On Fri, Feb 07, 2025 at 09:30:45PM -0500, Demi Marie Obenour wrote: > On Fri, Feb 07, 2025 at 07:07:11PM +0800, Huang, Honglei1 wrote: > > On 2025/2/7 2:21, Demi Marie Obenour wrote: > > > On Thu, Feb 06, 2025 at 06:53:55PM +0800, Huang, Honglei1 wrote: > > > > On 2025/1/31 8:33, Demi Marie Obenour wrote: > > > > > On Wed, Jan 29, 2025 at 03:54:59PM -0500, Demi Marie Obenour wrote: > > > > > > On 1/8/25 12:05 PM, Simona Vetter wrote: > > > > > > > On Fri, Dec 27, 2024 at 10:24:29AM +0800, Huang, Honglei1 wrote: > > > > > > > > > > > > > > > > On 2024/12/22 9:59, Demi Marie Obenour wrote: > > > > > > > > > On 12/20/24 10:35 AM, Simona Vetter wrote: > > > > > > > > > > On Fri, Dec 20, 2024 at 06:04:09PM +0800, Honglei Huang wrote: > > > > > > > > > > > From: Honglei Huang <Honglei1.Huang@xxxxxxx> > > > > > > > > > > > > > > > > > > > > > > A virtio-gpu userptr is based on HMM notifier. > > > > > > > > > > > Used for let host access guest userspace memory and > > > > > > > > > > > notice the change of userspace memory. > > > > > > > > > > > This series patches are in very beginning state, > > > > > > > > > > > User space are pinned currently to ensure the host > > > > > > > > > > > device memory operations are correct. > > > > > > > > > > > The free and unmap operations for userspace can be > > > > > > > > > > > handled by MMU notifier this is a simple and basice > > > > > > > > > > > SVM feature for this series patches. > > > > > > > > > > > The physical PFNS update operations is splited into > > > > > > > > > > > two OPs in here. The evicted memories won't be used > > > > > > > > > > > anymore but remap into host again to achieve same > > > > > > > > > > > effect with hmm_rang_fault. > > > > > > > > > > > > > > > > > > > > So in my opinion there are two ways to implement userptr that make sense: > > > > > > > > > > > > > > > > > > > > - pinned userptr with pin_user_pages(FOLL_LONGTERM). there is not mmu > > > > > > > > > > notifier > > > > > > > > > > > > > > > > > > > > - unpinnned userptr where you entirely rely on userptr and do not hold any > > > > > > > > > > page references or page pins at all, for full SVM integration. This > > > > > > > > > > should use hmm_range_fault ideally, since that's the version that > > > > > > > > > > doesn't ever grab any page reference pins. > > > > > > > > > > > > > > > > > > > > All the in-between variants are imo really bad hacks, whether they hold a > > > > > > > > > > page reference or a temporary page pin (which seems to be what you're > > > > > > > > > > doing here). In much older kernels there was some justification for them, > > > > > > > > > > because strange stuff happened over fork(), but with FOLL_LONGTERM this is > > > > > > > > > > now all sorted out. So there's really only fully pinned, or true svm left > > > > > > > > > > as clean design choices imo. > > > > > > > > > > > > > > > > > > > > With that background, why does pin_user_pages(FOLL_LONGTERM) not work for > > > > > > > > > > you? > > > > > > > > > > > > > > > > > > +1 on using FOLL_LONGTERM. Fully dynamic memory management has a huge cost > > > > > > > > > in complexity that pinning everything avoids. Furthermore, this avoids the > > > > > > > > > host having to take action in response to guest memory reclaim requests. > > > > > > > > > This avoids additional complexity (and thus attack surface) on the host side. > > > > > > > > > Furthermore, since this is for ROCm and not for graphics, I am less concerned > > > > > > > > > about supporting systems that require swappable GPU VRAM. > > > > > > > > > > > > > > > > Hi Sima and Demi, > > > > > > > > > > > > > > > > I totally agree the flag FOLL_LONGTERM is needed, I will add it in next > > > > > > > > version. > > > > > > > > > > > > > > > > And for the first pin variants implementation, the MMU notifier is also > > > > > > > > needed I think.Cause the userptr feature in UMD generally used like this: > > > > > > > > the registering of userptr always is explicitly invoked by user code like > > > > > > > > "registerMemoryToGPU(userptrAddr, ...)", but for the userptr release/free, > > > > > > > > there is no explicit API for it, at least in hsakmt/KFD stack. User just > > > > > > > > need call system call "free(userptrAddr)", then kernel driver will release > > > > > > > > the userptr by MMU notifier callback.Virtio-GPU has no other way to know if > > > > > > > > user has been free the userptr except for MMU notifior.And in UMD theres is > > > > > > > > no way to get the free() operation is invoked by user.The only way is use > > > > > > > > MMU notifier in virtio-GPU driver and free the corresponding data in host by > > > > > > > > some virtio CMDs as far as I can see. > > > > > > > > > > > > > > > > And for the second way that is use hmm_range_fault, there is a predictable > > > > > > > > issues as far as I can see, at least in hsakmt/KFD stack. That is the memory > > > > > > > > may migrate when GPU/device is working. In bare metal, when memory is > > > > > > > > migrating KFD driver will pause the compute work of the device in > > > > > > > > mmap_wirte_lock then use hmm_range_fault to remap the migrated/evicted > > > > > > > > memories to GPU then restore the compute work of device to ensure the > > > > > > > > correction of the data. But in virtio-GPU driver the migration happen in > > > > > > > > guest kernel, the evict mmu notifier callback happens in guest, a virtio CMD > > > > > > > > can be used