[SeaBIOS] [edk2] investigate features differences between ovmf and seabios with QEMU

Laszlo Ersek lersek at redhat.com
Mon Feb 24 12:59:30 CET 2014

On 02/24/14 05:19, Chen Fan wrote:
> Hi all,
>     I'm a novice with OVMF, so please pardon me if I make any mistake.
>     I have made a list about the feature differences between OVMF and
> Seabios after investigation using QEMU. I simply use seabios source code
> file name as the feature's name, then I made the following categorize
> about OVMF supported/unsupported features. maybe there are some defects
> at the list. and if this list miss some important features, please point
> it out to me.
>   I want to improve this list with you. so I'm welcome any comments and
> suggestions.
> Thanks,
> Chen
> -----------------------------------------------------------------------
>   A: OVMF supports feature which seabios support it
>      (This includes features which have similar bios)
> -----------------------------------------------------------------------
>   1. ata          2. blockcmd     3. dma         4. floppy
>   5. pci          6. pic          7. ps2port     8. rtc
>   9. timer        10. usb-ehci    11. usb-hid    12. usb-hub
>   13. usb-msc     14. usb-uhci    

I'll call these, in a hand-waving manner, "basic PC peripheral support",
and it's edk2 modules that support them, not directly OvmfPkg. IOW
OvmfPkg pulls in / utilizes drivers from the edk2 tree unmodified to
support these. For example, many drivers from under PcAtChipsetPkg/.

The end result is the same (ie. the OVMF.fd build you end up with
supports them), but it's good to be clear about the scope when saying OVMF.

Also, since we're talking "PC", I think we've never tried OVMF on q35
machine types.

>   15. virtio-blk 16. virtio-pci
>   17. virtio-ring 18. virtio-scsi

The virtio support we have could be a bit differently structured, but
mostly "yes".

> 19. pci bus    20. VGA-cirrus
>   21. ISA bridge  22. PCI IRQs    23. ISA        24. pci crs

Re ACPI (since you say PCI CRS): OVMF has some builtin tables, and the
PCI CRS is built dynamically (kudos for guidance from Gerd et al). It
works. When running on qemu machine types / qemu versions up to 1.6,
this is the only ACPI stuff OVMF exports.

I plan to post an OVMF patch soonish (when qemu 1.7.1 is out) that will
enable OVMF to download ACPI tables from qemu, benefiting from Michael
Tsirkin's qemu work. Since SeaBIOS already does that, this should (a)
synchronize ACPI between OVMF and SeaBIOS, (b) give some missing ACPI
features to guest OSes running on OVMF "for free", like PCI hotplug and

>   25. mtrr

Yes, although as I gather this is mostly cosmetic in KVM guests.

> 26. smbios

The SMBIOS integration that you might have become accustomed to between
qemu and SeaBIOS is not upstream OVMF.

I posted patches for that, I run them every day, and Gerd's RPMs
<http://www.kraxel.org/repos/> and IIRC Paolo's Copr builds
<http://copr-be.cloud.fedoraproject.org/results/bonzini/ovmf/> include
them. They are not in the upstream tree because Jordan and myself
disagree on how SMBIOS should be worked out in OVMF.

If I recall correctly, Jordan's point was to factor out SMBIOS code from
another client package (EmulatorPkg if I remember?...), librarize it in
a central edk2 location (like MdeModulePkg), and use this library /
integrate it with qemu in OVMF. My patches add custom / independent code

Jordan's suggestion would involve a lot of work, and modifying the
Mde*Pkg directories (aside from obvious small fixes) is very hard from a
project management POV. Spending cycles on this is not economic for me,
especially after having implemented the custom patches, so I'm not doing it.

>     27. smp

Not sure what this covers. Jordan has out-of-tree WIP for MP services,
and it's possible someone will help with it in the near future.

>        28. suspend

S3 is work in progress. It should make it into the tree soon.

>   29. block       30. boot        

OVMF supports qemu boot order specs, but the huge conceptual difference
between UEFI boot options and the traditional BIOS boot options / the
OpenFirmware expressive power keeps us revising details and fixing bugs.

