Re: [PATCH V4 04/19] machine: aux-ram-share option

[Steven Sistare <steven.sistare@oracle.com>]

On 12/5/2024 3:25 AM, Markus Armbruster wrote:
> Steve Sistare <steven.sistare@oracle.com> writes:
> 
>> Allocate auxilliary guest RAM as an anonymous file that is shareable
>> with an external process.  This option applies to memory allocated as
>> a side effect of creating various devices. It does not apply to
>> memory-backend-objects, whether explicitly specified on the command
>> line, or implicitly created by the -m command line option.
>>
>> This option is intended to support new migration modes, in which the
>> memory region can be transferred in place to a new QEMU process, by sending
>> the memfd file descriptor to the process.  Memory contents are preserved,
>> and if the mode also transfers device descriptors, then pages that are
>> locked in memory for DMA remain locked.  This behavior is a pre-requisite
>> for supporting vfio, vdpa, and iommufd devices with the new modes.
>>
>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> 
> [...]
> 
>> diff --git a/qemu-options.hx b/qemu-options.hx
>> index dacc979..02b9118 100644
>> --- a/qemu-options.hx
>> +++ b/qemu-options.hx
>> @@ -38,6 +38,9 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
>>       "                nvdimm=on|off controls NVDIMM support (default=off)\n"
>>       "                memory-encryption=@var{} memory encryption object to use (default=none)\n"
>>       "                hmat=on|off controls ACPI HMAT support (default=off)\n"
>> +#ifdef CONFIG_POSIX
>> +    "                aux-ram-share=on|off allocate auxiliary guest RAM as shared (default: off)\n"
>> +#endif
>>       "                memory-backend='backend-id' specifies explicitly provided backend for main RAM (default=none)\n"
>>       "                cxl-fmw.0.targets.0=firsttarget,cxl-fmw.0.targets.1=secondtarget,cxl-fmw.0.size=size[,cxl-fmw.0.interleave-granularity=granularity]\n",
>>       QEMU_ARCH_ALL)
>> @@ -101,6 +104,18 @@ SRST
>>           Enables or disables ACPI Heterogeneous Memory Attribute Table
>>           (HMAT) support. The default is off.
>>   
>> +#ifdef CONFIG_POSIX
>> +    ``aux-ram-share=on|off``
>> +        Allocate auxiliary guest RAM as an anonymous file that is
>> +        shareable with an external process.  This option applies to
>> +        memory allocated as a side effect of creating various devices.
>> +        It does not apply to memory-backend-objects, whether explicitly
>> +        specified on the command line, or implicitly created by the -m
>> +        command line option.
>> +
>> +        Some migration modes require aux-ram-share=on.
> 
> This leaves the one thing users really need to know unsaid: when exactly
> should users enable it.
> 
> "Some migration modes require aux-ram-share=on": do they enable it by
> default, or is that left to the user?  If the latter, why?
> 
> Please document the default, whatever it is.

How about:

   ``aux-ram-share=on|off`
       ...
       command line option.  The default is off.

       To use the cpr-transfer migration mode, you must set aux-ram-share=on.

cpr-transfer is a forward reference at this point in the series, so I will
move that last line to the "cpr-transfer mode" patch.

- Steve