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Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backe
From: |
Marcel Apfelbaum |
Subject: |
Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram |
Date: |
Thu, 1 Feb 2018 14:29:25 +0200 |
User-agent: |
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 |
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.
> 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
>>
>>> 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
>>
>
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Marcel Apfelbaum, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Eduardo Habkost, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram,
Marcel Apfelbaum <=
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Eduardo Habkost, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Marcel Apfelbaum, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Eduardo Habkost, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Marcel Apfelbaum, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Eduardo Habkost, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Marcel Apfelbaum, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Eduardo Habkost, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Marcel Apfelbaum, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Paolo Bonzini, 2018/02/01
- Re: [Qemu-devel] [PATCH V8 1/4] mem: add share parameter to memory-backend-ram, Michael S. Tsirkin, 2018/02/01