David Woodhouse <dwmw2@xxxxxxxxxxxxx> writes:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> In kvm_vcpu_block, the current task is set to TASK_INTERRUPTIBLE before
> making a final check whether the vCPU should be woken from HLT by any
> incoming interrupt.
>
> This is a problem for the get_user() in __kvm_xen_has_interrupt(), which
> really shouldn't be sleeping when the task state has already been set.
> I think it's actually harmless as it would just manifest itself as a
> spurious wakeup, but it's causing a debug warning:
>
> [ 230.963649] do not call blocking ops when !TASK_RUNNING; state=1 set at [<00000000b6bcdbc9>] prepare_to_swait_exclusive+0x30/0x80
>
> Fix the warning by turning it into an *explicit* spurious wakeup. When
> invoked with !task_is_running(current) (and we might as well add
> in_atomic() there while we're at it), just return 1 to indicate that
> an IRQ is pending, which will cause a wakeup and then something will
> call it again in a context that *can* sleep so it can fault the page
> back in.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 40da8ccd724f ("KVM: x86/xen: Add event channel interrupt vector upcall")
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> ---
> arch/x86/kvm/xen.c | 27 ++++++++++++++++++++++-----
> 1 file changed, 22 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 9ea9c3dabe37..8f62baebd028 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -190,6 +190,7 @@ void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, int state)
>
> int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
> {
> + int err;
> u8 rc = 0;
>
> /*
> @@ -216,13 +217,29 @@ int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
> if (likely(slots->generation == ghc->generation &&
> !kvm_is_error_hva(ghc->hva) && ghc->memslot)) {
> /* Fast path */
> - __get_user(rc, (u8 __user *)ghc->hva + offset);
> - } else {
> - /* Slow path */
> - kvm_read_guest_offset_cached(v->kvm, ghc, &rc, offset,
> - sizeof(rc));
> + pagefault_disable();
> + err = __get_user(rc, (u8 __user *)ghc->hva + offset);
> + pagefault_enable();
This reminds me of copy_from_user_nofault() -- can we use it instead maybe?
> + if (!err)
> + return rc;
> }
>
> + /* Slow path */
> +
> + /*
> + * This function gets called from kvm_vcpu_block() after setting the
> + * task to TASK_INTERRUPTIBLE, to see if it needs to wake immediately
> + * from a HLT. So we really mustn't sleep. If the page ended up absent
> + * at that point, just return 1 in order to trigger an immediate wake,
> + * and we'll end up getting called again from a context where we *can*
> + * fault in the page and wait for it.
> + */
> + if (in_atomic() || !task_is_running(current))
> + return 1;
> +
> + kvm_read_guest_offset_cached(v->kvm, ghc, &rc, offset,
> + sizeof(rc));
> +
> return rc;
> }
>
>
>
--
Vitaly
