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Re: [PATCH v2] machine: add missing doc for memory-backend option

From: Michal Privoznik
Subject: Re: [PATCH v2] machine: add missing doc for memory-backend option
Date: Fri, 15 Jan 2021 10:36:05 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 1/15/21 12:46 AM, Igor Mammedov wrote:
Add documentation for '-machine memory-backend' CLI option and
how to use it.

And document that x-use-canonical-path-for-ramblock-id,
is considered to be stable to make sure it won't go away by accident.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
  - add doc that x-use-canonical-path-for-ramblock-id is considered stable,
    (Peter Krempa <pkrempa@redhat.com>)
  backends/hostmem.c | 10 ++++++++++
  qemu-options.hx    | 28 +++++++++++++++++++++++++++-
  2 files changed, 37 insertions(+), 1 deletion(-)

@@ -96,6 +97,31 @@ SRST
          Enables or disables ACPI Heterogeneous Memory Attribute Table
          (HMAT) support. The default is off.
+     ``memory-backend='id'``
+        An alternative to legacy ``-mem-path`` and ``mem-prealloc`` options.
+        Allows to use a memory backend as main RAM.
+        For example:
+        ::
+        -object 
+        -machine memory-backend=pc.ram
+        -m 512M
+        Migration compatibility note:
+        a) as backend id one shall use value of 'default-ram-id', advertised by
+        machine type (available via ``query-machines`` QMP command)
+        b) for machine types 4.0 and older, user shall
+        use ``x-use-canonical-path-for-ramblock-id=on`` backend option
+        (this option must be considered stable, as if it didn't have the 'x-'
+        prefix including deprecation period, as long as 4.0 and older machine
+        types exists),
+        if migration to/from old QEMU (<5.0) is expected.
+        For example:
+        ::
+        -object 
+        -machine memory-backend=pc.ram
+        -m 512M

Igor, this doesn't correspond with your comment in bugzilla:


In fact, we had to turn the attribute OFF so that canonical path is not used. Isn't ON the default state anyway?


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