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

[Steven Sistare]

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