On Fri, Oct 07, 2022, Jarkko Sakkinen wrote: > On Thu, Oct 06, 2022 at 03:34:58PM +0000, Sean Christopherson wrote: > > On Thu, Oct 06, 2022, Jarkko Sakkinen wrote: > > > On Thu, Oct 06, 2022 at 05:58:03PM +0300, Jarkko Sakkinen wrote: > > > > On Thu, Sep 15, 2022 at 10:29:07PM +0800, Chao Peng wrote: > > > > > This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two > > > > > additional KVM memslot fields private_fd/private_offset to allow > > > > > userspace to specify that guest private memory provided from the > > > > > private_fd and guest_phys_addr mapped at the private_offset of the > > > > > private_fd, spanning a range of memory_size. > > > > > > > > > > The extended memslot can still have the userspace_addr(hva). When use, a > > > > > single memslot can maintain both private memory through private > > > > > fd(private_fd/private_offset) and shared memory through > > > > > hva(userspace_addr). Whether the private or shared part is visible to > > > > > guest is maintained by other KVM code. > > > > > > > > What is anyway the appeal of private_offset field, instead of having just > > > > 1:1 association between regions and files, i.e. one memfd per region? > > > > Modifying memslots is slow, both in KVM and in QEMU (not sure about Google's VMM). > > E.g. if a vCPU converts a single page, it will be forced to wait until all other > > vCPUs drop SRCU, which can have severe latency spikes, e.g. if KVM is faulting in > > memory. KVM's memslot updates also hold a mutex for the entire duration of the > > update, i.e. conversions on different vCPUs would be fully serialized, exacerbating > > the SRCU problem. > > > > KVM also has historical baggage where it "needs" to zap _all_ SPTEs when any > > memslot is deleted. > > > > Taking both a private_fd and a shared userspace address allows userspace to convert > > between private and shared without having to manipulate memslots. > > Right, this was really good explanation, thank you. > > Still wondering could this possibly work (or not): > > 1. Union userspace_addr and private_fd. No, because userspace needs to be able to provide both userspace_addr (shared memory) and private_fd (private memory) for a single memslot. > 2. Instead of introducing private_offset, use guest_phys_addr as the > offset. No, because that would force userspace to use a single private_fd for all of guest memory since it effectively means private_offset=0. And userspace couldn't skip over holes in guest memory, i.e. the size of the memfd would need to follow the max guest gpa. In other words, dropping private_offset could work, but it'd be quite kludgy and not worth saving 8 bytes.
