Re: [Qemu-devel] vhost-user: broken mem regions

[Damjan Marion (damarion)]

On 25 Jun 2014, at 17:50, Michael S. Tsirkin <address@hidden> wrote:

> On Wed, Jun 25, 2014 at 03:46:04PM +0000, Damjan Marion (damarion) wrote:
>> 
>> On 25 Jun 2014, at 17:29, Michael S. Tsirkin <address@hidden> wrote:
>> 
>>> On Wed, Jun 25, 2014 at 02:57:52PM +0000, Damjan Marion (damarion) wrote:
>>>> 
>>>> On 25 Jun 2014, at 16:27, Michael S. Tsirkin <address@hidden> wrote:
>>>> 
>>>>> On Wed, Jun 25, 2014 at 02:20:56PM +0000, Damjan Marion (damarion) wrote:
>>>>>> 
>>>>>> On 25 Jun 2014, at 16:13, Nikolay Nikolaev <address@hidden> wrote:
>>>>>> 
>>>>>>>>> - it will require changes on the user side also
>>>>>>>> 
>>>>>>>> why would it?
>>>>>>>> format seems unchanged, right?
>>>>>>> 
>>>>>>> yes, but it will happen that multiple regions have same FD so call to 
>>>>>>> mmap()
>>>>>>> should look different, I’m still playing with this on user side...
>>>>>>> but then you shoudl somehow accumulate the sizes and send just a single 
>>>>>>> fd, something along these lines. 
>>>>>> 
>>>>>> Yes, so I’m not very happy with that approach and looking if there is 
>>>>>> better proposal, 
>>>>>> or at least wider agreement how to address this issue.
>>>>>> 
>>>>>> Damjan
>>>>> 
>>>>> still not sure what the issue is ...
>>>>> 
>>>> 
>>>> No issue, just additional logic is needed on user side to calculate total 
>>>> size of shared regions and call mmap() once per FD.
>>>> 
>>>> Agree?
>>> 
>>> why not just call it multiple times? AFAIK linux handles this just fine.
>> 
>> I need to specify size when calling mmap(), so i need to run trough all 
>> regions and sum sizes before calling mmap().
> 
> You can map same file in many places.
> Just call mmap many times with offsets.

agree, but in that case I need to know offset, which i need to calculate by 
finding 1st region in the list.

I.e. My regions are:

nregions: 4
region:
        gpa = 0x100000000
        size = 3221225472
        ua = 0x2aab6ac00000
region:
        gpa = 0xFFFC0000
        size = 262144
        ua = 0x7fc13d200000
region:
        gpa = 0x0
        size = 655360
        ua = 0x2aaaaac00000
region:
        gpa = 0xC0000
        size = 3220439040
        ua = 0x2aaaaacc0000


So,  to calculate offset for 1st region I need to find region with GPA 0x0 
which is region 3.
Offset for region 1 is 0x2aab6ac00000 - 0x2aaaaac00000 = 0xC0000000.

BTW Any idea what to do with region 2, it doesn’t look like the one belonging 
to the same place, but qemu_get_ram_fd() returns same FD for it.