[PATCH] x86/irq: hide HYP in /proc/interrupts when not on Xen/Hyper-V

[PATCH] x86/irq: hide HYP in /proc/interrupts when not on Xen/Hyper-V

[Vitaly Kuznetsov]

Hypervisor callback interrupts are only accounted on Xen/Hyper-V and we
detect hypervisor's type in early boot. There is no point in having
always-zero HYP: line on other hypervisors or bare metal.

Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/kernel/irq.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 88b36648..0c82064 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -18,6 +18,7 @@
 #include <asm/mce.h>
 #include <asm/hw_irq.h>
 #include <asm/desc.h>
+#include <asm/hypervisor.h>
 
 #define CREATE_TRACE_POINTS
 #include <asm/trace/irq_vectors.h>
@@ -139,10 +140,14 @@ int arch_show_interrupts(struct seq_file *p, int prec)
 	seq_puts(p, "  Machine check polls\n");
 #endif
 #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)
-	seq_printf(p, "%*s: ", prec, "HYP");
-	for_each_online_cpu(j)
-		seq_printf(p, "%10u ", irq_stats(j)->irq_hv_callback_count);
-	seq_puts(p, "  Hypervisor callback interrupts\n");
+	if (x86_hyper == &x86_hyper_ms_hyperv ||
+	    x86_hyper == &x86_hyper_xen) {
+		seq_printf(p, "%*s: ", prec, "HYP");
+		for_each_online_cpu(j)
+			seq_printf(p, "%10u ",
+				   irq_stats(j)->irq_hv_callback_count);
+		seq_puts(p, "  Hypervisor callback interrupts\n");
+	}
 #endif
 	seq_printf(p, "%*s: %10u\n", prec, "ERR", atomic_read(&irq_err_count));
 #if defined(CONFIG_X86_IO_APIC)
-- 
2.4.3

Re: [PATCH] x86/irq: hide HYP in /proc/interrupts when not on Xen/Hyper-V

[Ingo Molnar]

* Vitaly Kuznetsov <vkuznets@redhat.com> wrote:

> Hypervisor callback interrupts are only accounted on Xen/Hyper-V and we
> detect hypervisor's type in early boot. There is no point in having
> always-zero HYP: line on other hypervisors or bare metal.
> 
> Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kernel/irq.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> index 88b36648..0c82064 100644
> --- a/arch/x86/kernel/irq.c
> +++ b/arch/x86/kernel/irq.c
> @@ -18,6 +18,7 @@
>  #include <asm/mce.h>
>  #include <asm/hw_irq.h>
>  #include <asm/desc.h>
> +#include <asm/hypervisor.h>
>  
>  #define CREATE_TRACE_POINTS
>  #include <asm/trace/irq_vectors.h>
> @@ -139,10 +140,14 @@ int arch_show_interrupts(struct seq_file *p, int prec)
>  	seq_puts(p, "  Machine check polls\n");
>  #endif
>  #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)
> -	seq_printf(p, "%*s: ", prec, "HYP");
> -	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->irq_hv_callback_count);
> -	seq_puts(p, "  Hypervisor callback interrupts\n");
> +	if (x86_hyper == &x86_hyper_ms_hyperv ||
> +	    x86_hyper == &x86_hyper_xen) {
> +		seq_printf(p, "%*s: ", prec, "HYP");
> +		for_each_online_cpu(j)
> +			seq_printf(p, "%10u ",
> +				   irq_stats(j)->irq_hv_callback_count);
> +		seq_puts(p, "  Hypervisor callback interrupts\n");
> +	}

So I think we should simplify this to:

	if (x86_hyper) {
		...
	}

this will print the HYP line on hypervisors that don't use 
HYPERVISOR_CALLBACK_VECTOR, but it will make it a lot more self-maintaining, we 
won't accidentally skip this line on hypervisors that start using the callback 
IRQ.

