On Thu, Oct 20, 2016 at 05:41:47PM +0200, Igor Mammedov wrote:
> On Thu, 20 Oct 2016 13:15:31 -0200
> Eduardo Habkost <ehabk...@redhat.com> wrote:
>
> > On Thu, Oct 20, 2016 at 04:17:35PM +0200, Igor Mammedov wrote:
> > > On Thu, 20 Oct 2016 11:47:18 -0200
> > > Eduardo Habkost <ehabk...@redhat.com> wrote:
> > >
> > > > On Thu, Oct 20, 2016 at 09:33:53PM +0800, Haozhong Zhang wrote:
> > > > > On 10/20/16 11:21 -0200, Eduardo Habkost wrote:
> > > > > > On Thu, Oct 20, 2016 at 02:34:12PM +0200, Igor Mammedov wrote:
> > > > > > > On Thu, 20 Oct 2016 14:13:01 +0800
> > > > > > > Haozhong Zhang <haozhong.zh...@intel.com> wrote:
> > > > > > >
> > > > > > > > If a file is used as the backend of memory-backend-file and its
> > > > > > > > size is
> > > > > > > > not identical to the property 'size', the file will be
> > > > > > > > truncated. For a
> > > > > > > > file used as the backend of vNVDIMM, its data is expected to be
> > > > > > > > persistent and the truncation may corrupt the existing data.
> > > > > > > I wonder if it's possible just skip 'size' property in your case
> > > > > > > instead
> > > > > > > 'notrunc' property. That way if size is not present one'd get
> > > > > > > actual size
> > > > > > > using get_file_size() and set 'size' to it?
> > > > > > > And if 'size' is provided and 'size' != file_size then error out.
> > > > > > >
> > > > > >
> > > > > > I think it is valid to start with a zero-size file and then let
> > > > > > QEMU extend it.
> > > > >
> > > > > For vNVDIMM, extending from zero-size file can be valid when a file is
> > > > > first used. However, it's not valid for the second and following use
> > > > > of the same file.
> > > > >
> > > > > > But I agree we should: 1) make 'size' optional as
> > > > > > you suggested; 2) never truncate the file to a smaller size.
> > > > > >
> > > > >
> > > > > I will add another patch for this. Is there any way in QEMU to decide
> > > > > whether a memory-backend-file object is used for vNVDIMM when the
> > > > > object is being created? Or 'size' can be optional for all kinds of
> > > > > usages?
> > > >
> > > > I believe 'size' can be optional for all usage, because at the
> > > > moment the memory allocation code asks the backend for a memory
> > > > region, it is supposed to know desired RAM size from the frontend
> > > > configuration (-numa, -m, or "size" property of pc-dimm).
> > >
> > > nope, currently the size propagates other way around
> > > from back-end to front-end and not backwards
> >
> > I'd say that this is a bug. Frontend size is guest ABI and
> > shouldn't be overridden by backend configuration if it's
> > explicitly set.
> frontend.size is always <= backend.size
>
> allocation specified when backend is created (-object/object_add)
> and front end size if needed/used is <= backend size
>
> so far code followed this design.
This would work, but it doesn't happen in the case of -numa:
$ ./x86_64-softmmu/qemu-system-x86_64 \
-object memory-backend-file,id=mem0,mem-path=/tmp/mempath,size=2G \
-numa node,size=1G,memdev=mem0 -m 1G
qemu-system-x86_64: total memory for NUMA nodes (0x80000000) should equal RAM
size (0x40000000)
That's a bug we need to fix.
--
Eduardo
