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Re: [libvirt] [PATCH 05/14] qemu: Implement NVDIMM



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 redhat com>
> > 
> > 
> >> +        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
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