lkml.org 
[lkml]   [2014]   [Jul]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] x86: Make x2APIC support depend on interrupt remapping or guest support
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.


\
 
 \ /
  Last update: 2014-07-06 18:01    [W:0.116 / U:0.032 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site