Thanks,

	Ingo

Re: [PATCH] x86/irq: hide HYP in /proc/interrupts when not on Xen/Hyper-V

[Andrew Jones]

On Sat, Jul 04, 2015 at 10:26:53AM +0200, Ingo Molnar wrote:
> 
> * Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> 
> > Hypervisor callback interrupts are only accounted on Xen/Hyper-V and we
> > detect hypervisor's type in early boot. There is no point in having
> > always-zero HYP: line on other hypervisors or bare metal.
> > 
> > Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> > ---
> >  arch/x86/kernel/irq.c | 13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> > index 88b36648..0c82064 100644
> > --- a/arch/x86/kernel/irq.c
> > +++ b/arch/x86/kernel/irq.c
> > @@ -18,6 +18,7 @@
> >  #include <asm/mce.h>
> >  #include <asm/hw_irq.h>
> >  #include <asm/desc.h>
> > +#include <asm/hypervisor.h>
> >  
> >  #define CREATE_TRACE_POINTS
> >  #include <asm/trace/irq_vectors.h>
> > @@ -139,10 +140,14 @@ int arch_show_interrupts(struct seq_file *p, int prec)
> >  	seq_puts(p, "  Machine check polls\n");
> >  #endif
> >  #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)
> > -	seq_printf(p, "%*s: ", prec, "HYP");
> > -	for_each_online_cpu(j)
> > -		seq_printf(p, "%10u ", irq_stats(j)->irq_hv_callback_count);
> > -	seq_puts(p, "  Hypervisor callback interrupts\n");
> > +	if (x86_hyper == &x86_hyper_ms_hyperv ||
> > +	    x86_hyper == &x86_hyper_xen) {
> > +		seq_printf(p, "%*s: ", prec, "HYP");
> > +		for_each_online_cpu(j)
> > +			seq_printf(p, "%10u ",
> > +				   irq_stats(j)->irq_hv_callback_count);
> > +		seq_puts(p, "  Hypervisor callback interrupts\n");
> > +	}
> 
> So I think we should simplify this to:
> 
> 	if (x86_hyper) {
> 		...
> 	}
> 
> this will print the HYP line on hypervisors that don't use 
> HYPERVISOR_CALLBACK_VECTOR, but it will make it a lot more self-maintaining, we 
> won't accidentally skip this line on hypervisors that start using the callback 
> IRQ.

This is a good point. When virt stuff creeps out, it's easy to lose track of
it. But, I would prefer that vmware and kvm guests don't have to have a
meaningless counter in /proc/interrupts. Also, for xen, I see that it only
matters for x86. ARM xen builds would also have a meaningless counter. Maybe
we should add a flags member to hypervisor_x86, and start using that in
these types of situations.

Thanks,
drew

Re: [PATCH] x86/irq: hide HYP in /proc/interrupts when not on Xen/Hyper-V

[Andrew Jones]

On Mon, Jul 06, 2015 at 03:57:33PM +0200, Andrew Jones wrote:
> On Sat, Jul 04, 2015 at 10:26:53AM +0200, Ingo Molnar wrote:
> > 
> > * Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> > 
> > > Hypervisor callback interrupts are only accounted on Xen/Hyper-V and we
> > > detect hypervisor's type in early boot. There is no point in having
> > > always-zero HYP: line on other hypervisors or bare metal.
> > > 
> > > Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
> > > Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> > > ---
> > >  arch/x86/kernel/irq.c | 13 +++++++++----
> > >  1 file changed, 9 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> > > index 88b36648..0c82064 100644
> > > --- a/arch/x86/kernel/irq.c
> > > +++ b/arch/x86/kernel/irq.c
> > > @@ -18,6 +18,7 @@
> > >  #include <asm/mce.h>
> > >  #include <asm/hw_irq.h>
> > >  #include <asm/desc.h>
> > > +#include <asm/hypervisor.h>
> > >  
> > >  #define CREATE_TRACE_POINTS
> > >  #include <asm/trace/irq_vectors.h>
> > > @@ -139,10 +140,14 @@ int arch_show_interrupts(struct seq_file *p, int prec)
> > >  	seq_puts(p, "  Machine check polls\n");
> > >  #endif
> > >  #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)
> > > -	seq_printf(p, "%*s: ", prec, "HYP");
> > > -	for_each_online_cpu(j)
> > > -		seq_printf(p, "%10u ", irq_stats(j)->irq_hv_callback_count);
> > > -	seq_puts(p, "  Hypervisor callback interrupts\n");
> > > +	if (x86_hyper == &x86_hyper_ms_hyperv ||
> > > +	    x86_hyper == &x86_hyper_xen) {
> > > +		seq_printf(p, "%*s: ", prec, "HYP");
> > > +		for_each_online_cpu(j)
> > > +			seq_printf(p, "%10u ",
> > > +				   irq_stats(j)->irq_hv_callback_count);
> > > +		seq_puts(p, "  Hypervisor callback interrupts\n");
> > > +	}
> > 
> > So I think we should simplify this to:
> > 
> > 	if (x86_hyper) {
> > 		...
> > 	}
> > 
> > this will print the HYP line on hypervisors that don't use 
> > HYPERVISOR_CALLBACK_VECTOR, but it will make it a lot more self-maintaining, we 
> > won't accidentally skip this line on hypervisors that start using the callback 
> > IRQ.
> 
> This is a good point. When virt stuff creeps out, it's easy to lose track of
> it. But, I would prefer that vmware and kvm guests don't have to have a
> meaningless counter in /proc/interrupts. Also, for xen, I see that it only
> matters for x86. ARM xen builds would also have a meaningless counter. Maybe
> we should add a flags member to hypervisor_x86, and start using that in
> these types of situations.
> 
Don't know why I just referred to ARM in the context of *x86*_hyper..., but
the flag idea might still be nice.

