Re: [Qemu-devel] [PATCH] xen: avoid tracking the region 0xa0000 - 0xbffff

[Jan Kiszka]

On 2011-06-14 13:50, Stefano Stabellini wrote:
> On Tue, 14 Jun 2011, Jan Kiszka wrote:
>> On 2011-06-14 12:54, Alexander Graf wrote:
>>>
>>> On 03.06.2011, at 17:56, <address@hidden> <address@hidden> wrote:
>>>
>>>> From: Stefano Stabellini <address@hidden>
>>>>
>>>> Xen can only do dirty bit tracking for one memory region, so we should
>>>> explicitly avoid trying to track the legacy VGA region between 0xa0000
>>>> and 0xbffff, rather than trying and failing.
>>>>
>>>> Signed-off-by: Stefano Stabellini <address@hidden>
>>>> ---
>>>> xen-all.c |    4 ++++
>>>> 1 files changed, 4 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/xen-all.c b/xen-all.c
>>>> index 9a5c3ec..1fdc2e8 100644
>>>> --- a/xen-all.c
>>>> +++ b/xen-all.c
>>>> @@ -218,6 +218,10 @@ static int xen_add_to_physmap(XenIOState *state,
>>>>     if (get_physmapping(state, start_addr, size)) {
>>>>         return 0;
>>>>     }
>>>> +    /* do not try to map legacy VGA memory */
>>>> +    if (start_addr >= 0xa0000 && start_addr + size <= 0xbffff) {
>>>
>>> I don't quite like the hardcoded range here. What exactly is the issue? The 
>>> fact that you can only map a single region? Then do a counter and fail when 
>>> it's > 1. If you don't want to map the VGA region as memory slot, why not 
>>> change the actual mapping code in the cirrus adapter?
>>
>> Err, please no "if (xen_enabled())" in that code. We just got rid of the
>> kvm_enabled() mess. And it doesn't scale, it would be required in e1000
>> as well e.g.
> 
> agreed

[Actually, e1000 is not using dirty logging but coalesced MMIO.]

> 
> 
>> BTW, if Xen is not able to track more than one dirty region, I think
>> it's time to fix that limitation. At some point it may no longer be
>> possible to work around it (who knows how the new memory API will look
>> like in this regard).
> 
> you are right, however it is not a simple fix and at present we don't
> actually need to track more than one region...

Well, you already miss dirty logged VGA/VBE memory access this way
(everything that goes to legacy VGA mem, not the framebuffer BAR). Grub
provides a really poor use experience in that mode.

Jan

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