Re: [Xen-devel] [PATCH] libxl: remove LIBXL_MAXMEM_CONSTANT

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 12 Mar 2015, Ian Campbell wrote:
> On Thu, 2015-03-12 at 11:02 +0000, Stefano Stabellini wrote:
> > On Thu, 26 Feb 2015, Ian Campbell wrote:
> > > On Thu, 2015-02-26 at 12:19 +0000, Stefano Stabellini wrote:
> > > > On Wed, 25 Feb 2015, Don Slutz wrote:
> > > > > On 02/25/15 10:07, Stefano Stabellini wrote:
> > > > > > LIBXL_MAXMEM_CONSTANT is used to increase the maxmem setting for a
> > > > > > domain by a constant amount. As it is not clear the reason why we 
> > > > > > should
> > > > > > be doing this, remove the constant.
> > > > > > 
> > > > > > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > > > > > CC: mlatimer@xxxxxxxx
> > > > > > CC: ian.campbell@xxxxxxxxxx
> > > > > > ---
> > > > > 
> > > > > I think that some sort of link to commit 901230f in QEMU:
> > > > > 
> > > > > ----
> > > > > commit 901230fd8ce053cc21312a2eca2f3ba9f1d103f2
> > > > > Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > > > > Date:   Wed Dec 3 08:15:19 2014 -0500
> > > > > 
> > > > >     xen-hvm: increase maxmem before calling xc_domain_populate_physmap
> > > > > 
> > > > >     Increase maxmem before calling xc_domain_populate_physmap_exact to
> > > > >     avoid the risk of running out of guest memory. This way we can 
> > > > > also
> > > > >     avoid complex memory calculations in libxl at domain construction
> > > > >     time.
> > > > > 
> > > > >     This patch fixes an abort() when assigning more than 4 NICs to a 
> > > > > VM.
> > > > > 
> > > > >     upstream-commit-id: c1d322e6048796296555dd36fdd102d7fa2f50bf
> > > > > 
> > > > >     Signed-off-by: Stefano Stabellini 
> > > > > <stefano.stabellini@xxxxxxxxxxxxx>
> > > > >     Signed-off-by: Don Slutz <dslutz@xxxxxxxxxxx>
> > > > > ----
> > > > > 
> > > > > Because after this patch and without a "correct" QEMU, the number of
> > > > > e1000 NICs a guest can use is less then 4.
> > > > 
> > > > That is true, in fact is not even a single emulated NIC in my tests.
> > > > I can either ask for a backport of
> > > > c1d322e6048796296555dd36fdd102d7fa2f50bf "xen-hvm: increase maxmem
> > > > before calling xc_domain_populate_physmap" to all QEMU stable branches,
> > > 
> > > It can't hurt to ask, I think?
> > > 
> > > > or we just have to keep this around for now and maybe just add a comment
> > > > on why it is needed.
> > > 
> > > (assuming they say no to the backports)
> > > 
> > > Could we at least make it x86/HVM specific?
> >  
> > The backport was made but only to 2.2 that is far too recent still. I
> > think we should keep the constant for the Xen 4.6 release. But we should
> > defenitely write a comment on what's for.
> 
> Can we also make it x86/HVM only?
> 
> What about scaling it with the number of NICs? Don't we already do
> something like that?

I would leave it like this for this release, then remove the constant
entirely at the beginning of the next one, when the required QEMU patch
is going to be more widespread.  We will have to introduce the concept
of minimum QEMU version supported by Xen, but at that point requiring
QEMU >= v2.2 won't be so bad. 


> > ---
> > 
> > Document what is the purpose of LIBXL_MAXMEM_CONSTANT
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > 
> > ---
> > 
> > http://bugs.xenproject.org/xen/bug/23
> > 
> > diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> > index 934465a..f9e04d8 100644
> > --- a/tools/libxl/libxl_internal.h
> > +++ b/tools/libxl/libxl_internal.h
> > @@ -89,6 +89,14 @@
> >  #define LIBXL_QEMU_BODGE_TIMEOUT 2
> >  #define LIBXL_XENCONSOLE_LIMIT 1048576
> >  #define LIBXL_XENCONSOLE_PROTOCOL "vt100"
> > +/* LIBXL_MAXMEM_CONSTANT is used to set maxmem higher than the actual 
> > amount of
> > + * memory of the domain by a constant amount at creation time.
> > + * The extra memory is allocated by upstream QEMU based device models up
> > + * to v2.1 for the ROM files of emulated network cards.
> > + *
> > + * v2.2 and later QEMUs are able to increase maxmem themselves whenever 
> > needed.
> > + * qemu-xen-traditional doesn't allocate memory for ROM files.
> > + */
> >  #define LIBXL_MAXMEM_CONSTANT 1024
> >  #define LIBXL_PV_EXTRA_MEMORY 1024
> >  #define LIBXL_HVM_EXTRA_MEMORY 2048
> 
> 

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