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SubjectRe: [RFC PATCH v3] Add rcu user eqs exception hooks for async page fault
On Fri, Nov 30, 2012 at 05:18:41PM +0800, Li Zhong wrote:
> 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();
> +
Why move rcu_irq_*() calls into kvm_async_pf_task_wait()?

> 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();
We can skip that if rcu_irq_*() will stay outside.

> 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

--
Gleb.


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