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

[David Hildenbrand <david@redhat.com>]

> @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>

-- 
Cheers,

David / dhildenb