Re: [PATCH] x86/irq: hide HYP in /proc/interrupts when not on Xen/Hyper-V

[Boris Ostrovsky]

On 07/06/2015 09:57 AM, Andrew Jones wrote:
> On Sat, Jul 04, 2015 at 10:26:53AM +0200, Ingo Molnar wrote:
>> * Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>
>>> Hypervisor callback interrupts are only accounted on Xen/Hyper-V and we
>>> detect hypervisor's type in early boot. There is no point in having
>>> always-zero HYP: line on other hypervisors or bare metal.
>>>
>>> Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
>>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>>> ---
>>>   arch/x86/kernel/irq.c | 13 +++++++++----
>>>   1 file changed, 9 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
>>> index 88b36648..0c82064 100644
>>> --- a/arch/x86/kernel/irq.c
>>> +++ b/arch/x86/kernel/irq.c
>>> @@ -18,6 +18,7 @@
>>>   #include <asm/mce.h>
>>>   #include <asm/hw_irq.h>
>>>   #include <asm/desc.h>
>>> +#include <asm/hypervisor.h>
>>>   
>>>   #define CREATE_TRACE_POINTS
>>>   #include <asm/trace/irq_vectors.h>
>>> @@ -139,10 +140,14 @@ int arch_show_interrupts(struct seq_file *p, int prec)
>>>   	seq_puts(p, "  Machine check polls\n");
>>>   #endif
>>>   #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)
>>> -	seq_printf(p, "%*s: ", prec, "HYP");
>>> -	for_each_online_cpu(j)
>>> -		seq_printf(p, "%10u ", irq_stats(j)->irq_hv_callback_count);
>>> -	seq_puts(p, "  Hypervisor callback interrupts\n");
>>> +	if (x86_hyper == &x86_hyper_ms_hyperv ||
>>> +	    x86_hyper == &x86_hyper_xen) {
>>> +		seq_printf(p, "%*s: ", prec, "HYP");
>>> +		for_each_online_cpu(j)
>>> +			seq_printf(p, "%10u ",
>>> +				   irq_stats(j)->irq_hv_callback_count);
>>> +		seq_puts(p, "  Hypervisor callback interrupts\n");
>>> +	}
>> So I think we should simplify this to:
>>
>> 	if (x86_hyper) {
>> 		...
>> 	}
>>
>> this will print the HYP line on hypervisors that don't use
>> HYPERVISOR_CALLBACK_VECTOR, but it will make it a lot more self-maintaining, we
>> won't accidentally skip this line on hypervisors that start using the callback
>> IRQ.
> This is a good point. When virt stuff creeps out, it's easy to lose track of
> it. But, I would prefer that vmware and kvm guests don't have to have a
> meaningless counter in /proc/interrupts. Also, for xen, I see that it only
> matters for x86. ARM xen builds would also have a meaningless counter. Maybe
> we should add a flags member to hypervisor_x86, and start using that in
> these types of situations.

This is all x86 arch code so I don't see how it will make any difference 
to Xen on ARM.


-boris

Re: [PATCH] x86/irq: hide HYP in /proc/interrupts when not on Xen/Hyper-V

[Vitaly Kuznetsov]

Ingo Molnar <mingo@kernel.org> writes:

> * Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
>> Hypervisor callback interrupts are only accounted on Xen/Hyper-V and we
>> detect hypervisor's type in early boot. There is no point in having
>> always-zero HYP: line on other hypervisors or bare metal.
>> 
>> Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  arch/x86/kernel/irq.c | 13 +++++++++----
>>  1 file changed, 9 insertions(+), 4 deletions(-)
>> 
>> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
>> index 88b36648..0c82064 100644
>> --- a/arch/x86/kernel/irq.c
>> +++ b/arch/x86/kernel/irq.c
>> @@ -18,6 +18,7 @@
>>  #include <asm/mce.h>
>>  #include <asm/hw_irq.h>
>>  #include <asm/desc.h>
>> +#include <asm/hypervisor.h>
>>  
>>  #define CREATE_TRACE_POINTS
>>  #include <asm/trace/irq_vectors.h>
>> @@ -139,10 +140,14 @@ int arch_show_interrupts(struct seq_file *p, int prec)
>>  	seq_puts(p, "  Machine check polls\n");
>>  #endif
>>  #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)
>> -	seq_printf(p, "%*s: ", prec, "HYP");
>> -	for_each_online_cpu(j)
>> -		seq_printf(p, "%10u ", irq_stats(j)->irq_hv_callback_count);
>> -	seq_puts(p, "  Hypervisor callback interrupts\n");
>> +	if (x86_hyper == &x86_hyper_ms_hyperv ||
>> +	    x86_hyper == &x86_hyper_xen) {
>> +		seq_printf(p, "%*s: ", prec, "HYP");
>> +		for_each_online_cpu(j)
>> +			seq_printf(p, "%10u ",
>> +				   irq_stats(j)->irq_hv_callback_count);
>> +		seq_puts(p, "  Hypervisor callback interrupts\n");
>> +	}
>
> So I think we should simplify this to:
>
> 	if (x86_hyper) {
> 		...
> 	}
>
> this will print the HYP line on hypervisors that don't use 
> HYPERVISOR_CALLBACK_VECTOR, but it will make it a lot more self-maintaining, we 
> won't accidentally skip this line on hypervisors that start using the callback 
> IRQ.

I was under an impression KVM and Vmware don't use these hypervisor
callback irqs 'by desing' and even if one day they start using it
someone will have to put inc_irq_stat() somewhere.

If the goal is to avoid having x86_hyper_ms_hyperv/x86_hyper_xen in
irq.c we can follow Andrew's suggestion and add a flag
(use_callback_irqs?) to struct hypervisor_x86.

Thanks,

-- 
  Vitaly

Re: [PATCH] x86/irq: hide HYP in /proc/interrupts when not on Xen/Hyper-V

[Ingo Molnar]

* Vitaly Kuznetsov <vkuznets@redhat.com> wrote:

> Ingo Molnar <mingo@kernel.org> writes:
> 
> > * Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> >
> >> Hypervisor callback interrupts are only accounted on Xen/Hyper-V and we
> >> detect hypervisor's type in early boot. There is no point in having
> >> always-zero HYP: line on other hypervisors or bare metal.
> >> 
> >> Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> >> ---
> >>  arch/x86/kernel/irq.c | 13 +++++++++----
> >>  1 file changed, 9 insertions(+), 4 deletions(-)
> >> 
> >> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> >> index 88b36648..0c82064 100644
> >> --- a/arch/x86/kernel/irq.c
> >> +++ b/arch/x86/kernel/irq.c
> >> @@ -18,6 +18,7 @@
> >>  #include <asm/mce.h>
> >>  #include <asm/hw_irq.h>
> >>  #include <asm/desc.h>
> >> +#include <asm/hypervisor.h>
> >>  
> >>  #define CREATE_TRACE_POINTS
> >>  #include <asm/trace/irq_vectors.h>
> >> @@ -139,10 +140,14 @@ int arch_show_interrupts(struct seq_file *p, int prec)
> >>  	seq_puts(p, "  Machine check polls\n");
> >>  #endif
> >>  #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)
> >> -	seq_printf(p, "%*s: ", prec, "HYP");
> >> -	for_each_online_cpu(j)
> >> -		seq_printf(p, "%10u ", irq_stats(j)->irq_hv_callback_count);
> >> -	seq_puts(p, "  Hypervisor callback interrupts\n");
> >> +	if (x86_hyper == &x86_hyper_ms_hyperv ||
> >> +	    x86_hyper == &x86_hyper_xen) {
> >> +		seq_printf(p, "%*s: ", prec, "HYP");
> >> +		for_each_online_cpu(j)
> >> +			seq_printf(p, "%10u ",
> >> +				   irq_stats(j)->irq_hv_callback_count);
> >> +		seq_puts(p, "  Hypervisor callback interrupts\n");
> >> +	}
> >
> > So I think we should simplify this to:
> >
> > 	if (x86_hyper) {
> > 		...
> > 	}
> >
> > this will print the HYP line on hypervisors that don't use 
> > HYPERVISOR_CALLBACK_VECTOR, but it will make it a lot more self-maintaining, we 
> > won't accidentally skip this line on hypervisors that start using the callback 
> > IRQ.
> 
> I was under an impression KVM and Vmware don't use these hypervisor
> callback irqs 'by desing' and even if one day they start using it
> someone will have to put inc_irq_stat() somewhere.
> 
> If the goal is to avoid having x86_hyper_ms_hyperv/x86_hyper_xen in
> irq.c we can follow Andrew's suggestion and add a flag
> (use_callback_irqs?) to struct hypervisor_x86.

