Re: [PATCH] KVM: optionally post write on ioeventfd write

[Sean Christopherson <seanjc@google.com>]

On Fri, Mar 06, 2026, Thanos Makatos wrote:
> @@ -656,7 +658,8 @@ struct kvm_ioeventfd {
>  	__u32 len;         /* 1, 2, 4, or 8 bytes; or 0 to ignore length */
>  	__s32 fd;
>  	__u32 flags;
> -	__u8  pad[36];
> +	__aligned_u64 post_addr; /* address to write to if POST_WRITE is set */
> +	__u8  pad[24];
>  };

I would prefer to have explicit padding:

  struct kvm_ioeventfd {
	__u64 datamatch;
	__u64 addr;        /* legal pio/mmio address */
	__u32 len;         /* 1, 2, 4, or 8 bytes; or 0 to ignore length */
	__s32 fd;
	__u32 flags;
	__u32 pad0;
	__u64 post_addr;   /* address to write to if POST_WRITE is set */
	__u8  pad[24];
  };

that way it's more obvious there's a hole we can use in the future.


> diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
> index fdec90e85467..7ab470981c31 100644
> --- a/tools/testing/selftests/kvm/Makefile.kvm
> +++ b/tools/testing/selftests/kvm/Makefile.kvm
> @@ -64,6 +64,7 @@ TEST_GEN_PROGS_COMMON += kvm_binary_stats_test
>  TEST_GEN_PROGS_COMMON += kvm_create_max_vcpus
>  TEST_GEN_PROGS_COMMON += kvm_page_table_test
>  TEST_GEN_PROGS_COMMON += set_memory_region_test
> +TEST_GEN_PROGS_COMMON += ioeventfd_test

Please name this ioeventfd_posted_write_test, as it isn't a generic ioeventfd
test.  And maintain the alphabetical sort.

Does this actually work on s390?  Which doesn't have MMIO...

> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 0e8b8a2c5b79..22bc49a41503 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -741,6 +741,7 @@ struct _ioeventfd {
>  	struct kvm_io_device dev;
>  	u8                   bus_idx;
>  	bool                 wildcard;
> +	void         __user *post_addr;
>  };
>  
>  static inline struct _ioeventfd *
> @@ -812,6 +813,9 @@ ioeventfd_write(struct kvm_vcpu *vcpu, struct kvm_io_device *this, gpa_t addr,
>  	if (!ioeventfd_in_range(p, addr, len, val))
>  		return -EOPNOTSUPP;
>  
> +	if (p->post_addr && len > 0 && __copy_to_user(p->post_addr, val, len))
> +		return -EFAULT;
> +
>  	eventfd_signal(p->eventfd);
>  	return 0;
>  }
> @@ -866,6 +870,7 @@ static int kvm_assign_ioeventfd_idx(struct kvm *kvm,
>  {
>  
>  	struct eventfd_ctx *eventfd;
> +	void __user *post_addr;
>  	struct _ioeventfd *p;
>  	int ret;
>  
> @@ -873,6 +878,16 @@ static int kvm_assign_ioeventfd_idx(struct kvm *kvm,
>  	if (IS_ERR(eventfd))
>  		return PTR_ERR(eventfd);
>  
> +	post_addr = u64_to_user_ptr(args->post_addr);
> +	if ((args->flags & KVM_IOEVENTFD_FLAG_POST_WRITE) &&
> +	    (!args->len || !post_addr ||
> +	     args->post_addr != untagged_addr(args->post_addr) ||
> +	     !access_ok(post_addr, args->len))) {
> +		/* In KVM’s ABI, post_addr must be non‑NULL. */

Maybe elaborate just a bit?  And I vote to drop the local "post_addr", even though
it's more "work", because it's super easy to overlook that KVM *must* do the cast
before checking for a NULL pointer.

E.g.

	/*
	 * Disallow posting to virtual address 0 so that KVM can treat
	 * a NULL user pointer as "no posted write".
	 */
	if ((args->flags & KVM_IOEVENTFD_FLAG_POST_WRITE) &&
	    (!args->len || !u64_to_user_ptr(args->post_addr) ||
	     args->post_addr != untagged_addr(args->post_addr) ||
	     !access_ok(u64_to_user_ptr(args->post_addr), args->len))) {
		ret = -EINVAL;
		goto fail;
	}