> 31. cdrom      32. output
>   33. resume      34. serial      35. usb        36. pciint
>   37. optionroms  38. xen         

The Xen community certainly cares about OVMF. For details you'll have to
talk to them.

> 39. vgahooks   40. PCI I/O protcol(pci
> bios)
>   41. DSDT table  42. SEC (post)  43. MADT table 44. font
>   45. mouse       46. disk        47. kbd        48. clock
>   49. memmap      50. Memory Allocation Services (pmm) 
>   51. System table (biosvar)      52. FW_CFG 

"Yes", in general. I'll just single out "font", which to me implies HII
in UEFI / edk2, and it's a beast.

> P.S:
>   the values ​​in parentheses represents the seabios feature.
> ---------------------------------------------------------------------
>   B: OVMF does not support features which seabios support it
> ---------------------------------------------------------------------
>   53. ahci        

Hmm. I think edk2 in general does support AHCI
(seeMdeModulePkg/Bus/Ata/AtaAtapiPassThru), we just don't include it. I
think we should have a good use case for drivers / features; more than
just "feature parity with SeaBIOS".

> 54. usb-ohci    55. esp-scsi   56. lsi-scsi
>   57. magasas

These look like various SCSI HBA's. edk2 probably has no drivers,
indeed. I'm not convinced we need to support them.

> 58. usb-uas     59. usb-xhci

I think the same applies to XHCI as to AHCI, see

>   60. cpu-hotplug
>   61. acpi-dbug   62. VGA-std     63. PIIX4 PM   64. pci-hotplug

I'm not sure if CPU and/or PCI hotplug is very useful during the
firmware's lifetime (and I don't know if the above SeaBIOS files imply
such support). If you mean the ACPI dependencies, they should come "for
free" once we have the ACPI download patch in place.

>   65. GPE         66. hpet        67. q35        68. pvpanic
>   69. processor table             70. smm

No SMM in OVMF; KVM doesn't support it (last I heard). edk2 has
extensive support for SMM otherwise.

>        71. pnp bios

Not sure what to make of this. I'll have a few words about CSM.

>   72. gdb

Source level debugging of OVMF is not solved / supported, at leat not by
tools in the upstream tree. There have been elaborate and working hacks
but they've gone stale. For me personally debug messages beat anything
and everything and lack of gdb is a non-issue.

> ---------------------------------------------------------------------
>   C: OVMF does not need to support feature which seabios support
>      (due to UEFI's spec/implementation)
> ---------------------------------------------------------------------
>   73. ramdisk    74. lzma decoder

Actually edk2 uses LZMA quite a bit. Most of the firmware image is

>  75. biostable   76. coreboot

There are efforts to support coreboot and OVMF in combination (google
CorebootPkg). I have no clue of the scope, and it's not upstream.

>   77. shadow     78. apm
>   D: OVMF supports feature which is required by UEFI spec
>   79. CSM        

David Woodhouse and Kevin O'Connor have done huge work to make this
work, covering both SeaBIOS and edk2/OVMF.

The edk2 patches are not in-tree yet. Actually the precise set of the
edk2 patches to include is not fully "agreed upon" yet. In my local tree
I carry the two critical patches from David on a topic branch. Gerd's
RPMs also provide a pure-UEFI *and* a CSM build of OVMF. (The flavor you
want must be specified at build time.)

David even managed to change the CSM specification -- additional fields
were needed between edk2 and SeaBIOS. v0.98 (IIRC) of the spec is
supposed to describe these too, but I'm not sure about the release
status of this version.

> 80. GPT           81. secureboot

Yes. (Side note: secure boot is per-spec incompatible with CSM.)

> ---------------------------------------------------------------------
>   E: OVMF does not support feature which is required by UEFI spec
> ---------------------------------------------------------------------
>   82. i18n

This is incorrect I'd think (it depends on the scope you have in mind of
course): edk2 has super-extensive support for HII, which covers fonts,
languages etc.

I'm sure I haven't been precise enough in this email, but it should help
ignite the discussion :)


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