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

[Daniel P . Berrangé]

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.

With regards,
Daniel
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