-----Original Message----- From: Christian König <christian.koenig@xxxxxxx> Sent: Friday, January 26, 2024 5:10 AM To: Zeng, Oak <oak.zeng@xxxxxxxxx>; David Airlie <airlied@xxxxxxxxxx> Cc: Ghimiray, Himal Prasad <himal.prasad.ghimiray@xxxxxxxxx>; Thomas.Hellstrom@xxxxxxxxxxxxxxx; Winiarski, Michal <michal.winiarski@xxxxxxxxx>; Felix Kuehling <felix.kuehling@xxxxxxx>; Welty, Brian <brian.welty@xxxxxxxxx>; Shah, Ankur N <ankur.n.shah@xxxxxxxxx>; dri- devel@xxxxxxxxxxxxxxxxxxxxx; intel-xe@xxxxxxxxxxxxxxxxxxxxx; Gupta, saurabhg <saurabhg.gupta@xxxxxxxxx>; Danilo Krummrich <dakr@xxxxxxxxxx>; Daniel Vetter <daniel@xxxxxxxx>; Brost, Matthew <matthew.brost@xxxxxxxxx>; Bommu, Krishnaiah <krishnaiah.bommu@xxxxxxxxx>; Vishwanathapura, Niranjana <niranjana.vishwanathapura@xxxxxxxxx> Subject: Re: Making drm_gpuvm work across gpu devices Hi Oak, you can still use SVM, but it should not be a design criteria for the kernel UAPI. In other words the UAPI should be designed in such a way that the GPU virtual address can be equal to the CPU virtual address of a buffer, but can also be different to support use cases where this isn't the case.Terminology: SVM: any technology which can achieve a shared virtual address space b/t cpu and devices. The virtual address space can be managed by user space or kernel space. Intel implemented a SVM, based on the BO-centric gpu driver (gem-create, vm-bind) where virtual address space is managed by UMD. System allocator: another way of implement SVM. User just use malloc'ed memory for gpu submission. Virtual address space is managed by Linux core mm. In practice, we leverage HMM to implement system allocator. This article described details of all those different model: https://developer.nvidia.com/blog/simplifying-gpu-application-development-with-heterogeneous-memory-management/ Our programming model allows a mixture use of system allocator (even though system allocator is ) and traditional vm_bind (where cpu address can != gpu address). Let me re-post the pseudo codes: 1. Fd0 = open(/"dev/dri/render0") 2. Fd1 = open("/dev/dri/render1") 3. Fd3 = open("/dev/dri/xe-svm") 4. Gpu_Vm0 =xe_vm_create(fd0) 5. Gpu_Vm1 = xe_vm_create(fd1) 6. Queue0 = xe_exec_queue_create(fd0, gpu_vm0) 7. Queue1 = xe_exec_queue_create(fd1, gpu_vm1) 8. ptr = malloc() 9. bo = xe_bo_create(fd0) 10. Vm_bind(bo, gpu_vm0, va)//va is from UMD, cpu can access bo with same or different va. It is UMD's responsibility that va doesn't conflict with malloc'ed PTRs. 11. Xe_exec(queue0, ptr)//submit gpu job which use ptr, on card0 12. Xe_exec(queue1, ptr)//submit gpu job which use ptr, on card1 13. Xe_exec(queue0, va)//submit gpu job which use va, on card0 In above codes, the va used in vm_bind (line 10, Intel's API to bind an object to a va for GPU access) can be different from the CPU address when cpu access the same object. But whenever user use malloc'ed ptr for GPU submission (line 11, 12, so called system allocator), it implies CPU and GPUs use the same ptr to access. In above vm_bind, it is user/UMD's responsibility to guarantee that vm_bind va doesn't conflict with malloc'ed ptr. Otherwise it is treated as programming error. I think this design still meets your design restrictions.
Well why do you need this "Fd3 = open("/dev/dri/xe-svm")" ?
As far as I see fd3 isn't used anywhere. What you can do is to bind parts of your process address space to your driver connections (fd1, fd2 etc..) with a vm_bind(), but this should *not* come because of implicitely using some other file descriptor in the process.
As far as I can see this design is exactly what failed so badly with KFD.
Regards,
Christian.
