On 01/05/2016 11:18 AM, Yang Zhang wrote: > On 2016/1/4 14:22, Jason Wang wrote: >> >> >> On 01/04/2016 09:39 AM, Yang Zhang wrote: >>> On 2015/12/31 15:13, Jason Wang wrote: >>>> This patch tries to implement an device IOTLB for vhost. This could be >>>> used with for co-operation with userspace(qemu) implementation of >>>> iommu for a secure DMA environment in guest. >>>> >>>> The idea is simple. When vhost meets an IOTLB miss, it will request >>>> the assistance of userspace to do the translation, this is done >>>> through: >>>> >>>> - Fill the translation request in a preset userspace address (This >>>> address is set through ioctl VHOST_SET_IOTLB_REQUEST_ENTRY). >>>> - Notify userspace through eventfd (This eventfd was set through ioctl >>>> VHOST_SET_IOTLB_FD). >>>> >>>> When userspace finishes the translation, it will update the vhost >>>> IOTLB through VHOST_UPDATE_IOTLB ioctl. Userspace is also in charge of >>>> snooping the IOTLB invalidation of IOMMU IOTLB and use >>>> VHOST_UPDATE_IOTLB to invalidate the possible entry in vhost. >>> >>> Is there any performance data shows the difference with IOTLB >>> supporting? >> >> Basic testing show it was slower than without IOTLB. >> >>> I doubt we may see performance decrease since the flush code path is >>> longer than before. >>> >> >> Yes, it also depend on the TLB hit rate. >> >> If lots of dynamic mappings and unmappings are used in guest (e.g normal >> Linux driver). This method should be much more slower since: >> >> - lots of invalidation and its path is slow. >> - the hit rate is low and the high price of userspace assisted address >> translation. >> - limitation of userspace IOMMU/IOTLB implementation (qemu's vtd >> emulation simply empty all entries when it's full). >> >> Another method is to implement kernel IOMMU (e.g vtd). But I'm not sure >> vhost is the best place to do this, since vhost should be architecture >> independent. Maybe we'd better to do it in kvm or have a pv IOMMU >> implementation in vhost. > > Actually, i have the kernel IOMMU(virtual vtd) patch which can pass > though the physical device to L2 guest on hand. A little bit confused, I believe the first step is to exporting an IOMMU to L1 guest for it to use for a assigned device? > But it is just a draft patch which was written several years ago. If > there is real requirement for it, I can rebase it and send out it for > review. Interesting but I think the goal is different. This patch tries to make vhost/virtio works with emulated IOMMU. > >> >> Another side, if fixed mappings were used in guest, (e.g dpdk in guest). >> We have the possibility to have 100% hit rate with almost no >> invalidation, the performance penalty should be ignorable, this should >> be the main use case for this patch. >> >> The patch is just a prototype for discussion. Any other ideas are >> welcomed. >> >> Thanks >> > > -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
