On Fri, Oct 15, 2021 at 11:27:02AM +0200, David Hildenbrand (da...@redhat.com) wrote: > > > On 15.10.21 11:10, david.dai wrote: > > On Wed, Oct 13, 2021 at 10:33:39AM +0200, David Hildenbrand > > (da...@redhat.com) wrote: > >> > >> CAUTION: This email originated from outside of the organization. Do not > >> click links or open attachments unless you recognize the sender and know > >> the > >> content is safe. > >> > >> > >> On 13.10.21 10:13, david.dai wrote: > >>> On Mon, Oct 11, 2021 at 09:43:53AM +0200, David Hildenbrand > >>> (da...@redhat.com) wrote: > >>>> > >>>> > >>>> > >>>>>> virito-mem currently relies on having a single sparse memory region > >>>>>> (anon > >>>>>> mmap, mmaped file, mmaped huge pages, mmap shmem) per VM. Although we > >>>>>> can > >>>>>> share memory with other processes, sharing with other VMs is not > >>>>>> intended. > >>>>>> Instead of actually mmaping parts dynamically (which can be quite > >>>>>> expensive), virtio-mem relies on punching holes into the backend and > >>>>>> dynamically allocating memory/file blocks/... on access. > >>>>>> > >>>>>> So the easy way to make it work is: > >>>>>> > >>>>>> a) Exposing the CXL memory to the buddy via dax/kmem, esulting in > >>>>>> device > >>>>>> memory getting managed by the buddy on a separate NUMA node. > >>>>>> > >>>>> > >>>>> Linux kernel buddy system? how to guarantee other applications don't > >>>>> apply memory > >>>>> from it > >>>> > >>>> Excellent question. Usually, you would online the memory to ZONE_MOVABLE, > >>>> such that even if some other allocation ended up there, that it could > >>>> get migrated somewhere else. > >>>> > >>>> For example, "daxctl reconfigure-device" tries doing that as default: > >>>> > >>>> https://pmem.io/ndctl/daxctl-reconfigure-device.html > >>>> > >>>> However, I agree that we might actually want to tell the system to not > >>>> use this CPU-less node as fallback for other allocations, and that we > >>>> might not want to swap out such memory etc. > >>>> > >>>> > >>>> But, in the end all that virtio-mem needs to work in the hypervisor is > >>>> > >>>> a) A sparse memmap (anonymous RAM, memfd, file) > >>>> b) A way to populate memory within that sparse memmap (e.g., on fault, > >>>> using madvise(MADV_POPULATE_WRITE), fallocate()) > >>>> c) A way to discard memory (madvise(MADV_DONTNEED), > >>>> fallocate(FALLOC_FL_PUNCH_HOLE)) > >>>> > >>>> So instead of using anonymous memory+mbind, you can also mmap a sparse > >>>> file > >>>> and rely on populate-on-demand. One alternative for your use case would > >>>> be > >>>> to create a DAX filesystem on that CXL memory (IIRC that should work) > >>>> and > >>>> simply providing virtio-mem with a sparse file located on that > >>>> filesystem. > >>>> > >>>> Of course, you can also use some other mechanism as you might have in > >>>> your approach, as long as it supports a,b,c. > >>>> > >>>>> > >>>>>> > >>>>>> b) (optional) allocate huge pages on that separate NUMA node. > >>>>>> c) Use ordinary memory-device-ram or memory-device-memfd (for huge > >>>>>> pages), > >>>>>> *bidning* the memory backend to that special NUMA node. > >>>>>> > >>>>> "-object memory-backend/device-ram or memory-device-memfd, id=mem0, > >>>>> size=768G" > >>>>> How to bind backend memory to NUMA node > >>>>> > >>>> > >>>> I think the syntax is "policy=bind,host-nodes=X" > >>>> > >>>> whereby X is a node mask. So for node "0" you'd use "host-nodes=0x1" for > >>>> "5" > >>>> "host-nodes=0x20" etc. > >>>> > >>>>>> > >>>>>> This will dynamically allocate memory from that special NUMA node, > >>>>>> resulting > >>>>>> in the virtio-mem device completely being backed by that device memory, > >>>>>> being able to dynamically resize the memory allocation. > >>>>>> > >>>>>> > >>>>>> Exposing an actual devdax to the virtio-mem device, shared by multiple > >>>>>> VMs > >>>>>> isn't really what we want and won't work without major design changes. > >>>>>> Also, > >>>>>> I'm not so sure it's a very clean design: exposing memory belonging to > >>>>>> other > >>>>>> VMs to unrelated QEMU processes. This sounds like a serious security > >>>>>> hole: > >>>>>> if you managed to escalate to the QEMU process from inside the VM, you > >>>>>> can > >>>>>> access unrelated VM memory quite happily. You want an abstraction > >>>>>> in-between, that makes sure each VM/QEMU process only sees private > >>>>>> memory: > >>>>>> for example, the buddy via dax/kmem. > >>>>>> > >>>>> Hi David > >>>>> Thanks for your suggestion, also sorry for my delayed reply due to my > >>>>> long vacation. > >>>>> How does current virtio-mem dynamically attach memory to guest, via > >>>>> page fault? > >>>> > >>>> Essentially you have a large sparse mmap. Withing that mmap, memory is > >>>> populated on demand. Instead if mmap/munmap you perform a single large > >>>> mmap and then dynamically populate memory/discard memory. > >>>> > >>>> Right now, memory is populated via page faults on access. This is > >>>> sub-optimal when dealing with limited resources (i.e., hugetlbfs, > >>>> file blocks) and you might run out of backend memory. > >>>> > >>>> I'm working on a "prealloc" mode, which will preallocate/populate memory > >>>> necessary for exposing the next block of memory to the VM, and which > >>>> fails gracefully if preallocation/population fails in the case of such > >>>> limited resources. > >>>> > >>>> The patch resides on: > >>>> https://github.com/davidhildenbrand/qemu/tree/virtio-mem-next > >>>> > >>>> commit ded0e302c14ae1b68bdce9059dcca344e0a5f5f0 > >>>> Author: David Hildenbrand <da...