Re: Bug in protected mode segments?

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
> Mathias Gottschlag wrote:
>>
>> Grub inits the CPU with no paging and flat segmenting, I now activate 
>> segments to move the code to 0xE0000000 and later activate paging and 
>> reset the segments. When I run this code on qemu or on real hardware 
>> (Athlon XP), everything works well, but on kvm I get several hangs. 
>> For example I try to write to 0xE00B8000 to write into the VGA 
>> framebuffer. This addres should now get translated back to 0xB8000 by 
>> the segment which has the base address 0x20000000, and this 
>> definately works on real hardware, but on kvm I only notice a hang at 
>> the instruction which writes at that address (kvm still responds, but 
>> doesn't update eip anymore, execution stops.
>
> I looked briefly and there didn't appear to be any explicit checks for 
> wrap around but I think that it will work correctly since we're always 
> using at least unsigned long in the host.  My suspicion is that we're 
> somehow getting something wrong with MMIO decoding.

Well, unsigned long is 64-bits on a 64-bit host, so the generated 
address will be 0x1000b8000 instead of 0xb8000.  So the problem here is 
likely to be a missing wraparound, rather than a wraparound.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.