On 05/01/2013 07:15:53 PM, Marcelo Tosatti wrote:
On Fri, Apr 26, 2013 at 07:53:38PM -0500, Scott Wood wrote:
> diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
> index 1020119..506c87d 100644
> --- a/arch/powerpc/kvm/booke.c
> +++ b/arch/powerpc/kvm/booke.c
> @@ -832,6 +832,8 @@ int kvmppc_handle_exit(struct kvm_run *run,
struct kvm_vcpu *vcpu,
> {
> int r = RESUME_HOST;
> int s;
> + int idx = 0; /* silence bogus uninitialized warning */
> + bool need_srcu = false;
>
> /* update before a new last_exit_type is rewritten */
> kvmppc_update_timing_stats(vcpu);
> @@ -847,6 +849,20 @@ int kvmppc_handle_exit(struct kvm_run *run,
struct kvm_vcpu *vcpu,
> run->exit_reason = KVM_EXIT_UNKNOWN;
> run->ready_for_interrupt_injection = 1;
>
> + /*
> + * Don't get the srcu lock unconditionally, because kvm_ppc_pv()
> + * can call kvm_vcpu_block(), and kvm_ppc_pv() is shared with
> + * book3s, so dropping the srcu lock there would be awkward.
> + */
> + switch (exit_nr) {
> + case BOOKE_INTERRUPT_ITLB_MISS:
> + case BOOKE_INTERRUPT_DTLB_MISS:
> + need_srcu = true;
> + }
This is not good practice (codepaths should either hold srcu or not
hold
it, unconditionally).
How is it different from moving the srcu lock into individual cases of
the switch? I just did it this way to make it easier to add new
exception types if necessary (e.g. at the time I thought I'd end up
adding exceptions which lead to instruction emulation, but I ended up
acquiring the lock further down the path in that case).
Can you give more details of the issue? (not obvious)
ITLB/DTLB miss call things like gfn_to_memslot() which need the lock
(but don't grab it themselves -- that seems like the real bad practice
here...). The syscall exceptions can't have the SRCU lock held,
because they call kvmppc_kvm_pv which can call kvm_vcpu_block() (yes,
you can sleep with SRCU, but not indefinitely...). kvmppc_kvm_pv is
shared with book3s code, so adding code to drop the srcu lock there
would be a problem since book3s doesn't hold the SRCU lock then...
-Scott
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