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

From: Nuno Das Neves <nunodasneves@linux.microsoft.com>
To: Sean Christopherson <seanjc@google.com>,
	Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Tianrui Zhao <zhaotianrui@loongson.cn>,
	Bibo Mao <maobibo@loongson.cn>,
	Huacai Chen <chenhuacai@kernel.org>,
	Anup Patel <anup@brainfault.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@kernel.org>,
	Joel Fernandes <joelagnelf@nvidia.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Uladzislau Rezki <urezki@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	kvm@vger.kernel.org, loongarch@lists.linux.dev,
	kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
	linux-hyperv@vger.kernel.org, rcu@vger.kernel.org,
	Mukesh R <mrathor@linux.microsoft.com>
Subject: Re: [PATCH v2 1/7] Drivers: hv: Handle NEED_RESCHED_LAZY before transferring to guest
Date: Thu, 28 Aug 2025 16:56:20 -0700	[thread overview]
Message-ID: <0b4eb0cc-657f-4cdb-8255-e3b8f6b14077@linux.microsoft.com> (raw)
In-Reply-To: <20250828000156.23389-2-seanjc@google.com>

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@linux.microsoft.com>
> Cc: Mukesh R <mrathor@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@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@linux.microsoft.com>
Reviewed-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>

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