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

[Jike Song <jike.song@intel.com>]

On 05/07/2016 12:16 AM, Kirti Wankhede wrote:
> 
> 
> 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).
> 

[Thanks for the kind explanation, I spent some time to dig & recall the details]


So this consists of two checks:

	1) vm_end >= vm_start
	2) fault_addr >= vm_start && fault_addr <= vm_end

>>>> 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.
> 

Thanks.

For the vma with a vm_ops, and each time only one pfn to proceed, can
we replace remap_pfn_range with vm_insert_pfn? I had a quick check on
kernel repo, it seems that remap_pfn_range is only called from fops.mmap,
not from vma->vm_ops.fault.

>>>
>>> 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).

No. I'd like QEMU to be aware that only a *portion* of the physical BAR1
is available for the vGPU, like:

	pGPU	: 1GB size BAR1
	vGPU	: 128MB size BAR1

QEMU has the information of the available size for a particular vGPU,
calling mmap() with that.

I'd say that your implementation is nice and flexible, but in order to
ensure whatever level a resource QoS, you have to account it from the
device-model (where validate_map_request is implemented), right?

How about making QEMU be aware that only a portion of MMIO is available?
Would appreciate hearing your opinion on this. Thanks!


> Thanks,
> Kirti
>

--
Thanks,
Jike