
> > > > +{
> > > > +    unsigned int count, i;
> > > > +    int rc;
> > > > +
> > > > +    /* For QEMU upstream we always need to return the number
> > > > +     * of cpus present to QEMU whether they are online or not;
> > > > +     * otherwise QEMU won't accept the saved state.
> > > > +     */
> > > > +    rc = libxl__qmp_query_cpus(gc, domid, &count);
> > > > +    if (rc) {
> > > > +        LOG(ERROR, "fail to get number of cpus for domain %d", domid);
> > > > +        goto out;
> > > > +    }
> > > > +
> > > > +    for (i = 0; i < count; i++)
> > > > +        libxl_bitmap_set(map, i);
> > > 
> > > What if I have cpu 1, 7 and 42 online, but all the other offline?
> > > 
> > 
> > I have more or less the same question when I wrote this patch. At that
> > time I thought the avail_vcpus was only used for generating -smp option
> > to QEMU.
> 
> Right, and -smp does not let you set a bitmap... I guess having a count
> for this purpose is enough.
> 
> > In your example, you will have -smp 3,maxvcpus=$Y. I think the migration
> > should still succeed. Furthermore, the cpu-add operation doesn't care,
> > so it probably won't have visible effect.
> 
> What does cpu-add not care about?
> 

Never mind. This is too wrong to explain. I was thinking about some
other stuff when writing that.

The libxl API does care about the bitmap, but QEMU parameter doesn't.

> > I agree it would be good to set the exact bits though -- if you can tell
> > me which field to test. 
> > 
> 
> From the example below, I guess it would be "CPU".

Sounds good to me.

Wei.

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