Re: [PATCH] entry/kvm: Make vCPU tasks exit to userspace when a livepatch is pending

[Sean Christopherson <seanjc@google.com>]

On Tue, May 03, 2022, Seth Forshee wrote:
> A livepatch migration for a task can only happen when the task is
> sleeping or it exits to userspace. This may happen infrequently for a
> heavily loaded vCPU task, leading to livepatch transition failures.
> 
> Fake signals will be sent to tasks which fail to migrate via stack
> checking. This will cause running vCPU tasks to exit guest mode, but
> since no signal is pending they return to guest execution without
> exiting to userspace. Fix this by treating a pending livepatch migration
> like a pending signal, exiting to userspace with EINTR. This allows the
> migration to complete, and userspace should re-excecute KVM_RUN to
> resume guest execution.
> 
> In my testing, systems where livepatching would timeout after 60 seconds
> were able to load livepatches within a couple of seconds with this
> change.
> 
> Signed-off-by: Seth Forshee <sforshee@digitalocean.com>
> ---
>  kernel/entry/kvm.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
> index 9d09f489b60e..efe4b791c253 100644
> --- a/kernel/entry/kvm.c
> +++ b/kernel/entry/kvm.c
> @@ -14,7 +14,12 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
>  				task_work_run();
>  		}
>  
> -		if (ti_work & _TIF_SIGPENDING) {
> +		/*
> +		 * When a livepatch migration is pending, force an exit to

Can the changelog and comment use terminology other than migration?  Maybe "transition"?
That seems to be prevelant through the livepatch code and docs.  There are already
too many meanings for "migration" in KVM, e.g. live migration, page migration, task/vCPU
migration, etc...

> +		 * userspace as though a signal is pending to allow the
> +		 * migration to complete.
> +		 */
> +		if (ti_work & (_TIF_SIGPENDING | _TIF_PATCH_PENDING)) {

_TIF_PATCH_PENDING needs to be in XFER_TO_GUEST_MODE_WORK too, otherwise there's
no guarantee KVM will see the flag and invoke xfer_to_guest_mode_handle_work().

>  			kvm_handle_signal_exit(vcpu);
>  			return -EINTR;
>  		}
> -- 
> 2.32.0
>