Yeah, a flag would be nicer, and that flag should also be used somewhere else - 
for example in the IRQ vector init code or so?

I.e. make it only possible to use the callback vector if the flag is set properly 
- or something like that. That way there will never be a mismatch between flag and 
reality.

Thanks,

	Ingo

Re: [PATCH] x86/irq: hide HYP in /proc/interrupts when not on Xen/Hyper-V

[Vitaly Kuznetsov]

Ingo Molnar <mingo@kernel.org> writes:

> * Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
>> Ingo Molnar <mingo@kernel.org> writes:
>> 
>> > * Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>> >
>> >> Hypervisor callback interrupts are only accounted on Xen/Hyper-V and we
>> >> detect hypervisor's type in early boot. There is no point in having
>> >> always-zero HYP: line on other hypervisors or bare metal.
>> >> 
>> >> Suggested-by: Radim Krčmář <rkrcmar@redhat.com>
>> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> >> ---
>> >>  arch/x86/kernel/irq.c | 13 +++++++++----
>> >>  1 file changed, 9 insertions(+), 4 deletions(-)
>> >> 
>> >> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
>> >> index 88b36648..0c82064 100644
>> >> --- a/arch/x86/kernel/irq.c
>> >> +++ b/arch/x86/kernel/irq.c
>> >> @@ -18,6 +18,7 @@
>> >>  #include <asm/mce.h>
>> >>  #include <asm/hw_irq.h>
>> >>  #include <asm/desc.h>
>> >> +#include <asm/hypervisor.h>
>> >>  
>> >>  #define CREATE_TRACE_POINTS
>> >>  #include <asm/trace/irq_vectors.h>
>> >> @@ -139,10 +140,14 @@ int arch_show_interrupts(struct seq_file *p, int prec)
>> >>  	seq_puts(p, "  Machine check polls\n");
>> >>  #endif
>> >>  #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)
>> >> -	seq_printf(p, "%*s: ", prec, "HYP");
>> >> -	for_each_online_cpu(j)
>> >> -		seq_printf(p, "%10u ", irq_stats(j)->irq_hv_callback_count);
>> >> -	seq_puts(p, "  Hypervisor callback interrupts\n");
>> >> +	if (x86_hyper == &x86_hyper_ms_hyperv ||
>> >> +	    x86_hyper == &x86_hyper_xen) {
>> >> +		seq_printf(p, "%*s: ", prec, "HYP");
>> >> +		for_each_online_cpu(j)
>> >> +			seq_printf(p, "%10u ",
>> >> +				   irq_stats(j)->irq_hv_callback_count);
>> >> +		seq_puts(p, "  Hypervisor callback interrupts\n");
>> >> +	}
>> >
>> > So I think we should simplify this to:
>> >
>> > 	if (x86_hyper) {
>> > 		...
>> > 	}
>> >
>> > this will print the HYP line on hypervisors that don't use 
>> > HYPERVISOR_CALLBACK_VECTOR, but it will make it a lot more self-maintaining, we 
>> > won't accidentally skip this line on hypervisors that start using the callback 
>> > IRQ.
>> 
>> I was under an impression KVM and Vmware don't use these hypervisor
>> callback irqs 'by desing' and even if one day they start using it
>> someone will have to put inc_irq_stat() somewhere.
>> 
>> If the goal is to avoid having x86_hyper_ms_hyperv/x86_hyper_xen in
>> irq.c we can follow Andrew's suggestion and add a flag
>> (use_callback_irqs?) to struct hypervisor_x86.
>
> Yeah, a flag would be nicer, and that flag should also be used somewhere else - 
> for example in the IRQ vector init code or so?
>
> I.e. make it only possible to use the callback vector if the flag is set properly 
> - or something like that. That way there will never be a mismatch between flag and 
> reality.

Actually we already have the flag :-) Both Xen and Hyper-V do
alloc_intr_gate(HYPERVISOR_CALLBACK_VECTOR, ...) so we can simply do
test_bit(HYPERVISOR_CALLBACK_VECTOR, used_vectors) here. It feels a
little bit hackish but it checks what we need to check.

-- 
  Vitaly