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

[Damjan Marion (damarion)]

On 26 Jun 2014, at 09:56, Michael S. Tsirkin <address@hidden> wrote:

> On Thu, Jun 26, 2014 at 07:44:24AM +0000, Damjan Marion (damarion) wrote:
>> 
>> On 26 Jun 2014, at 09:13, Michael S. Tsirkin <address@hidden> wrote:
>> 
>>> On Wed, Jun 25, 2014 at 09:52:09PM +0000, Damjan Marion (damarion) wrote:
>>>> 
>>>> On 25 Jun 2014, at 18:44, Paolo Bonzini <address@hidden> wrote:
>>>> 
>>>>>> nregions: 4
>>>>>> region:
>>>>>>  gpa = 0x100000000
>>>>>>  size = 3221225472
>>>>>>  ua = 0x2aab6ac00000
>>>>> 
>>>>> High memory, above 3 gigabytes.
>>>>> 
>>>>>> region:
>>>>>>  gpa = 0xFFFC0000
>>>>>>  size = 262144
>>>>>>  ua = 0x7fc13d200000
>>>>> 
>>>>> This is the BIOS.  There shouldn't be any FD for this one, it
>>>>> is not allocated in hugetlbfs.
>>>>> 
>>>>>> region:
>>>>>>  gpa = 0x0
>>>>>>  size = 655360
>>>>>>  ua = 0x2aaaaac00000
>>>>>> region:
>>>>>>  gpa = 0xC0000
>>>>>>  size = 3220439040
>>>>>>  ua = 0x2aaaaacc0000
>>>>> 
>>>>> Together, it's the first 3 GB of memory.
>>>>> 
>>>>> I understand now what you mean.  Yeah, the format should be changed
>>>>> to include the offset (why does vhost-user need the ua at all?
>>>>> perhaps the offset can replace the ua).
>>>>> 
>>>>>> 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.
>>>>> 
>>>>> This must be a bug.  I would have expected qemu_get_ram_fd to return -1
>>>>> here, so no descriptor should be passed to vhost-user.
>>>> 
>>>> Problem is inside qemu_get_ram_block():
>>>> 
>>>> 
>>>>   if (block && addr - block->offset < block->length) {
>>>>       goto found;
>>>>   }
>>>> 
>>>> this is true if we have > 4G of RAM allocated.
>>> 
>>> Hmm I don't get it. Why is it always true for >4G RAM?
>> 
>> This check assumes that guest memory is mapped to RAM block 
>> without gaps. as BIOS is mapped to guest address space
>> near 4G this check will return true if block size is bigger than
>> BIOS address.
> 
> 
> But then offset is > addr so addr - offset is a huge value no?

With 6GB allocated to VM, I’see:

block->offset = 0
block->length = 0x180000000

So if addr = 0xFFFC0000 then it will be false positive.


> 
>>> 
>>>> 
>>>> Any advice how to fix this?
>>>> 
>>>>