@redhat.com> > >>>> Date: Mon Aug 2 19:51:36 2021 +0200 > >>>> > >>>> virtio-mem: support "prealloc=on" option > >>>> Especially for hugetlb, but also for file-based memory backends, > >>>> we'd > >>>> like to be able to prealloc memory, especially to make user errors > >>>> less > >>>> severe: crashing the VM when there are not sufficient huge pages > >>>> around. > >>>> A common option for hugetlb will be using > >>>> "reserve=off,prealloc=off" for > >>>> the memory backend and "prealloc=on" for the virtio-mem device. This > >>>> way, no huge pages will be reserved for the process, but we can > >>>> recover > >>>> if there are no actual huge pages when plugging memory. > >>>> Signed-off-by: David Hildenbrand <da...@redhat.com> > >>>> > >>>> > >>>> -- > >>>> Thanks, > >>>> > >>>> David / dhildenb > >>>> > >>> > >>> Hi David, > >>> > >>> After read virtio-mem code, I understand what you have expressed, please > >>> allow me to describe > >>> my understanding to virtio-mem, so that we have a aligned view. > >>> > >>> Virtio-mem: > >>> Virtio-mem device initializes and reserved a memory area(GPA), later > >>> memory dynamically > >>> growing/shrinking will not exceed this scope, memory-backend-ram has > >>> mapped anon. memory > >>> to the whole area, but no ram is attached because Linux have a policy > >>> to delay allocation. > >> > >> Right, but it can also be any sparse file (memory-backend-memfd, > >> memory-backend-file). > >> > >>> When virtio-mem driver apply to dynamically add memory to guest, it > >>> first request a region > >>> from the reserved memory area, then notify virtio-mem device to record > >>> the information > >>> (virtio-mem device doesn't make real memory allocation). After received > >>> response from > >> > >> In the upcoming prealloc=on mode I referenced, the allocation will happen > >> before the guest is notified about success and starts using the memory. > >> > >> With vfio/mdev support, the allocation will happen nowadays already, when > >> vfio/mdev is notified about the populated memory ranges (see > >> RamDiscardManager). That's essentially what makes virtio-mem device > >> passthrough work. > >> > >>> virtio-mem deivce, virtio-mem driver will online the requested region > >>> and add it to Linux > >>> page allocator. Real ram allocation will happen via page fault when > >>> guest cpu access it. > >>> Memory shrink will be achieved by madvise() > >> > >> Right, but you could write a custom virtio-mem driver that pools this > >> memory > >> differently. > >> > >> Memory shrinking in the hypervisor is either done using madvise(DONMTNEED) > >> or fallocate(FALLOC_FL_PUNCH_HOLE) > >> > >>> > >>> Questions: > >>> 1. heterogeneous computing, memory may be accessed by CPUs on host side > >>> and device side. > >>> Memory delayed allocation is not suitable. Host software(for > >>> instance, OpenCL) may > >>> allocate a buffer to computing device to place the computing result > >>> in. > >> > >> That works already with virtio-mem with vfio/mdev via the RamDiscardManager > >> infrastructure introduced recently. With "prealloc=on", the delayed memory > >> allocation can also be avoided without vfio/mdev. > >> > >>> 2. we hope build ourselves page allocator in host kernel, so it can offer > >>> customized mmap() > >>> method to build va->pa mapping in MMU and IOMMU. > >> > >> Theoretically, you can wire up pretty much any driver in QEMU like > >> vfio/mdev > >> via the RamDiscardManager. From there, you can issue whatever syscall you > >> need to popualte memory when plugging new memory blocks. All you need to > >> support is a sparse mmap and a way to populate/discard memory. > >> Populate/discard could be wired up in QEMU virtio-mem code as you need it. > >> > >>> 3. some potential requirements also require our driver to manage memory, > >>> so that page size > >>> granularity can be controlled to fit small device iotlb cache. > >>> CXL has bias mode for HDM(host managed device memory), it needs > >>> physical address to make > >>> bias mode switch between host access and device access. These tell us > >>> driver manage memory > >>> is mandatory. > >> > >> I think if you write your driver in a certain way and wire it up in QEMU > >> virtio-mem accordingly (e.g., using a new memory-backend-whatever), that > >> shouldn't be an issue. > >> > > > > Thanks a lot, so let me have a try. > > Let me know if you need some help or run into issues! Further, if we > need spec extensions to handle some additional requirements, that's also > not really an issue. > > I certainly don't want you to use virtio-mem by any means. However > "virtual pci device to dynamically attach memory to QEMU" is essentially > what virtio-mem was does :) . As it's already compatible with vfio/mdev > and soon has full support for dealing with limited resources > (preallocation support, VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE), it feels > like a good fit for your use case as well, although some details are > left to be figured out. > > (also, virtio-mem solved a lot of the issues related to guest memory > dumping, VM snapshotting/migration, and how to make it consumable by > upper layers like libvirt -- so you would get that for almost free as well) > >
Yes, if virtio-mem satisfy our requirements, of course we will employ it. If any question, I will contact you for help. Thanks, David
