On Tue, Nov 7, 2023 at 5:27 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote: > > On Tue, Nov 07, 2023 at 01:34:34PM -0800, David Matlack wrote: > > On Tue, Nov 7, 2023 at 1:10 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote: > > Thanks Oliver. Maybe I'm being dense but I'm still not understanding > > how VGIC and UFFD interact :). I understand that VGIC is unaware of > > UFFD, but fundamentally they must interact in some way during > > post-copy. Can you spell out the sequence of events? > > Well it doesn't help that my abbreviated explanation glosses over some > details. So here's the verbose explanation, and I'm sure Marc will have > a set of corrections too :) I meant there's no _explicit_ interaction > between UFFD and the various bits of GIC that need to touch guest > memory. > > The GIC redistributors contain a set of MMIO registers that are > accessible through the KVM_GET_DEVICE_ATTR and KVM_SET_DEVICE_ATTR > ioctls. Writes to these are reflected directly into the KVM > representation, no biggie there. > > One of the registers (GICR_PENDBASER) is a pointer to guest memory, > containing a bitmap of pending LPIs managed by the redistributor. The > ITS takes this to the extreme, as it is effectively a bunch of page > tables for interrupts. All of this state actually lives in a KVM > representation, and is only flushed out to guest memory when explicitly > told to do so by userspace. > > On the target, we reread all the info when rebuilding interrupt > translations when userspace calls KVM_DEV_ARM_ITS_RESTORE_TABLES. All of > these guest memory accesses go through kvm_read_guest() and I expect the > usual UFFD handling for non-present pages kicks in from there. Thanks for the longer explanation. Yes kvm_read_guest() eventually calls __copy_from_user() which will trigger a page fault and UserfaultFD will notify userspace and wait for the page to become present. In the KVM-specific proposal I outlined, calling kvm_read_guest() will ultimately result in a check of the VM's present bitmap and KVM will nnotify userspace and wait for the page to become present if it's not, before calling __copy_from_user(). So I don't expect a KVM-specific solution to have any increased maintenance burden for VGIC (or any other widgets).
