ASSERT in QemuVideoDxe driver during reset

["Wang, Jian J" <jian.j.wang@intel.com>]

Hi guys,

I found an ASSERT issue in function InstallVbeShim() in QemuVideoDxe driver during reset. The assert statement is like below.

    ASSERT (Int0x10->Segment == 0x0000);
    ASSERT (Int0x10->Offset  == 0x0000);

This happened after I have enabled NULL pointer access detection feature, in which page 0 (4K)  is disabled. And because of page 0 disabled, I have to skip the memory clearing for page 0 in DXE core. Otherwise it will cause page fault exception there. It seems that QEMU may clear all its memory at startup. Skipping the action of clearing page 0 in core won't cause ASSERT issue in QemuVideoDxe, for the first time boot. But QemuVideoDxe will write int10 vector at memory 0x10 and QEMU will not clear all its memory during warm boot. ASSERT will be triggered after reset.

It's easy to fix this issue but there're some subtle situations which I'm not quite certain. I'd like your opinions for them.

Here're my thoughts on several solutions:
a) Remove the ASSERT statement in InstallVbeShim(). But I'm sure if it is safe to do so because I don't quite understand the purpose of the ASSERT. 
b) Instead of skipping clearing page 0, enable it, do clearing and then disable it. The problem here is that CPU arch protocol is not ready at that time. I have to "manually" do page operation, which might be non-portable and a little bit odd in DXE core.
c) Move code clearing page 0 from DXE core to another place wherever appropriate, like DxeIpl or cpu driver. But I think there's a good reason to put code there before.

Thanks,
Wang, Jian J