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 <nunodasne...@linux.microsoft.com>
> Cc: Mukesh R <mrat...@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 <sea...@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 <nunodasne...@linux.microsoft.com>
Reviewed-by: Nuno Das Neves <nunodasne...@linux.microsoft.com>
