KVM: s390: Rework guest entry logic

In __vcpu_run() and do_vsie_run(), we enter an RCU extended quiescent
state (EQS) by calling guest_enter_irqoff(), which lasts until
__vcpu_run() calls guest_exit_irqoff(). However, between the two we
enable interrupts and may handle interrupts during the EQS. As the IRQ
entry code will not wake RCU in this case, we may run the core IRQ code
and IRQ handler without RCU watching, leading to various potential
problems.

It is necessary to unmask (host) interrupts around entering the guest,
as entering the guest via SIE will not automatically unmask these. When
a host interrupt is taken from a guest, it is taken via its regular
host IRQ handler rather than being treated as a direct exit from SIE.
Due to this, we cannot simply mask interrupts around guest entry, and
must handle interrupts during this window, waking RCU as required.

Additionally, between guest_enter_irqoff() and guest_exit_irqoff(), we
use local_irq_enable() and local_irq_disable() to unmask interrupts,
violating the ordering requirements for RCU/lockdep/tracing around
entry/exit sequences. Further, since this occurs in an instrumentable
function, it's possible that instrumented code runs during this window,
with potential usage of RCU, etc.

To fix the RCU wakeup problem, an s390 implementation of
arch_in_rcu_eqs() is added which checks for PF_VCPU in current->flags.
PF_VCPU is set/cleared by guest_timing_{enter,exit}_irqoff(), which
surround the actual guest entry.

To fix the remaining issues, the lower-level guest entry logic is moved
into a shared noinstr helper function using the
guest_state_{enter,exit}_irqoff() helpers. These perform all the
lockdep/RCU/tracing manipulation necessary, but as sie64a() does not
enable/disable interrupts, we must do this explicitly with the
non-instrumented arch_local_irq_{enable,disable}() helpers:

        guest_state_enter_irqoff()

        arch_local_irq_enable();
        sie64a(...);
        arch_local_irq_disable();

        guest_state_exit_irqoff();

[ajd@linux.ibm.com: rebase, fix commit message]

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20250708092742.104309-3-ajd@linux.ibm.com
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Message-ID: <20250708092742.104309-3-ajd@linux.ibm.com>

2 files changed, 13 insertions, 0 deletions

diff --git a/arch/s390/include/asm/entry-common.h b/arch/s390/include/asm/entry-common.h
index 35555c944630..979af986a8fe 100644
--- a/arch/s390/include/asm/entry-common.h
+++ b/arch/s390/include/asm/entry-common.h
@@ -59,4 +59,14 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 
 #define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare
 
+static __always_inline bool arch_in_rcu_eqs(void)
+{
+	if (IS_ENABLED(CONFIG_KVM))
+		return current->flags & PF_VCPU;
+
+	return false;
+}
+
+#define arch_in_rcu_eqs arch_in_rcu_eqs
+
 #endif
diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
index cb89e54ada25..f870d09515cc 100644
--- a/arch/s390/include/asm/kvm_host.h
+++ b/arch/s390/include/asm/kvm_host.h
@@ -716,6 +716,9 @@ extern char sie_exit;
 bool kvm_s390_pv_is_protected(struct kvm *kvm);
 bool kvm_s390_pv_cpu_is_protected(struct kvm_vcpu *vcpu);
 
+extern int kvm_s390_enter_exit_sie(struct kvm_s390_sie_block *scb,
+				   u64 *gprs, unsigned long gasce);
+
 extern int kvm_s390_gisc_register(struct kvm *kvm, u32 gisc);
 extern int kvm_s390_gisc_unregister(struct kvm *kvm, u32 gisc);