Re: [Qemu-devel] [PATCH RFC] vga: flag vga ram for notifiers

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/31/2011 04:37 PM, Michael S. Tsirkin wrote:
> On Thu, Mar 31, 2011 at 04:32:44PM -0500, Anthony Liguori wrote:
>> On 03/31/2011 04:26 PM, Michael S. Tsirkin wrote:
>>> On Thu, Mar 31, 2011 at 02:29:50PM -0500, Anthony Liguori wrote:
>>>> On 03/31/2011 02:18 PM, Peter Maydell wrote:
>>>>> On 31 March 2011 20:01, Anthony Liguori<anthony@codemonkey.ws>    wrote:
>>>>>> VGA is just another device.  It happens to be that we treat VGA device
>>>>>> memory as something that behaves like ram occassionally but that does not
>>>>>> make it RAM.
>>>>> So, to ask a dumb question, what does make something RAM?
>>>> It's a made up concept that we use to make device performance faster.
>>>>
>>>> Basically, RAM should include all of the memory that a reasonable
>>>> device (that we control) would DMA to and has a relatively stable
>>>> mapping.
>>>>
>>>>> My take on RAM is that RAM is just another device; the only
>>>>> difference is that you want to be able to implement fast
>>>>> paths that go straight(ish) to target memory; but that's
>>>>> an optimisation detail, not something that makes RAM
>>>>> conceptually different from other devices...
>>>> Right, the trouble is, if you want to treat RAM like any other
>>>> device, you can't get stable mappings to it which is bad for
>>>> something like vhost-net.
>>>>
>>>> Regards,
>>>>
>>>> Anthony Liguori
>>> Not only that I guess. Removing the VGA memory with the baloon
>>> will likely also be a bad idea.
>> It's just the equivalent of a memset(0).  It would be a silly thing
>> for a guest to do but not somethign to be concerned about.
>>
>> Regards,
>>
>> Anthony Liguori
>>
>
> BTW, what is IO_MEM_NOTDIRTY?

TCG needs to keep track of dirty memory in order to be able to deal with 
self modifying code.  IO_MEM_NOTDIRTY is the state that RAM takes before 
the first write.  I don't think IO_MEM_NOTDIRTY would ever show up in 
the l1_phys_map but it will show up in the iotlb.  The iotlb uses the 
same dispatch code as the l1_phys_map though.

Regards,

Anthony Liguori

> I thought another way would be to replace IO_MEM_RAM with
> IO_MEM_DEVICE_SHADOW_RAM in these cases but no idea
> what the right value for that enum would be.
>