Re: [PATCH] x86: Make x2APIC support depend on interrupt remapping or guest support

[Gleb Natapov <gleb@kernel.org>]

On Sun, Jul 06, 2014 at 05:24:27PM +0200, Jan Kiszka wrote:
> On 2014-07-06 17:12, Gleb Natapov wrote:
> > On Sat, Jul 05, 2014 at 09:47:54AM +0200, Jan Kiszka wrote:
> >> From: Jan Kiszka <jan.kiszka@siemens.com>
> >>
> >> We are able to use x2APIC mode in the absence of interrupt remapping on
> >> certain hypervisors. So it if fine to disable IRQ_REMAP without having
> >> to give up x2APIC support.
> > FWIW I did similar thing back when I added x2apic to KVM:
> > http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-06/msg14579.html
> > But was advised against it.
> 
> I don't get the point from that thread.
It is not architectural to use x2apic without irq remapping, so Suresh
from Intel felt that it should not be allowed to create a kernel that
can run on real HW and has non architectural configuration compiled in,
though only when running on a VM x2apic will ever be enabled without irq
remapping. I didn't argue about it to much back then. If HYPERVISOR_GUEST
guaranties that resulting kernel can run only on a VM (does it?) this
objection does not hold any more.

> 
> However, this change is just formalizing a configuration that is already
> being used: there is no interrupt remapping available with KVM (yet),
> but we do run our guests in x2APIC mode most of the time. Interrupt
> remapping code in kernels tailored as KVM guest is dead code, right now.
Yes. I am just saying why things are the way they are.

> 
> Jan
> 
> > 
> >>
> >> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> >> ---
> >>  arch/x86/Kconfig | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> >> index a8f749e..30a9987 100644
> >> --- a/arch/x86/Kconfig
> >> +++ b/arch/x86/Kconfig
> >> @@ -310,7 +310,7 @@ config SMP
> >>  
> >>  config X86_X2APIC
> >>  	bool "Support x2apic"
> >> -	depends on X86_LOCAL_APIC && X86_64 && IRQ_REMAP
> >> +	depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST)
> >>  	---help---
> >>  	  This enables x2apic support on CPUs that have this feature.
> >>  
> >>
> >>
> > 
> > 
> > 
> > --
> > 			Gleb.
> > 
> 
> 



--
			Gleb.