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Subject[RFC PATCH v3] Add rcu user eqs exception hooks for async page fault
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This patch adds user eqs exception hooks for async page fault page not
present code path, to exit the user eqs and re-enter it as necessary.

Async page fault is different from other exceptions that it may be
triggered from idle process, so we still need rcu_irq_enter() and
rcu_irq_exit() to exit cpu idle eqs when needed, to protect the code
that needs use rcu.

As Frederic pointed out it would be safest and simplest to protect the
whole kvm_async_pf_task_wait(). Otherwise, "we need to check all the
code there deeply for potential RCU uses and ensure it will never be
extended later to use RCU.".

However, We'd better re-enter the cpu idle eqs if we get the exception
in cpu idle eqs, by calling rcu_irq_exit() before native_safe_halt().

So the patch does what Frederic suggested for rcu_irq_*() API usage
here, except that I moved the rcu_irq_*() pair originally in
do_async_page_fault() into kvm_async_pf_task_wait().

That's because, I think it's better to have rcu_irq_*() pairs to be in
one function ( rcu_irq_exit() after rcu_irq_enter() ), especially here,
kvm_async_pf_task_wait() has other callers, which might cause
rcu_irq_exit() be called without a matching rcu_irq_enter() before it,
which is illegal if the cpu happens to be in rcu idle state.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
---
arch/x86/kernel/kvm.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 4180a87..342b00b 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -42,6 +42,7 @@
#include <asm/apic.h>
#include <asm/apicdef.h>
#include <asm/hypervisor.h>
+#include <asm/rcu.h>

static int kvmapf = 1;

@@ -112,6 +113,8 @@ void kvm_async_pf_task_wait(u32 token)
DEFINE_WAIT(wait);
int cpu, idle;

+ rcu_irq_enter();
+
cpu = get_cpu();
idle = idle_cpu(cpu);
put_cpu();
@@ -123,6 +126,8 @@ void kvm_async_pf_task_wait(u32 token)
hlist_del(&e->link);
kfree(e);
spin_unlock(&b->lock);
+
+ rcu_irq_exit();
return;
}

@@ -147,13 +152,16 @@ void kvm_async_pf_task_wait(u32 token)
/*
* We cannot reschedule. So halt.
*/
+ rcu_irq_exit();
native_safe_halt();
+ rcu_irq_enter();
local_irq_disable();
}
}
if (!n.halted)
finish_wait(&n.wq, &wait);

+ rcu_irq_exit();
return;
}
EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
@@ -247,10 +255,10 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
break;
case KVM_PV_REASON_PAGE_NOT_PRESENT:
/* page is swapped out by the host. */
- rcu_irq_enter();
+ exception_enter(regs);
exit_idle();
kvm_async_pf_task_wait((u32)read_cr2());
- rcu_irq_exit();
+ exception_exit(regs);
break;
case KVM_PV_REASON_PAGE_READY:
rcu_irq_enter();
--
1.7.11.4


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