[Qemu-devel] Use of flash for x86 BIOS (was: [PATCH 0/2] Implement migration support for pflash_cfi01)

[Markus Armbruster]

[Note cc: Jordan, who added flash to x86 in commit bd183c79]

Peter Maydell <address@hidden> writes:

> These patches implement migration support for pflash_cfi01.
> The first patch just drops some useless state so we don't
> have to think about it for migration.
>
> NB that pflash_cfi01 is used in the x86 pc model. I think this
> means that migration while the BIOS is accessing the flash
> wouldn't have worked properly. Since migration from a device
> with no vmstate to one with vmstate works OK this shouldn't
> break cross-version migration. However x86 maintainers may
> wish to review and confirm this for themselves...

x86 maintainers may wish to *switch it off* until it's done fully and
properly, by setting "pc-sysfw" property "rom_only" to 1.

x86 flash use was added in series "[PATCH v10 0/8] PC system flash
support", commit 4732dcaf..1b89fafe, v1.1.  Core code is
pc_system_firmware_init().  Works as follows, as far as I can tell:

* Create a "pc-sysfw" device with default properties.

  This device does nothing in particular, and is *not* guest-visible.
  It appears to be used just as a property container.

* If (default) property "rom_only" is non-zero, map the BIOS ROM the
  traditional way.  Get its contents from the file named by -bios,
  default "bios.bin".

  This is the case for isapc as well as pc-1.0 and older.

* Else, if KVM is enabled, putting BIOS in flash doesn't actually work
  (KVM restriction), so force "rom_only" to one, and map the BIOS ROM
  the traditional way, as above.

  Fatal error if the user defined drive if=pflash,bus=0,unit=0, because
  that's valid only when we're using flash for the BIOS ROM.

  I understand this case is meant to go away when the KVM restriction
  gets lifted.

* Else, map the BIOS ROM as flash device.

  Note: for now, this case applies only when KVM is disabled.

  - If the user did not define said pflash drive, create one with BIOS
    ROM contents (get it the same way as above).  Failure is fatal.

  - Create a "cfi.pflash01" block device backed by the pflash drive.

What does this mean?

1. The "pc-sysfw" device is mostly harmless.

2. Enabling/disabling KVM is guest-visible!  With KVM disabled, you get
   a flash memory device.  With KVM enabled, you get a ROM.  Not good;
   KVM should be as transparent as possible to the guest.

   I raised this issue last August, Jordan told me he's working on
   it[*], and I let the matter rest then.  That was a mistake.

[*] http://lists.nongnu.org/archive/html/qemu-devel/2012-08/msg03178.html