Re: [PATCH v1 1/3] softmmu/physmem: fallback to opening guest RAM file as readonly in a MAP_PRIVATE mapping

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Aug 17, 2023 at 04:37:52PM +0200, David Hildenbrand wrote:
> On 17.08.23 16:37, Daniel P. Berrangé wrote:
> > On Thu, Aug 17, 2023 at 04:30:16PM +0200, David Hildenbrand wrote:
> > > 
> > > > @Stefan, do you have any concern when we would do 1) ?
> > > > 
> > > > As far as I can tell, we have to set the nvdimm to "unarmed=on" either way:
> > > > 
> > > > +   "unarmed" controls the ACPI NFIT NVDIMM Region Mapping Structure "NVDIMM
> > > > +   State Flags" Bit 3 indicating that the device is "unarmed" and cannot accept
> > > > +   persistent writes. Linux guest drivers set the device to read-only when this
> > > > +   bit is present. Set unarmed to on when the memdev has readonly=on.
> > > > 
> > > > So changing the behavior would not really break the nvdimm use case.
> > > 
> > > Looking into the details, this seems to be the right thing to do.
> > > 
> > > This is what I have now as patch description, that also highlights how libvirt
> > > doesn't even make use of readonly=true.
> > > 
> > > 
> > >  From 42f272ace68e0cd660a8448adb5aefb3b9dd7005 Mon Sep 17 00:00:00 2001
> > > From: David Hildenbrand <david@redhat.com>
> > > Date: Thu, 17 Aug 2023 12:09:07 +0200
> > > Subject: [PATCH v2 2/4] backends/hostmem-file: Make share=off,readonly=on
> > >   result in RAM instead of ROM
> > > 
> > > For now, "share=off,readonly=on" would always result in us opening the
> > > file R/O and mmap'ing the opened file MAP_PRIVATE R/O -- effectively
> > > turning it into ROM.
> > > 
> > > As documented, readonly only specifies that we want to open the file R/O:
> > > 
> > >      @readonly: if true, the backing file is opened read-only; if false,
> > >          it is opened read-write.  (default: false)
> > > 
> > > Especially for VM templating, "share=off" is a common use case. However,
> > > that use case is impossible with files that lack write permissions,
> > > because "share=off,readonly=off" will fail opening the file, and
> > > "share=off,readonly=on" will give us ROM instead of RAM.
> > > 
> > > With MAP_PRIVATE we can easily open the file R/O and mmap it R/W, to
> > > turn it into COW RAM: private changes don't affect the file after all and
> > > don't require write permissions.
> > > 
> > > This implies that we only get ROM now via "share=on,readonly=on".
> > > "share=off,readonly=on" will give us RAM.
> > > 
> > > The sole user of ROM via memory-backend-file are R/O NVDIMMs. They
> > > also require "unarmed=on" to be set for the nvdimm device.
> > > 
> > > With this change, R/O NVDIMMs will continue working even if
> > > "share=off,readonly=on" was specified similar to when simply
> > > providing ordinary RAM to the nvdimm device and setting "unarmed=on".
> > > 
> > > Note that libvirt seems to default for a "readonly" nvdimm to
> > > * -object memory-backend-file,share=off (implying readonly=off)
> > > * -device nvdimm,unarmed=on
> > > And never seems to even set "readonly=on" for memory-backend-file. So
> > > this change won't affect libvirt, they already always get COW RAM -- not
> > > modifying the underlying file but opening it R/O.
> > > 
> > > If someone really wants ROM, they can just use "share=on,readonly=on".
> > > After all, there is not relevant difference between a R/O MAP_SHARED
> > > file mapping and a R/O MAP_PRIVATE file mapping.
> > > 
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> > 
> > This still leaves the patch having a warn_report() which I think is
> > undesirable to emit in a valid / supported use case.
> 
> No warning.
> 
> Please elaborate on "valid/supported use case".

The usage scenario that this patch aims to enable. IIUC, it will follow
the codepath that leads to the warn_report() call in this patch.

With regards,
Daniel
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