for notify host but as lack of mmap_write_lock protection in > > > > > > > > host kernel, host will hold invalid data for a short period of time, this > > > > > > > > may lead to some issues. And it is hard to fix as far as I can see. > > > > > > > > > > > > > > > > I will extract some APIs into helper according to your request, and I will > > > > > > > > refactor the whole userptr implementation, use some callbacks in page > > > > > > > > getting path, let the pin method and hmm_range_fault can be choiced > > > > > > > > in this series patches. > > > > > > > > > > > > > > Ok, so if this is for svm, then you need full blast hmm, or the semantics > > > > > > > are buggy. You cannot fake svm with pin(FOLL_LONGTERM) userptr, this does > > > > > > > not work. > > > > > > > > > > > > > > The other option is that hsakmt/kfd api is completely busted, and that's > > > > > > > kinda not a kernel problem. > > > > > > > -Sima > > > > > > > > > > > > On further thought, I believe the driver needs to migrate the pages to > > > > > > device memory (really a virtio-GPU blob object) *and* take a FOLL_LONGTERM > > > > > > pin on them. The reason is that it isn’t possible to migrate these pages > > > > > > back to "host" memory without unmapping them from the GPU. For the reasons > > > > > > I mention in [1], I believe that temporarily revoking access to virtio-GPU > > > > > > blob objects is not feasible. Instead, the pages must be treated as if > > > > > > they are permanently in device memory until guest userspace unmaps them > > > > > > from the GPU, after which they must be migrated back to host memory. > > > > > > > > > > Discussion on IRC indicates that migration isn't reliable. This is > > > > > because Linux core memory management is largely lock-free for > > > > > performance reasons, so there is no way to prevent temporary elevation > > > > > of a page's reference count. A page with an elevated reference count > > > > > cannot be migrated. > > > > > > > > > > The only alternative I can think of is for the hypervisor to perform the > > > > > migration. The hypervisor can revoke the guest's access to the page > > > > > without the guest's consent or involvement. The host can then replace > > > > > the page with one of its own pages, which might be on the CPU or GPU. > > > > > Further migration between the CPU and GPU is controlled by the host > > > > > kernel-mode driver (KMD) and host kernel memory management. The guest > > > > > kernel driver must take a FOLL_LONGTERM pin before telling the host to > > > > > use the pages, but that is all. > > > > > > > > > > On KVM, this should be essentially automatic, as guest memory really is > > > > > just host userspace memory. On Xen, this requires that the backend > > > > > domain can revoke fronted access to _any_ frontend page, or at least > > > > > frontend pages that have been granted to the backend. The backend will > > > > > then need to be able to handle page faults for the frontend pages, and > > > > > replace the frontend pages with its own pages at will. Furthermore, > > > > > revoking pages that the backend has installed into the frontend must > > > > > never fail, because the backend will panic if it does fail. > > > > > > > > > > Sima, is putting guest pages under host kernel control the only option? > > > > > I thought that this could be avoided by leaving the pages on the CPU if > > > > > migration fails, but that won't work because there will be no way to > > > > > migrate them to the GPU later, causing performance problems that would > > > > > be impossible to debug. Is waiting (possibly forever) on migration to > > > > > finish an option? Otherwise, this might mean extra complexity in the > > > > > Xen hypervisor, as I do not believe the primitives needed are currently > > > > > available. Specifically, in addition to the primitives discussed at Xen > > > > > Project Summit 2024, the backend also needs to intercept access to, and > > > > > replace the contents of, arbitrary frontend-controlled pages. > > > > > > > > Hi Demi, > > > > > > > > I agree that to achieve the complete SVM feature in virtio-GPU, it is > > > > necessary to have the hypervisor deeply involved and add new features. > > > > It needs solid design, I saw the detailed reply in a another thread, it > > > > is very helpful,looking forward to the response from the Xen/hypervisor > > > > experts. > > > > > > From further discussion with Sima, I suspect that virtio-GPU cannot > > > support SVM with reasonable performance. Native contexts have such good > > > performance for graphics workloads because graphics workloads very rarely > > > perform blocking waits for host GPU operations to complete, so one can > > > make all frequently-used operations asynchronous and therefore hide the > > > guest <=> host latency. SVM seems to require many synchronous GPU > > > operations, and I believe those will severely harm performance with > > > virtio-GPU. > > > > > > If you need full SVM for your workloads, I recommend using hardware > > > SR-IOV. This should allow the guest to perform GPU virtual memory > > > operations without host involvement, which I expect will be much faster > > > than paravirtualizing these operations. Scalable I/O virtualization > > > might also work, but it might also require paravirtualizing the > > > performance-critical address-space operations unless the hardware has > > > stage 2 translation tables. > > > > > > > Yes I