linux-kernel - Re: What differences and relations between SVM, HSA, HMM and Unified Memory?

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Date:   Mon, 17 Jul 2017 13:52:46 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Yisheng Xie <xieyisheng1@...wei.com>,
        "Wuzongyong (Cordius Wu, Euler Dept)" <wuzongyong1@...wei.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "Wanzongshun (Vincent)" <wanzongshun@...wei.com>,
        "oded.gabbay@....com" <oded.gabbay@....com>, liubo95@...wei.com
Subject: Re: What differences and relations between SVM, HSA, HMM and Unified
 Memory?

On 17/07/17 12:57, Yisheng Xie wrote:
> Hi Jean-Philippe，
> 
> On 2017/6/12 19:37, Jean-Philippe Brucker wrote:
>> Hello,
>>
>> On 10/06/17 05:06, Wuzongyong (Cordius Wu, Euler Dept) wrote:
>>> Hi,
>>>
>>> Could someone explain differences and relations between the SVM(Shared
>>> Virtual Memory, by Intel), HSA(Heterogeneous System Architecture, by AMD),
>>> HMM(Heterogeneous Memory Management, by Glisse) and UM(Unified Memory, by
>>> NVIDIA) ? Are these in the substitutional relation?
>>>
>>> As I understand it, these aim to solve the same thing, sharing pointers
>>> between CPU and GPU(implement with ATS/PASID/PRI/IOMMU support). So far,
>>> SVM and HSA can only be used by integrated gpu. And, Intel declare that
>>> the root ports doesn’t not have the required TLP prefix support, resulting
>>>  that SVM can’t be used by discrete devices. So could someone tell me the
>>> required TLP prefix means what specifically?>
>>> With HMM, we can use allocator like malloc to manage host and device
>>> memory. Does this mean that there is no need to use SVM and HSA with HMM,
>>> or HMM is the basis of SVM and HAS to implement Fine-Grained system SVM
>>> defined in the opencl spec?
>>
>> I can't provide an exhaustive answer, but I have done some work on SVM.
>> Take it with a grain of salt though, I am not an expert.
>>
>> * HSA is an architecture that provides a common programming model for CPUs
>> and accelerators (GPGPUs etc). It does have SVM requirement (I/O page
>> faults, PASID and compatible address spaces), though it's only a small
>> part of it.
>>
>> * Similarly, OpenCL provides an API for dealing with accelerators. OpenCL
>> 2.0 introduced the concept of Fine-Grained System SVM, which allows to
>> pass userspace pointers to devices. It is just one flavor of SVM, they
>> also have coarse-grained and non-system. But they might have coined the
>> name, and I believe that in the context of Linux IOMMU, when we talk about
>> "SVM" it is OpenCL's fine-grained system SVM.
>> [...]
>>
>> While SVM is only about virtual address space,
> As you mentioned, SVM is only about virtual address space, I'd like to know how to
> manage the physical address especially about device's RAM, before HMM?
> 
> When OpenCL alloc a SVM pointer like:
>     void* p = clSVMAlloc (
>         context, // an OpenCL context where this buffer is available
>         CL_MEM_READ_WRITE | CL_MEM_SVM_FINE_GRAIN_BUFFER,
>         size, // amount of memory to allocate (in bytes)
>         0 // alignment in bytes (0 means default)
>     );
> 
> where this RAM come from， device RAM or host RAM?

Sorry, I'm not familiar with OpenCL/GPU drivers. It is up to them to
decide where to allocate memory for clSVMAlloc. My SMMU work would deal
with fine-grained *system* SVM, the kind that can be obtained from malloc
and doesn't require a call to clSVMAlloc. Hopefully others on this list or
linux-mm might be able to help you.

Thanks,
Jean

> Thanks
> Yisheng Xie
> 
>> HMM deals with physical
>> storage. If I understand correctly, HMM allows to transparently use device
>> RAM from userspace applications. So upon an I/O page fault, the mm
>> subsystem will migrate data from system memory into device RAM. It would
>> differ from "pure" SVM in that you would use different page directories on
>> IOMMU and MMU sides, and synchronize them using MMU notifiers. But please
>> don't take this at face value, I haven't had time to look into HMM yet.
>>
>> Thanks,
>> Jean
>>
>> .
>>
>