Jan Kiszka <jan.kiszka@xxxxxxxxxxx> writes:
> On 2012-09-11 05:02, Kevin O'Connor wrote:
>> On Mon, Sep 10, 2012 at 11:25:38AM +0200, Jan Kiszka wrote:
>>> On 2012-09-09 17:45, Avi Kivity wrote:
>>>> On 09/07/2012 11:50 AM, Jan Kiszka wrote:
>>>>>
>>>>>> + } else {
>>>>>> + cpu_physical_memory_rw(run->mmio.phys_addr,
>>>>>> + run->mmio.data,
>>>>>> + run->mmio.len,
>>>>>> + run->mmio.is_write);
>>>>>> + }
>>>>>> +
>>>>>> ret = 0;
>>>>>> break;
>>>>>> case KVM_EXIT_IRQ_WINDOW_OPEN:
>>>>>>
>>>>>
>>>>> Great to see this feature for KVM finally! I'm just afraid that this
>>>>> will finally break good old isapc - due to broken Seabios. KVM used to
>>>>> "unbreak" it as it didn't respect write protections. ;)
>>>>
>>>> Can you describe the breakage?
>>>
>>> Try "qemu -machine isapc [-enable-kvm]". Seabios is writing to some
>>> read-only marked area. Don't recall where precisely.
>>
>> On boot, QEMU marks the memory at 0xc0000-0x100000 as read-only.
>
> Only the remapped BIOS ROM (0xe0000-0xfffff) is read-only. And that's
> where SeaBIOS apparently wants to write to.
>
>> SeaBIOS then makes the area read-write, performs its init, and then
>> makes portions of it read-only before launching the OS.
>
> What does it do if there is no PAM? Nothing?
>
>>
>> The registers SeaBIOS uses to make the memory read-write are on a PCI
>> device. On isapc, this device is not reachable, and thus SeaBIOS
>> can't make the memory writable.
>
> On isapc, this device and all the PAM does not even exist.
>
>>
>> The easiest way to fix this is to change QEMU to boot with the area
>> read-write. There's no real gain in booting with the memory read-only
>> as the first thing SeaBIOS does is make it read-write.
>
> Considering SeaBIOS, that is true. If Seabios depends inherently on
> shadow ROMs and as we have no real chipset for isapc to control
> shadowing behavior, that will likely be the best option. Can have a
> look.
I've never really understood this.
Why do we need ISAPC? An ISA-only OS would still be okay on a system
with an i440fx and no PCI devices, no?
I think that makes a lot more sense because then SeaBIOS doesn't have to
deal with the notion of ISAPC.
Regards,
Anthony Liguori
>
> Jan
>
> --
> Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
> Corporate Competence Center Embedded Linux
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