Since Paolo has removed irq-enable-operation in vmx_handle_external_intr
(KVM: x86: use guest_exit_irqoff), the original comment about the IF bit
in rflags is incorrect now.
Signed-off-by: Longpeng(Mike) <[email protected]>
---
arch/x86/kvm/vmx.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index cf1b16d..9fa3c76 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -8647,9 +8647,12 @@ static void vmx_handle_external_intr(struct kvm_vcpu *vcpu)
register void *__sp asm(_ASM_SP);
/*
- * If external interrupt exists, IF bit is set in rflags/eflags on the
- * interrupt stack frame, and interrupt will be enabled on a return
- * from interrupt handler.
+ * If external interrupt exists, fakes an interrupt stack and jump to
+ * idt table to let real handler to handle it. Because most of bits in
+ * rflags are cleared when VM exit(Intel SDM volum 3, chapter 27.5.3),
+ * the IF bit is 0 in rflags on the interrupt stack frame, so interrupt
+ * is still disabled when return from the irq handler, but it will be
+ * enabled later by the caller.
*/
if ((exit_intr_info & (INTR_INFO_VALID_MASK | INTR_INFO_INTR_TYPE_MASK))
== (INTR_INFO_VALID_MASK | INTR_TYPE_EXT_INTR)) {
--
2.7.4
2016-10-10 08:23+0800, Longpeng(Mike):
> Since Paolo has removed irq-enable-operation in vmx_handle_external_intr
> (KVM: x86: use guest_exit_irqoff), the original comment about the IF bit
> in rflags is incorrect now.
>
> Signed-off-by: Longpeng(Mike) <[email protected]>
> ---
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -8647,9 +8647,12 @@ static void vmx_handle_external_intr(struct kvm_vcpu *vcpu)
> register void *__sp asm(_ASM_SP);
>
> /*
> - * If external interrupt exists, IF bit is set in rflags/eflags on the
> - * interrupt stack frame, and interrupt will be enabled on a return
> - * from interrupt handler.
Good catch, thanks.
We want to change it, but I think that the new comment is an overkill.
I am generally not a fan of code comments that describe what the code
does; code speaks for itself and it is better to fix the code, e.g.
split into well named functions, instead of duplicating it.
> + * If external interrupt exists, fakes an interrupt stack and jump to
> + * idt table to let real handler to handle it.
This is the duplication I was talking about. If the corresponding part
of the code is not obvious, it would be better to rework it instead.
> Because most of bits in
> + * rflags are cleared when VM exit(Intel SDM volum 3, chapter 27.5.3),
> + * the IF bit is 0 in rflags on the interrupt stack frame, so interrupt
> + * is still disabled when return from the irq handler, but it will be
> + * enabled later by the caller.
This part is acceptable as it gives a new information code, yet the
function does not modify flags, which makes it unremarkable.
And dependencies on the caller would be better described in a header
(if we cannot express them well in the code).
The most comment-worthy thing about this function is the reason why we
execute the interrupt handler manually, i.e. the dependency on
VM_EXIT_ACK_INTR_ON_EXIT, but that is easy to tell from the commit
message and convenient access to git history is essential in a workflow,
so providing a leeway could be counter-productive.
I would go with no comment for now.
Hi Radim,
On 2016/10/12 2:23, Radim Krčmář wrote:
> This part is acceptable as it gives a new information code, yet the
> function does not modify flags, which makes it unremarkable.
> And dependencies on the caller would be better described in a header
> (if we cannot express them well in the code).
>
> The most comment-worthy thing about this function is the reason why we
> execute the interrupt handler manually, i.e. the dependency on
> VM_EXIT_ACK_INTR_ON_EXIT, but that is easy to tell from the commit
> message and convenient access to git history is essential in a workflow,
> so providing a leeway could be counter-productive.
>
> I would go with no comment for now.
>
> .
>
Thanks for your patience, and your advice is useful for me.
In addition, the comment below is misleading too, hope you can fix it
simultaneously.
/* Interrupt is enabled by handle_external_intr() */
kvm_x86_ops->handle_external_intr(vcpu);
--
Regards,
Longpeng(Mike)
2016-10-12 09:15+0800, Longpeng (Mike):
> On 2016/10/12 2:23, Radim Krčmář wrote:
>> This part is acceptable as it gives a new information code, yet the
>> function does not modify flags, which makes it unremarkable.
>> And dependencies on the caller would be better described in a header
>> (if we cannot express them well in the code).
>>
>> The most comment-worthy thing about this function is the reason why we
>> execute the interrupt handler manually, i.e. the dependency on
>> VM_EXIT_ACK_INTR_ON_EXIT, but that is easy to tell from the commit
>> message and convenient access to git history is essential in a workflow,
>> so providing a leeway could be counter-productive.
>>
>> I would go with no comment for now.
>
> Thanks for your patience, and your advice is useful for me.
I appreciate the patch, I just didn't want to repeat the same mistake
that you were fixing in the patch, which made me go into rambling mode.
Please send v2 with a simpler code comment (or no comment).
And you are more than welcome to improve the code even further!
> In addition, the comment below is misleading too, hope you can fix it
> simultaneously.
>
> /* Interrupt is enabled by handle_external_intr() */
> kvm_x86_ops->handle_external_intr(vcpu);
Yep, this comment should have been expressed in a function name.
Paolo already fixed it in 1a6982353db9 ("KVM: x86: remove stale
comments").