Additionally to what Dave wrote I can summarize a few things I have learned while working on the AMD GPU drivers in the last decade or so: 1. Userspace requirements are *not* relevant for UAPI or even more general kernel driver design. 2. What should be done is to look at the hardware capabilities and try to expose those in a save manner to userspace. 3. The userspace requirements are then used to validate the kernel driver and especially the UAPI design to ensure that nothing was missed. The consequence of this is that nobody should ever use things like Cuda, Vulkan, OpenCL, OpenGL etc.. as argument to propose a certain UAPI design. What should be done instead is to say: My hardware works in this and that way -> we want to expose it like this -> because that enables us to implement the high level API in this and that way. Only this gives then a complete picture of how things interact together and allows the kernel community to influence and validate the design.What you described above is mainly bottom up. I know other people do top down, or whole system vertical HW-SW co-design. I don't have strong opinion here. Regards, OakThis doesn't mean that you need to throw away everything, but it gives a clear restriction that designs are not nailed in stone and for example you can't use something like a waterfall model. Going to answer on your other questions separately. Regards, Christian. Am 25.01.24 um 06:25 schrieb Zeng, Oak:Hi Dave, Let me step back. When I wrote " shared virtual address space b/t cpu and allgpu devices is a hard requirement for our system allocator design", I meant this is not only Intel's design requirement. Rather this is a common requirement for both Intel, AMD and Nvidia. Take a look at cuda driver API definition of cuMemAllocManaged (search this API on https://docs.nvidia.com/cuda/cuda- driver-api/group__CUDA__MEM.html#group__CUDA__MEM), it said:"The pointer is valid on the CPU and on all GPUs in the system that supportmanaged memory."This means the program virtual address space is shared b/t CPU and all GPUdevices on the system. The system allocator we are discussing is just one step advanced than cuMemAllocManaged: it allows malloc'ed memory to be shared b/t CPU and all GPU devices.I hope we all agree with this point. With that, I agree with Christian that in kmd we should make driver code per-device based instead of managing all devices in one driver instance. Our system allocator (and generally xekmd)design follows this rule: we make xe_vm per device based - one device is *not* aware of other device's address space, as I explained in previous email. I started this email seeking a one drm_gpuvm instance to cover all GPU devices. I gave up this approach (at least for now) per Danilo and Christian's feedback: We will continue to have per device based drm_gpuvm. I hope this is aligned with Christian but I will have to wait for Christian's reply to my previous email.I hope this clarify thing a little. Regards, Oak-----Original Message----- From: dri-devel <dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf OfDavidAirlie Sent: Wednesday, January 24, 2024 8:25 PM To: Zeng, Oak <oak.zeng@xxxxxxxxx> Cc: Ghimiray, Himal Prasad <himal.prasad.ghimiray@xxxxxxxxx>; Thomas.Hellstrom@xxxxxxxxxxxxxxx; Winiarski, Michal <michal.winiarski@xxxxxxxxx>; Felix Kuehling <felix.kuehling@xxxxxxx>;Welty,Brian <brian.welty@xxxxxxxxx>; Shah, Ankur N <ankur.n.shah@xxxxxxxxx>;dri-devel@xxxxxxxxxxxxxxxxxxxxx; intel-xe@xxxxxxxxxxxxxxxxxxxxx; Gupta, saurabhg <saurabhg.gupta@xxxxxxxxx>; Danilo Krummrich <dakr@xxxxxxxxxx>; Daniel Vetter <daniel@xxxxxxxx>; Brost, Matthew <matthew.brost@xxxxxxxxx>;Bommu,Krishnaiah <krishnaiah.bommu@xxxxxxxxx>; Vishwanathapura, Niranjana <niranjana.vishwanathapura@xxxxxxxxx>; Christian König <christian.koenig@xxxxxxx> Subject: Re: Making drm_gpuvm work across gpu devicesFor us, Xekmd doesn't need to know it is running under bare metal orvirtualized environment. Xekmd is always a guest driver. All the virtual address used in xekmd is guest virtual address. For SVM, we require all the VF devices share one single shared address space with guest CPU program. So all thedesignworks in bare metal environment can automatically work under virtualized environment. +@Shah, Ankur N +@Winiarski, Michal to backup me if I amwrong.Again, shared virtual address space b/t cpu and all gpu devices is a hardrequirement for our system allocator design (which means malloc’ed memory, cpu stack variables, globals can be directly used in gpu program. Same requirement as kfd SVM design). This was aligned with our user spacesoftwarestack. Just to make a very general point here (I'm hoping you listen to Christian a bit more and hoping he replies in more detail), but just because you have a system allocator design done, it doesn't in any way enforce the requirements on the kernel driver to accept that design. Bad system design should be pushed back on, not enforced in implementation stages. It's a trap Intel falls into regularly since they say well we already agreed this design with the userspace team and we can't change it now. This isn't acceptable. Design includes upstream discussion and feedback, if you say misdesigned the system allocator (and I'm not saying you definitely have), and this is pushing back on that, then you have to go fix your system architecture. KFD was an experiment like this, I pushed back on AMD at the start saying it was likely a bad plan, we let it go and got a lot of experience in why it was a bad design. Dave.
