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Add rcu user eqs exception hooks for async page fault
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> Signed-off-by: Gleb Natapov <gleb@redhat.com>
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1 changed files with 10 additions and 2 deletions
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@ -43,6 +43,7 @@
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#include <asm/apicdef.h>
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#include <asm/hypervisor.h>
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#include <asm/kvm_guest.h>
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#include <asm/context_tracking.h>
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static int kvmapf = 1;
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@ -121,6 +122,8 @@ void kvm_async_pf_task_wait(u32 token)
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struct kvm_task_sleep_node n, *e;
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DEFINE_WAIT(wait);
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rcu_irq_enter();
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spin_lock(&b->lock);
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e = _find_apf_task(b, token);
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if (e) {
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@ -128,6 +131,8 @@ void kvm_async_pf_task_wait(u32 token)
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hlist_del(&e->link);
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kfree(e);
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spin_unlock(&b->lock);
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rcu_irq_exit();
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return;
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}
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@ -152,13 +157,16 @@ void kvm_async_pf_task_wait(u32 token)
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/*
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* We cannot reschedule. So halt.
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*/
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rcu_irq_exit();
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native_safe_halt();
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rcu_irq_enter();
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local_irq_disable();
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}
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}
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if (!n.halted)
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finish_wait(&n.wq, &wait);
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rcu_irq_exit();
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return;
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}
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EXPORT_SYMBOL_GPL(kvm_async_pf_task_wait);
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@ -252,10 +260,10 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
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break;
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case KVM_PV_REASON_PAGE_NOT_PRESENT:
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/* page is swapped out by the host. */
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rcu_irq_enter();
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exception_enter(regs);
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exit_idle();
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kvm_async_pf_task_wait((u32)read_cr2());
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rcu_irq_exit();
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exception_exit(regs);
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break;
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case KVM_PV_REASON_PAGE_READY:
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rcu_irq_enter();
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