From: Austin Kim <[email protected]>
Normally local variable 'flags' is defined out of for loop,
when 'flags' is used as the second parameter in a call to
spinlock_irq[save/restore] function.
So it had better declear local variable 'flags' ahead of for loop.
Signed-off-by: Austin Kim <[email protected]>
---
arch/arm64/kvm/vgic/vgic-mmio.c | 2 +-
arch/arm64/kvm/vgic/vgic-v4.c | 5 ++---
2 files changed, 3 insertions(+), 4 deletions(-)
diff --git a/arch/arm64/kvm/vgic/vgic-mmio.c b/arch/arm64/kvm/vgic/vgic-mmio.c
index 48c6067fc5ec..ae32428d9f87 100644
--- a/arch/arm64/kvm/vgic/vgic-mmio.c
+++ b/arch/arm64/kvm/vgic/vgic-mmio.c
@@ -232,11 +232,11 @@ unsigned long vgic_mmio_read_pending(struct kvm_vcpu *vcpu,
u32 intid = VGIC_ADDR_TO_INTID(addr, 1);
u32 value = 0;
int i;
+ unsigned long flags;
/* Loop over all IRQs affected by this read */
for (i = 0; i < len * 8; i++) {
struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, intid + i);
- unsigned long flags;
bool val;
raw_spin_lock_irqsave(&irq->irq_lock, flags);
diff --git a/arch/arm64/kvm/vgic/vgic-v4.c b/arch/arm64/kvm/vgic/vgic-v4.c
index c1845d8f5f7e..a0c743f83bbe 100644
--- a/arch/arm64/kvm/vgic/vgic-v4.c
+++ b/arch/arm64/kvm/vgic/vgic-v4.c
@@ -116,7 +116,7 @@ static void vgic_v4_enable_vsgis(struct kvm_vcpu *vcpu)
{
struct its_vpe *vpe = &vcpu->arch.vgic_cpu.vgic_v3.its_vpe;
int i;
-
+ unsigned long flags;
/*
* With GICv4.1, every virtual SGI can be directly injected. So
* let's pretend that they are HW interrupts, tied to a host
@@ -125,7 +125,6 @@ static void vgic_v4_enable_vsgis(struct kvm_vcpu *vcpu)
for (i = 0; i < VGIC_NR_SGIS; i++) {
struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, i);
struct irq_desc *desc;
- unsigned long flags;
int ret;
raw_spin_lock_irqsave(&irq->irq_lock, flags);
@@ -158,11 +157,11 @@ static void vgic_v4_enable_vsgis(struct kvm_vcpu *vcpu)
static void vgic_v4_disable_vsgis(struct kvm_vcpu *vcpu)
{
int i;
+ unsigned long flags;
for (i = 0; i < VGIC_NR_SGIS; i++) {
struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, i);
struct irq_desc *desc;
- unsigned long flags;
int ret;
raw_spin_lock_irqsave(&irq->irq_lock, flags);
--
2.20.1
On Sat, 12 Jun 2021 12:00:14 +0100,
Austin Kim <[email protected]> wrote:
>
> From: Austin Kim <[email protected]>
>
> Normally local variable 'flags' is defined out of for loop,
> when 'flags' is used as the second parameter in a call to
> spinlock_irq[save/restore] function.
>
> So it had better declear local variable 'flags' ahead of for loop.
Why better? Reducing the scope of a variable is in general good
practice. Do you see any material advantage in moving this variable
out of the loop? Does the compiler generate better code?
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
2021년 6월 12일 (토) 오후 8:10, Marc Zyngier <[email protected]>님이 작성:
>
> On Sat, 12 Jun 2021 12:00:14 +0100,
> Austin Kim <[email protected]> wrote:
> >
> > From: Austin Kim <[email protected]>
> >
> > Normally local variable 'flags' is defined out of for loop,
> > when 'flags' is used as the second parameter in a call to
> > spinlock_irq[save/restore] function.
> >
> > So it had better declear local variable 'flags' ahead of for loop.
>
> Why better? Reducing the scope of a variable is in general good
> practice. Do you see any material advantage in moving this variable
> out of the loop? Does the compiler generate better code?
First all of, thanks for feedback.
I checked how the compiler generate assembly code(before/after) using
objdump utility.
And then found out that compiler generates the same assembly code.
<compiler version: gcc-linaro-7.5.0-2019.12-x86_64_aarch64>
ffff80001005f5c8 <vgic_mmio_read_pending>:
ffff80001005f5c8: d503233f paciasp
...
ffff80001005f63c: 97ffe9af bl ffff800010059cf8 <vgic_get_irq>
ffff80001005f640: aa0003f3 mov x19, x0
ffff80001005f644: 943886c3 bl ffff800010e81150 <_raw_spin_lock_irqsave>
Let me keep in mind how compiler generate assembly code with new patch,
which leads to material advantage.
BR,
Austin Kim
>
> Thanks,
>
> M.
>
> --
> Without deviation from the norm, progress is not possible.