[SeaBIOS] [Qemu-devel] [edk2] (PAM stuff) reset doesn't work on OVMF + SeaBIOS CSM

[Laszlo Ersek]

On 02/19/13 19:41, Gleb Natapov wrote:
> On Tue, Feb 19, 2013 at 06:35:03PM +0000, David Woodhouse wrote:
>> On Tue, 2013-02-19 at 20:13 +0200, Gleb Natapov wrote:
>>>
>>>> I take it you mean copy 0xfffe0000 to 0xe0000?  That would not be
>>> fun.
>>>> SeaBIOS would need to detect that it's in the state (it's definitely
>>>> not correct to do that on real-hardware or on "working" kvm
>>>> instances), then setup a trampoline somewhere outside of
>>>> 0xe0000-0xfffff to do the memcpy, jump to that trampoline, copy the
>>>> memory, restore segment registers, and then jump to 0xfffffff0.
>>>> That's a lot of kvm specific code to add to seabios as a workaround
>>>> and it seems fragile anyway.
>>>>
>>> Isn't this exactly what qemu_prep_reset() is doing now?
>>
>> No. It doesn't do the trampoline thing because it doesn't *have* to;
>> it's copying an identical copy of the code back over itself.
>>
> Ah, yes of course. So does CSM takes the whole 0xe0000-0xfffff segment or
> it leaves OVMF code there somewhere. CSM reset code can jump into OVMF
> code in 0xe0000-0xfffff range and let it do the copy.

I think the only thing you could know about the UEFI environment
(call-wise or jump-wise) while in the CSM is ReverseThunkCallSegment /
ReverseThunkCallOffset. Theoretically those allow the CSM to call back
into EfiCompatibility (32-bit protected mode environment).

Some problems:
- Using the reverse thunk might only be allowed if we ended up in real
mode coming through the forward thunk to begin with. When qemu/kvm
simply jumps to 0xffff0 (reset request from guest OS), this doesn't hold.

- Currently no reverse thunk functions are defined in the CSM spec, and
the implementation in TianoCore seems ... absent. The directory
"IntelFrameworkModulePkg/Csm/LegacyBiosDxe/Ipf" appears to contain some
incomplete Itanium assembly.

Anyway David is fixing qemu to reset the PAMs at hard reset, so OVMF
should show up again in the f-segment, accomodating older kvm hosts.

Laszlo