Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram

[Marcel Apfelbaum <marcel@redhat.com>]

On 01/02/2018 16:24, Michael S. Tsirkin wrote:
> On Thu, Feb 01, 2018 at 02:29:25PM +0200, Marcel Apfelbaum wrote:
>> On 01/02/2018 14:10, Eduardo Habkost wrote:
>>> On Thu, Feb 01, 2018 at 07:36:50AM +0200, Marcel Apfelbaum wrote:
>>>> On 01/02/2018 4:22, Michael S. Tsirkin wrote:
>>>>> On Wed, Jan 31, 2018 at 09:34:22PM -0200, Eduardo Habkost wrote:
>>> [...]
>>>>>> BTW, what's the root cause for requiring HVAs in the buffer?
>>>>>
>>>>> It's a side effect of the kernel/userspace API which always wants
>>>>> a single HVA/len pair to map memory for the application.
>>>>>
>>>>>
>>>>
>>>> Hi Eduardo and Michael,
>>>>
>>>>>>  Can
>>>>>> this be fixed?
>>>>>
>>>>> I think yes.  It'd need to be a kernel patch for the RDMA subsystem
>>>>> mapping an s/g list with actual memory. The HVA/len pair would then just
>>>>> be used to refer to the region, without creating the two mappings.
>>>>>
>>>>> Something like splitting the register mr into
>>>>>
>>>>> mr = create mr (va/len) - allocate a handle and record the va/len
>>>>>
>>>>> addmemory(mr, offset, hva, len) - pin memory
>>>>>
>>>>> register mr - pass it to HW
>>>>>
>>>>> As a nice side effect we won't burn so much virtual address space.
>>>>>
>>>>
>>>> We would still need a contiguous virtual address space range (for post-send)
>>>> which we don't have since guest contiguous virtual address space
>>>> will always end up as non-contiguous host virtual address space.
>>>>
>>>> I am not sure the RDMA HW can handle a large VA with holes.
>>>
>>> I'm confused.  Why would the hardware see and care about virtual
>>> addresses? 
>>
>> The post-send operations bypasses the kernel, and the process
>> puts in the work request GVA addresses.
> 
> To be more precise, it's the guest supplied IOVA that is sent to the card.
> 
>>> How exactly does the hardware translates VAs to
>>> PAs? 
>>
>> The HW maintains a page-directory like structure different form MMU
>> VA -> phys pages
>>
>>> What if the process page tables change?
>>>
>>
>> Since the page tables the HW uses are their own, we just need the phys
>> page to be pinned.
>>
>>>>
>>>> An alternative would be 0-based MR, QEMU intercepts the post-send
>>>> operations and can substract the guest VA base address.
>>>> However I didn't see the implementation in kernel for 0 based MRs
>>>> and also the RDMA maintainer said it would work for local keys
>>>> and not for remote keys.
>>>
>>> This is also unexpected: are GVAs visible to the virtual RDMA
>>> hardware? 
>>
>> Yes, explained above
>>
>>> Where does the QEMU pvrdma code translates GVAs to
>>> GPAs?
>>>
>>
>> During reg_mr (memory registration commands)
>> Then it registers the same addresses to the real HW.
>> (as Host virtual addresses)
>>
>> Thanks,
>> Marcel
> 
> 
> The full fix would be to allow QEMU to map a list of
> pages to a guest supplied IOVA.
> 

Agreed, we are trying to influence the RDMA discussion
on the new API in this direction.

Thanks,
Marcel

>>>>
>>>>> This will fix rdma with hugetlbfs as well which is currently broken.
>>>>>
>>>>>
>>>>
>>>> There is already a discussion on the linux-rdma list:
>>>>     https://www.spinics.net/lists/linux-rdma/msg60079.html
>>>> But it will take some (actually a lot of) time, we are currently talking about
>>>> a possible API. And it does not solve the re-mapping...
>>>>
>>>> Thanks,
>>>> Marcel
>>>>
>>>>>> -- 
>>>>>> Eduardo
>>>>
>>>