On Thu, Feb 23, 2017 at 11:16:17AM +0100, Michal Privoznik wrote: > On 02/23/2017 11:02 AM, Daniel P. Berrange wrote: > > On Thu, Feb 23, 2017 at 10:02:48AM +0100, Michal Privoznik wrote: > >> So, majority of the code is just ready as-is. Well, with one > >> slight change: differentiate between dimm and nvdimm in places > >> like device alias generation, generating the command line and so > >> on. > >> > >> Speaking of the command line, we also need to append 'nvdimm=on' > >> to the '-machine' argument so that the nvdimm feature is > >> advertised in the ACPI tables properly. > >> > >> Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx> > > > > > >> + if (prealloc && > >> + virJSONValueObjectAdd(props, > >> + "b:prealloc", true, > >> + NULL) < 0) > >> + goto cleanup; > > > > As discussed on IRC, using prealloc with QEMU causes it to memset() > > the first byte of every page of memory to 0. With NVDIMM this is > > obviously corrupting application data stored in the NVDIMM. > > > > Obviously this needs fixing in QEMU, but I would think that libvirt > > needs to block use of prealloc==true when running against existing > > broken QEMU versions, otherwise users are going to be rather upset > > to see their data corrupted on every boot > > They are, but should we work around broken QEMUs? And if so - how do we > detect whether the qemu we are dealing with is broken or already fixed > in that respect? Most likely we'll just have todo a version number check I think, since fixing this isn't going to involve any externally visible change to QEMU. > On the other hand, we can just not set prealloc true for nvdimm. That > will have one downside though - after qemu mmaps() the file, kernel is > not forced to create a private copy of the pages. If the guest has requested prealloc, then I don't think we can ignore it for NVDIMM, because its a clear violation of the memory guarantee we're claiming to provide apps. Thus I think we've no option but to report an error if we see prealloc + broken QEMU Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://entangle-photo.org -o- http://search.cpan.org/~danberr/ :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
