Re: [PATCH] kvm: x86: lapic: remove one redundant judging condition

[Chen, Tiejun]

On 2014/11/5 18:22, Paolo Bonzini wrote:

On 05/11/2014 10:03, Tiejun Chen wrote:

Finally we always return highest_irr so its unnecessary to return -1
after check if highest_irr == -1.

Signed-off-by: Tiejun Chen <tiejun.c...@intel.com>
---
  arch/x86/kvm/lapic.c | 3 +--
  1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 5f574b4..e6a7eb6 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1638,8 +1638,7 @@ int kvm_apic_has_interrupt(struct kvm_vcpu *vcpu)

        apic_update_ppr(apic);
        highest_irr = apic_find_highest_irr(apic);
-       if ((highest_irr == -1) ||
-           ((highest_irr & 0xF0) <= kvm_apic_get_reg(apic, APIC_PROCPRI)))
+       if ((highest_irr & 0xF0) <= kvm_apic_get_reg(apic, APIC_PROCPRI))
                return -1;
        return highest_irr;
  }

I think the code is clearer without this change.

The two returns mean:

- return -1: no interrupt to inject

- return highest_irr: inject this interrupt

With IRR equal to all zeroes (highest_irr = -1), your patch would make
the "if" always false ("current PPR is low, can inject the interrupt"),
but computing highest_irr & 0xF0 would make no sense if highest_irr ==
-1.

Yeah, you're right so here is just a little confusion to read.

Actually what this code is doing looks like,

@@ -1638,7 +1638,7 @@ int kvm_apic_has_interrupt(struct kvm_vcpu *vcpu)

        apic_update_ppr(apic);
        highest_irr = apic_find_highest_irr(apic);
-       if ((highest_irr == -1) ||
+       if ((highest_irr != -1) &&
            ((highest_irr & 0xF0) <= kvm_apic_get_reg(apic, APIC_PROCPRI)))
                return -1;
        return highest_irr;

But it's really no big deal so we can keep the original alive.

Thanks
Tiejun

To put it another way, imagine the code looked like this:

static inline int int_prio(int vector)
{
        WARN_ON(vector == -1);
        return vector & 0xF0;
}
...

        apic_update_ppr(apic);
        highest_irr = apic_find_highest_irr(apic);
        if (highest_irr == -1 ||
            int_prio(highest_irr) <= kvm_apic_get_reg(apic, APIC_PROCPRI))
                return -1;
        return highest_irr;

Then removing the check on highest_irr == -1 would trigger a warning.

Paolo

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