[PATCH v2 1/7] Drivers: hv: Handle NEED_RESCHED_LAZY before transferring to guest

[Nuno Das Neves]

On 8/27/2025 5:01 PM, Sean Christopherson wrote:
> Check for NEED_RESCHED_LAZY, not just NEED_RESCHED, prior to transferring
> control to a guest.  Failure to check for lazy resched can unnecessarily
> delay rescheduling until the next tick when using a lazy preemption model.
> 
> Note, ideally both the checking and processing of TIF bits would be handled
> in common code, to avoid having to keep three separate paths synchronized,
> but defer such cleanups to the future to keep the fix as standalone as
> possible.
> 
> Cc: Nuno Das Neves <nunodasneves at linux.microsoft.com>
> Cc: Mukesh R <mrathor at linux.microsoft.com>
> Fixes: 621191d709b1 ("Drivers: hv: Introduce mshv_root module to expose /dev/mshv to VMMs")
> Fixes: 64503b4f4468 ("Drivers: hv: Introduce mshv_vtl driver")
> Signed-off-by: Sean Christopherson <seanjc at google.com>
> ---
>  drivers/hv/mshv_common.c    | 2 +-
>  drivers/hv/mshv_root_main.c | 3 ++-
>  drivers/hv/mshv_vtl_main.c  | 3 ++-
>  3 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hv/mshv_common.c b/drivers/hv/mshv_common.c
> index 6f227a8a5af7..eb3df3e296bb 100644
> --- a/drivers/hv/mshv_common.c
> +++ b/drivers/hv/mshv_common.c
> @@ -151,7 +151,7 @@ int mshv_do_pre_guest_mode_work(ulong th_flags)
>  	if (th_flags & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
>  		return -EINTR;
>  
> -	if (th_flags & _TIF_NEED_RESCHED)
> +	if (th_flags & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
>  		schedule();
>  
>  	if (th_flags & _TIF_NOTIFY_RESUME)
> diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
> index 932932cb91ea..0d849f09160a 100644
> --- a/drivers/hv/mshv_root_main.c
> +++ b/drivers/hv/mshv_root_main.c
> @@ -484,7 +484,8 @@ mshv_vp_wait_for_hv_kick(struct mshv_vp *vp)
>  static int mshv_pre_guest_mode_work(struct mshv_vp *vp)
>  {
>  	const ulong work_flags = _TIF_NOTIFY_SIGNAL | _TIF_SIGPENDING |
> -				 _TIF_NEED_RESCHED  | _TIF_NOTIFY_RESUME;
> +				 _TIF_NEED_RESCHED  | _TIF_NEED_RESCHED_LAZY |
> +				 _TIF_NOTIFY_RESUME;
>  	ulong th_flags;
>  
>  	th_flags = read_thread_flags();
> diff --git a/drivers/hv/mshv_vtl_main.c b/drivers/hv/mshv_vtl_main.c
> index dc6594ae03ad..12f5e77b7095 100644
> --- a/drivers/hv/mshv_vtl_main.c
> +++ b/drivers/hv/mshv_vtl_main.c
> @@ -728,7 +728,8 @@ static int mshv_vtl_ioctl_return_to_lower_vtl(void)
>  	preempt_disable();
>  	for (;;) {
>  		const unsigned long VTL0_WORK = _TIF_SIGPENDING | _TIF_NEED_RESCHED |
> -						_TIF_NOTIFY_RESUME | _TIF_NOTIFY_SIGNAL;
> +						_TIF_NOTIFY_RESUME | _TIF_NOTIFY_SIGNAL |
> +						_TIF_NEED_RESCHED_LAZY;
>  		unsigned long ti_work;
>  		u32 cancel;
>  		unsigned long irq_flags;

Tested by compiling with CONFIG_PREEMPT_LAZY=y and booting a guest. For
the test I added a check to confirm _TIF_NEED_RESCHED_LAZY was set and
honored.

Looks good, thanks.

Tested-by: Nuno Das Neves <nunodasneves at linux.microsoft.com>
Reviewed-by: Nuno Das Neves <nunodasneves at linux.microsoft.com>