On Thu, Feb 20, 2020 at 09:39:05PM -0800, Tian, Kevin wrote: > > From: Chia-I Wu <olvaffe@xxxxxxxxx> > > Sent: Friday, February 21, 2020 12:51 PM > > If you think it is the best for KVM to inspect hva to determine the memory > > type with page granularity, that is reasonable and should work for us too. > > The userspace can do something (e.g., add a GPU driver dependency to the > > hypervisor such that the dma-buf is imported as a GPU memory and mapped > > using > > vkMapMemory) or I can work with dma-buf maintainers to see if dma-buf's > > semantics can be changed. > > I think you need consider the live migration requirement as Paolo pointed out. > The migration thread needs to read/write the region, then it must use the > same type as GPU process and guest to read/write the region. In such case, > the hva mapped by Qemu should have the desired type as the guest. However, > adding GPU driver dependency to Qemu might trigger some concern. I'm not > sure whether there is generic mechanism though, to share dmabuf fd between GPU > process and Qemu while allowing Qemu to follow the desired type w/o using > vkMapMemory... Alternatively, KVM could make KVM_MEM_DMA and KVM_MEM_LOG_DIRTY_PAGES mutually exclusive, i.e. force a transition to WB memtype for the guest (with appropriate zapping) when migration is activated. I think that would work? > Note this is orthogonal to whether introducing a new uapi or implicitly checking > hva to favor guest memory type. It's purely about Qemu itself. Ideally anyone > with the desire to access a dma-buf object should follow the expected semantics. > It's interesting that dma-buf sub-system doesn't provide a centralized > synchronization about memory type between multiple mmap paths.
