Non-KVM folks,
I am hoping to route this through the KVM tree (6.17 or later), as the non-KVM changes should be glorified nops. Please holler if you object to that idea.
Hyper-V folks in particular, let me know if you want a stable topic branch/tag, e.g. on the off chance you want to make similar changes to the Hyper-V code, and I'll make sure that happens.
As for what this series actually does...
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 obviously 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).
v3: - Retain WQ_FLAG_EXCLUSIVE in mshv_eventfd.c, which snuck in between v1 and v2. [Peter] - Use EXPORT_SYMBOL_GPL. [Peter] - Move WQ_FLAG_EXCLUSIVE out of add_wait_queue_priority() in a prep patch so that the affected subsystems are more explicitly documented (and then immediately drop the flag from drivers/xen/privcmd.c, which amusingly hides that file from the diff stats).
v2: - https://lore.kernel.org/all/20250519185514.2678456-1-seanjc@google.com - Use guard(spinlock_irqsave). [Prateek]
v1: https://lore.kernel.org/all/20250401204425.904001-1-seanjc@google.com
Sean Christopherson (13): 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: Drop WQ_FLAG_EXCLUSIVE from add_wait_queue_priority() xen: privcmd: Don't mark eventfd waiter as EXCLUSIVE sched/wait: Add a waitqueue helper for fully exclusive priority waiters KVM: Disallow binding multiple irqfds to an eventfd with a priority waiter 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
drivers/hv/mshv_eventfd.c | 8 ++ include/linux/kvm_irqfd.h | 1 - include/linux/wait.h | 2 + kernel/sched/wait.c | 22 ++- tools/testing/selftests/kvm/Makefile.kvm | 1 + 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 +++++++++++++----- 10 files changed, 302 insertions(+), 65 deletions(-) create mode 100644 tools/testing/selftests/kvm/irqfd_test.c
base-commit: 45eb29140e68ffe8e93a5471006858a018480a45