On Fri, Oct 21, 2016 at 11:31:41AM +0200, Igor Mammedov wrote: > On Thu, 20 Oct 2016 14:56:58 -0200 > Eduardo Habkost <ehabk...@redhat.com> wrote: > > > On Thu, Oct 20, 2016 at 05:35:38PM +0200, Igor Mammedov wrote: > > > On Thu, 20 Oct 2016 12:47:34 -0200 > > > Eduardo Habkost <ehabk...@redhat.com> wrote: > > > > > > > On Thu, Oct 20, 2016 at 04:15:21PM +0200, Igor Mammedov wrote: > > > > > On Thu, 20 Oct 2016 11:56:10 -0200 > > > > > Eduardo Habkost <ehabk...@redhat.com> wrote: > > > > > > > > > > > On Thu, Oct 20, 2016 at 03:42:15PM +0200, Igor Mammedov wrote: > > > > > > > On Thu, 20 Oct 2016 21:11:38 +0800 > > > > > > > Haozhong Zhang <haozhong.zh...@intel.com> wrote: > > > > > > > > > > > > > > > On 10/20/16 14:34 +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 don't know how this can be implemented in QEMU. Specially, > > > > > > > > how does > > > > > > > > the memory-backend-file know it's used for vNVDIMM, so that it > > > > > > > > can > > > > > > > > skip the 'size' property? > > > > > > > Does memory-backend-file needs to know that it's used by NVDIMM? > > > > > > > Looking at nvdimm_realize it doesn't as it's assumes > > > > > > > hostemem_size == pmem_size + label_size > > > > > > > > > > > > > > I'd make hostmem_file.size optional and take size from file > > > > > > > and if 'size' is specified explictly require it to mach file size. > > > > > > > It's generic and has nothing to do with nvdimm. > > > > > > > > > > > > We can take size from file, or take size from the > > > > > > host_memory_backend_get_memory() callers. > > > > > > > > > > > > Enumerating all sizes that QEMU can use as input: > > > > > > > > > > > > A) Backend file size > > > > > > B) memory backend "size" option > > > > > > C) frontend-provided size (-numa size, -m, or pc-dimm "size" > > > > > > property) > > > > > -numa size affect only anon memory not backend backed one, for > > > > > backend baked memory we use memdev where size comes from backend > > > > > > > > > > pc-dimm.size is readonly and isn't supposed to influence backend.size > > > > > > > > > > I'd drop C option > > > > > > > > If C is not present, it should be, as it affects the guest ABI > > > > (and the ABI must never depend on the host you are running or > > > > backend configuration, only on the frontend configuration). > > > I've meant that C should not affect behavior of backend. > > > > > > > If we are dropping -numa size in favor of the > > > > memory-backend-provided size, that's a bug. > > > -numa size is not applicable here as it's not using backends, > > > when backends are used it's -numa memdev instead in which case > > > numa_info[nodenr].node_mem = object_property_get_int(o, "size", NULL); > > > > > > > OK. My suggestion is to change this to not ignore the size > > option, but to validate it and/or do whatever necessary to get a > > MemoryRegion of the right size (your suggestion below to split > > the memory region would work too). > > > > In other words, this should work: > > > > $ qemu -object memory-backend-file,id=mem0,mem-path=/tmp/mempath,size=2G \ > > -numa node,size=2G,memdev=mem0 -m 2G > > > > this must error out: > > > > $ qemu -object memory-backend-file,id=mem0,mem-path=/tmp/mempath,size=2G \ > > -numa node,size=4G,memdev=mem0 -m 2G > > > > and this should either error out, or result in a 1GB NUMA node > > (but never in a 2G NUMA node): > > > > $ qemu -object memory-backend-file,id=mem0,mem-path=/tmp/mempath,size=2G \ > > -numa node,size=1G,memdev=mem0 -m 1G > Looks like a bit of over-engineering and mixing together backend with > frontend, > I don't see a compelling reason to support > -numa size=X,memdev=foo > as just memdev=foo is sufficient > > so I'd rather error out if user adds 'size' to -numa memdev=foo
Erroring out would be good enough. I won't argue to allow people to use only a portion of the backend-provided memory unless we have a real use case for it. > > [...] > > > Probably there's only one case where behavior would be different > > than what I was thinking. I would like to be possible to specify > > only C (frontend size), and omit both A and B. For example: > > > > $ mkdir /tmp/mempath > > $ qemu -object memory-backend-file,id=mem0,mem-path=/tmp/mempath \ > > -numa node,size=1G,memdev=mem0 -m 1G > > > > I would like this command to create a 1G file inside /tmp/mempath > > automatically, without the need for an explicit size=1G argument > > to memory-backend-file. > This is still C where frontend manages allocation of backend > (i.e. changes backend's 'size' property) vs being just consumer > of whatever backend provides. I don't want it to change the 'size' property of the backend, I just want to allow the backend to allocate memory on demand in case size was not specified, to make the command-line interface simpler and less error-prone. > Purpose of backend is to create complete/initialized hostside > object regardless of which frontend uses it. > I see all properties of backend (including sizes) as belonging > to hostside which shouldn't be influenced by whatever frontend > it's used. And these properties are specified/managed only by mgmt > side explicitly when creating a backend (either via CLI or monitor/QMP). I agree completely with the above paragraph, but we probably have different definitions of "complete/initialized". To me, "complete" doesn't need to include allocation (unless prealloc is required), and it's OK to let the backend allocate memory on demand to simplify the command-line interface. > > In your example I'd error out with: > "memory-backend-file: requires 'size' to create file in /tmp/mempath" This is what should happen today, yes. But I would like to make 'size' optional later, to make the interface simpler. Anyway, let's wait until I have an actual RFC (which may take a while as it's not high priority to me), otherwise it would be just a theoretical discussion. In the meantime, we can change the code to error out on the examples I gave. -- Eduardo