think so, the SR-IOV or some other hardware virtualization are clean > > design for ROCm/compute currently. But actually those hardware features > > supported solution also have their own limitation, like high hardware cost > > and the performance decreasing caused by different guest VMs hardware > > workload schedule. We are trying a low-cost, high-performance virtualization > > solution, it appears to be difficult to support full feature VS SR-IOV at > > present. But it doesn't prevent us from enabling part of functions. > > > > > > So for the current virito-GPU userptr implementation, It can not support the > > > > full SVM feature, it just can only let GPU access the user space memory, > > > > maybe can be called by userptr feature. I think I will finish this small > > > > part firstly and then to try to complete the whole SVM feature. > > > > > > I think you will still have problems if the host is able to migrate > > > pages in any way. This requires that the host install an MMU notifier > > > for the pages it has received from the guest, which in turn implies that > > > the host must be able to prevent the guest from accessing the pages. > > > If the pages are used in grant table operations, this isn't possible. > > > > > > If you are willing to have the pages be pinned on the host side things > > > are much simpler. Such pages will always be in system memory, and will > > > never be able to migrate to VRAM. This will result in a performance > > > penalty and will likely require explicit prefetching by programs using > > > ROCm, but this may be acceptable for your use-cases. The performance > > > penalty is the same as that with XNACK disabled, which is the case for > > > all RDNA2+ GPUs, so all code that aims to be portable to recent consumer > > > hardware will have to account for it anyway. > > > > Totally agreed. Actually memory migrating to VRAM is very common in GFX > > side, but in ROCm/KFD, maybe it can be disabled and not often used as far as > > I know. ROCm/KFD always uses SDMA to transfer or copy data maybe this is > > faster than migrating to VRAM (needs further verification). > > But we have some method to workaround it. Really thanks for your reminding. > > I think you will do okay if you treat virtio-GPU as providing a virtual > GPU with no XNACK support. XNACK is necessary for migrating pages > between GPU and CPU based on demand, and it is this migration that is > so hard to implement. Furthermore, I highly doubt that the combination > of AMDKFD and the hardware you are targeting even supports XNACK. > > At Xen Project Summit 2024, AMD mentioned that it wanted to enable both > rendering (Vulkan/OpenGL) and compute (ROCm) with virtio-GPU native > contexts under Xen. The only GPUs for which AMDKFD will enable XNACK > support are GFX9 GPUs, which are GCN and CDNA. Shipping a GCN GPU in a > new design would be very unusual and CDNA (Instinct) accelerators do not > support graphics, so either AMD is using separate devices for compute > and graphics or the workloads will run with no XNACK support. Since you > mention HW cost as an important consideration, I suspect the latter. > > I believe that the Instinct accelerators that support XNACK also support > SR-IOV, but if you wish to combine XNACK and virtio-GPU, this should be > possible subject to caveats. The main caveat is that under no > circumstances can the host's Xen driver install an MMU notifier for > pages that the guest can use in grant table operations or DMA. A driver > that installs an MMU notifier promises that it can block access to > pages in a bounded amount of time, and if the guest can DMA to the pages > or grant them to other domains this is not possible. Without the Xen > driver installing an MMU notifier, there is no way for the pages to be > migrated to the GPU without risking use-after-free or at least data > corruption. Instead, one of the following options will be needed: > > 1. hipMallocManaged() allocates the memory from the backend using the > Map primitive discussed elsewhere. Such memory is not mappable in > the IOMMU (if there is an assigned PCI device) and cannot be used for > grant table operations. Memory allocated via system allocators > (anonymous pages) is not able to be migrated. > > 2. The frontend uses shadow buffers for all I/O. This allows the > backend to use a new Steal primitive to revoke the guest's accesses > to anonymous pages and handle page faults accordingly. > > 3. Same as 2 except that the frontend allocates all memory (except > bounce buffers) from the backend, just like a KVM guest does, rather > than from the Xen toolstack. > > 4. The frontend tries to migrate the pages to backend-provided ones, and > falls back to leaving them pinned on the CPU. The frontend's MMU > notifier tells the backend to stop accessing the pages, blocking > until the backend confirms this. The frontend then moves the pages > to its own memory and returns from the notifier. This may require > new AMDKFD APIs. > > 5. Same as 4 except that the frontend uses hmm_range_fault to move the > pages to the backend in response to GPU page faults. This requires a > frontend <-> backend round-trip for each fault (slooooow) so a new > fast mechanism for this might be needed. > -- > Sincerely, > Demi Marie Obenour (she/her/hers) > Invisible Things Lab CC Simona Vetter -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab
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