Hi David,
On 06/17/19 12:52, David Woodhouse wrote:
> On Mon, 2014-05-12 at 20:21 +0400, Mike Maslenkin wrote:
>>>
>>>>> + Segment0 = 0;
>>>>> + Segment0Pages = 1;
>>>>> + Status = gBS->AllocatePages (AllocateAddress, EfiReservedMemoryType,
>>>>> + Segment0Pages, &Segment0);
>>>>> + if (EFI_ERROR (Status)) {
>>>>> + goto RestorePam1;
>>>>> + }
>>>>
>>>> If CSM is enabled, we will fail to allocate, right?
>>
>> Allocation at LegacyBiosInstall() function will fail, but no one cares
>> about it and MemoryAddress remains uninitialized. This is because uefi
>> video driver is being initialized earlier.
>
> Right... at the time the above code runs, the CSM has already been
> initialised and installed a stub 'iret' handler for INT 10h, which in
> my case happens to be at F000:F065.
>
> Because the CSM chose to put that in the F-segment not the E-segment,
> that happens not to trigger the check for an existing handler:
>
> //
> // Check if a video BIOS handler has been installed previously -- we
> // shouldn't override a real video BIOS with our shim, nor our own shim if
> // it's already present.
> //
> Handler = (Int0x10->Segment << 4) + Int0x10->Offset;
> if (Handler >= SegmentC && Handler < SegmentF) {
> DEBUG ((EFI_D_INFO, "%a: Video BIOS handler found at %04x:%04x\n",
> __FUNCTION__, Int0x10->Segment, Int0x10->Offset));
> return;
> }
>
> So InstallVbeShim() goes ahead and copies the shim to the C-segment and
> points the INT10 vector to it (at C000:0200 it seems).
>
> Later, LegacyBiosInstallRom() shadows the video OpROM, stomping on the
> shim. The very *next* thing it does before actually invoking the newly-
> shadowed OpROM is make an INT 10h call to put the display into a plain
> text mode. Which blows up since there's nothing useful at C000:0200 any
> more.
>
>
> There are a few ways we could fix this...
>
> If I just move that PrepareToScanRom hook invocation (that sets the
> text mode) to happen before the CopyMem() of the shadowing, that makes
> things work again. But mostly by luck.
>
> If I change the check in InstallVbeShim() to be '<= SegmentF' then the
> VBE shim won't install itself even over the CSM's iret stub. This is
> basically equivalent to making the VBE Shim refuse to install if
> CSM_ENABLE is set. And might be the right thing to do, since the VBE
> Shim isn't enough to actually make legacy code work.
>
> It might also work if you were to allocate the space for the VBE shim
> so that we don't later try to shadow the real ROM to the same location.
>
> Or maybe we should be letting the legacy BIOS video driver take
> precedence if the CSM has a video BIOS, and not letting the native
> drivers bind at all?
>
In 2013, you submitted the following patch:
OvmfPkg: Don't build in QemuVideoDxe when we have CSM
The thread starts here:
https://www.mail-archive.com/[email protected]/msg01871.html
After an update:
http://mid.mail-archive.com/[email protected]
I had given my R-b:
http://mid.mail-archive.com/[email protected]
But, the patch was never merged.
The commit hash referenced in those messages still works (pointing into your
personal repo):
http://git.infradead.org/users/dwmw2/edk2.git/commitdiff/22253c949e5
Can you resubmit that patch please?
Thanks,
Laszlo
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