Re: [Qemu-devel] [RFC PATCH v3 1/3] vGPU Core driver

[Kirti Wankhede]

On 5/6/2016 5:44 PM, Jike Song wrote:
> On 05/05/2016 05:06 PM, Tian, Kevin wrote:
>>> From: Kirti Wankhede
>>>
>>>  >> + * @validate_map_request:      Validate remap pfn request
>>>  >> + *                             @vdev: vgpu device structure
>>>  >> + *                             @virtaddr: target user address to start 
>>> at
>>>  >> + *                             @pfn: physical address of kernel 
>>> memory, GPU
>>>  >> + *                             driver can change if required.
>>>  >> + *                             @size: size of map area, GPU driver can 
>>> change
>>>  >> + *                             the size of map area if desired.
>>>  >> + *                             @prot: page protection flags for this 
>>> mapping,
>>>  >> + *                             GPU driver can change, if required.
>>>  >> + *                             Returns integer: success (0) or error 
>>> (< 0)
>>>  >
>>>  > Was not at all clear to me what this did until I got to patch 2, this
>>>  > is actually providing the fault handling for mmap'ing a vGPU mmio BAR.
>>>  > Needs a better name or better description.
>>>  >
>>>
>>> If say VMM mmap whole BAR1 of GPU, say 128MB, so fault would occur when
>>> BAR1 is tried to access then the size is calculated as:
>>> req_size = vma->vm_end - virtaddr
> Hi Kirti,
> 
> virtaddr is the faulted one, vma->vm_end the vaddr of the mmap-ed 128MB BAR1?
> 
> Would you elaborate why (vm_end - fault_addr) results the requested size? 
> 
> 

If first access is at start address of mmaped address, fault_addr is
vma->vm_start. Then (vm_end - vm_start) is the size mmapped region.

req_size should not exceed (vm_end - vm_start).


>>> Since GPU is being shared by multiple vGPUs, GPU driver might not remap
>>> whole BAR1 for only one vGPU device, so would prefer, say map one page
>>> at a time. GPU driver returns PAGE_SIZE. This is used by
>>> remap_pfn_range(). Now on next access to BAR1 other than that page, we
>>> will again get a fault().
>>> As the name says this call is to validate from GPU driver for the size
>>> and prot of map area. GPU driver can change size and prot for this map area.
> 
> If I understand correctly, you are trying to share a physical BAR among
> multiple vGPUs, by mapping a single pfn each time, when fault happens?
> 

Yes.

>>
>> Currently we don't require such interface for Intel vGPU. Need to think about
>> its rationale carefully (still not clear to me). Jike, do you have any 
>> thought on
>> this?
> 
> We need the mmap method of vgpu_device to be implemented, but I was
> expecting something else, like calling remap_pfn_range() directly from
> the mmap.
>

Calling remap_pfn_range directly from mmap means you would like to remap
pfn for whole BAR1 during mmap, right?

In that case, don't set validate_map_request() and access start of mmap
address, so that on first access it will do remap_pfn_range() for
(vm_end - vm_start).

Thanks,
Kirti


>>
>> Thanks
>> Kevin
>>
> 
> --
> Thanks,
> Jike
>