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Re: [Qemu-devel] Standard vga breakage

From: Stefan Weil
Subject: Re: [Qemu-devel] Standard vga breakage
Date: Sun, 15 Nov 2009 21:16:50 +0100
User-agent: Mozilla-Thunderbird (X11/20090707)

malc schrieb:
> On Sun, 15 Nov 2009, Gleb Natapov wrote:
>> On Sun, Nov 15, 2009 at 06:22:40PM +0300, malc wrote:
>>> On Sun, 15 Nov 2009, Blue Swirl wrote:
>>>> On Sun, Nov 15, 2009 at 3:25 AM, malc <address@hidden> wrote:
>>>>> debian-503-i386-netinst.iso when booted on a machine with `-vga
>>>>> std' just
>>>>> shows black screen (after bios' setup screen)
>>>> Just some observations:
>>>> - if you press enter, installer starts and Linux is able to fix the
>>>> display
>>>> - the screen is fine with -bios pcbios.bin, so the issue is related
>>>> to SeaBIOS
>>> Yes i forgot to mention that -bios works, also worth noting is that
>>> before
>>> the commit that fixed cirrus -bios hack also made it work.

Here is a summary of my analysis of this problem:

Graphics mode memory (ram 0xe0000000 ...) is not mapped
in QEMU address space (which results in unassigned
memory writes) for standard pci vga (-vga std), so users
don't get vga display output with grub (in graphical mode),
debian installer (in graphical mode) or similar boot software.

This happens for PC system emulation using SeaBIOS.
It won't happen when using Bochs BIOS.

Other system emulations which can run standard pci vga
seem to suffer from the same problem: when I run mips
malta using -vga std, it won't display anything on the vga
screen (it works with cirrus pci vga).

Malta has no vga bios, so the error might be in QEMU's
vga code (hw/vga-pci.c). Maybe wrong initial PCI mappings?
Maybe this has always be wrong (which would explain why
git bisect does not work).

pci_vga_init() is called by sun4u, ppc_prep, ppc_oldworld,
ppc_newworld, pc and mips, so all these emulations
share the same potential problem.

Bochs BIOS and PC Linux obviously fix the problem.


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