Re: [Qemu-devel] [PATCH 3/3] support readonly memory feature in qemu

[Anthony Liguori <aliguori@us.ibm.com>]

Jan Kiszka <jan.kiszka@siemens.com> 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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