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SubjectRe: [RFC PATCH 1/5] entry: Pass pt_regs to irqentry_exit_cond_resched()
Date
On Wed, Aug 10 2022 at 01:18, Thomas Gleixner wrote:
> Both clearly demonstrate how well tested this XEN_PV muck is which means
> we should just delete it.
>
> Anyway. This wants the fix below.

which is not sufficient. If CONFIG_PREEMPT_DYNAMIC is set then
CONFIG_PREEMPTION is set too, which makes the preemtible hypercall magic
a complete NOP. So if the dynamic call is disabled, then such a several
seconds (sic!) hypercall cannot be preempted at all.

Something like the below then. Oh well...

Thanks,

tglx
---
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -253,7 +253,8 @@ SYSCALL_DEFINE0(ni_syscall)
}

#ifdef CONFIG_XEN_PV
-#ifndef CONFIG_PREEMPTION
+
+#if !defined(CONFIG_PREEMPTION) || defined(CONFIG_PREEMPT_DYNAMIC)
/*
* Some hypercalls issued by the toolstack can take many 10s of
* seconds. Allow tasks running hypercalls via the privcmd driver to
@@ -283,9 +284,18 @@ static __always_inline void restore_inhc
{
__this_cpu_write(xen_in_preemptible_hcall, inhcall);
}
+
+static __always_inline void xenpv_irqentry_exit_cond_resched(void)
+{
+ instrumentation_begin();
+ raw_irqentry_exit_cond_resched();
+ trace_hardirqs_on();
+ instrumentation_end();
+}
#else
static __always_inline bool get_and_clear_inhcall(void) { return false; }
static __always_inline void restore_inhcall(bool inhcall) { }
+static __always_inline void xenpv_irqentry_exit_cond_resched(void) { }
#endif

static void __xen_pv_evtchn_do_upcall(struct pt_regs *regs)
@@ -306,11 +316,11 @@ static void __xen_pv_evtchn_do_upcall(st

instrumentation_begin();
run_sysvec_on_irqstack_cond(__xen_pv_evtchn_do_upcall, regs);
+ instrumentation_end();

inhcall = get_and_clear_inhcall();
if (inhcall && !WARN_ON_ONCE(state.exit_rcu)) {
- irqentry_exit_cond_resched();
- instrumentation_end();
+ xenpv_irqentry_exit_cond_resched();
restore_inhcall(inhcall);
} else {
instrumentation_end();
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -416,6 +416,7 @@ irqentry_state_t noinstr irqentry_enter(
* Conditional reschedule with additional sanity checks.
*/
void raw_irqentry_exit_cond_resched(void);
+
#ifdef CONFIG_PREEMPT_DYNAMIC
#if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
#define irqentry_exit_cond_resched_dynamic_enabled raw_irqentry_exit_cond_resched
--- a/include/xen/xen-ops.h
+++ b/include/xen/xen-ops.h
@@ -194,7 +194,7 @@ bool xen_running_on_version_or_later(uns
void xen_efi_runtime_setup(void);


-#if defined(CONFIG_XEN_PV) && !defined(CONFIG_PREEMPTION)
+#if defined(CONFIG_XEN_PV) && (!defined(CONFIG_PREEMPTION) || defined(CONFIG_PREEMPT_DYNAMIC))

DECLARE_PER_CPU(bool, xen_in_preemptible_hcall);

--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -385,6 +385,7 @@ void raw_irqentry_exit_cond_resched(void
preempt_schedule_irq();
}
}
+
#ifdef CONFIG_PREEMPT_DYNAMIC
#if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
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