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hostmem-file: add offset option

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Add an option for hostmem-file to start the memory object at an offset
into the target file. This is useful if multiple memory objects reside
inside the same target file, such as a device node.

In particular, it's useful to map guest memory directly into /dev/mem
for experimentation.

To make this work consistently, also fix up all places in QEMU that
expect fd offsets to be 0.

Signed-off-by: Alexander Graf <graf@amazon.com>
Message-Id: <20230403221421.60877-1-graf@amazon.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
This commit is contained in:
Alexander Graf
2023-04-03 22:14:21 +00:00
committed by David Hildenbrand
parent 886c0453cb
commit 4b870dc4d0
9 changed files with 69 additions and 9 deletions
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6
qemu-options.hx
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@@ -4949,7 +4949,7 @@ SRST
they are specified. Note that the 'id' property must be set. These
objects are placed in the '/objects' path.
``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align,readonly=on|off``
``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align,offset=offset,readonly=on|off``
Creates a memory file backend object, which can be used to back
the guest RAM with huge pages.
@@ -5019,6 +5019,10 @@ SRST
such cases, users can specify the required alignment via this
option.
The ``offset`` option specifies the offset into the target file
that the region starts at. You can use this parameter to back
multiple regions with a single file.
The ``pmem`` option specifies whether the backing file specified
by ``mem-path`` is in host persistent memory that can be
accessed using the SNIA NVM programming model (e.g. Intel