[00/12] KVM: Make irqfd registration globally unique

Message

Sean Christopherson April 1, 2025, 8:44 p.m. UTC

Xen folks, I Cc'd y'all because "sched/wait: Drop WQ_FLAG_EXCLUSIVE from
add_wait_queue_priority()" changes the behavior of add_wait_queue_priority(),
which Xen's privcmd uses.  Unless I've misread the code, the Xen behavior
isn't actually affected, but extra eyeballs and any testing you can provide
would be much appreciated.


Rework KVM's irqfd registration to require that an eventfd is bound to at
most one irqfd throughout the entire system.  KVM currently disallows
binding an eventfd to multiple irqfds for a single VM, but doesn't reject
attempts to bind an eventfd to multiple VMs.

This is obvious an ABI change, but I'm fairly confident that it won't
break userspace, because binding an eventfd to multiple irqfds hasn't
truly worked since commit e8dbf19508a1 ("kvm/eventfd: Use priority waitqueue
to catch events before userspace").  A somewhat undocumented, and perhaps
even unintentional, side effect of suppressing eventfd notifications for
userspace is that the priority+exclusive behavior also suppresses eventfd
notifications for any subsequent waiters, even if they are priority waiters.
I.e. only the first VM with an irqfd+eventfd binding will get notifications.

And for IRQ bypass, a.k.a. device posted interrupts, globally unique
bindings are a hard requirement (at least on x86; I assume other archs are
the same).  KVM and the IRQ bypass manager kinda sorta handle this, but in
the absolute worst way possible (IMO).  Instead of surfacing an error to
userspace, KVM silently ignores IRQ bypass registration errors.

The motivation for this series is to harden against userspace goofs.  AFAIK,
we (Google) have never actually had a bug where userspace tries to assign
an eventfd to multiple VMs, but the possibility has come up in more than one
bug investigation (our intra-host, a.k.a. copyless, migration scheme
transfers eventfds from the old to the new VM when updating the host VMM).

Sean Christopherson (12):
  KVM: Use a local struct to do the initial vfs_poll() on an irqfd
  KVM: Acquire SCRU lock outside of irqfds.lock during assignment
  KVM: Initialize irqfd waitqueue callback when adding to the queue
  KVM: Add irqfd to KVM's list via the vfs_poll() callback
  KVM: Add irqfd to eventfd's waitqueue while holding irqfds.lock
  sched/wait: Add a waitqueue helper for fully exclusive priority
    waiters
  KVM: Disallow binding multiple irqfds to an eventfd with a priority
    waiter
  sched/wait: Drop WQ_FLAG_EXCLUSIVE from add_wait_queue_priority()
  KVM: Drop sanity check that per-VM list of irqfds is unique
  KVM: selftests: Assert that eventfd() succeeds in Xen shinfo test
  KVM: selftests: Add utilities to create eventfds and do KVM_IRQFD
  KVM: selftests: Add a KVM_IRQFD test to verify uniqueness requirements

 include/linux/kvm_irqfd.h                     |   1 -
 include/linux/wait.h                          |   2 +
 kernel/sched/wait.c                           |  24 +++-
 tools/testing/selftests/kvm/Makefile.kvm      |   4 +
 tools/testing/selftests/kvm/arm64/vgic_irq.c  |  12 +-
 .../testing/selftests/kvm/include/kvm_util.h  |  40 ++++++
 tools/testing/selftests/kvm/irqfd_test.c      | 130 ++++++++++++++++++
 .../selftests/kvm/x86/xen_shinfo_test.c       |  21 +--
 virt/kvm/eventfd.c                            | 130 +++++++++++++-----
 9 files changed, 299 insertions(+), 65 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/irqfd_test.c


base-commit: 782f9feaa9517caf33186dcdd6b50a8f770ed29b