[Xen-devel] Re: [PATCH] xen: HVM X2APIC support

[Sheng Yang <sheng@xxxxxxxxxxxxxxx>]

On Thursday 02 December 2010 21:54:55 Stefano Stabellini wrote:
> On Thu, 2 Dec 2010, Sheng Yang wrote:
> > On Thursday 02 December 2010 14:28:16 Jeremy Fitzhardinge wrote:
> > > On 12/01/2010 07:03 PM, Sheng Yang wrote:
> > > > This patch is similiar to Gleb Natapov's patch for KVM, which enable
> > > > the hypervisor to emulate x2apic feature for the guest. By this way,
> > > > the emulation of lapic would be simpler with x2apic interface(MSR),
> > > > and faster.
> > > 
> > > We have a set of patches to directly use event channels from within hvm
> > > domains, completely bypassing the apic altogether.  Do we need this as
> > > well?
> > 
> > This is for other HVMs. And the pvhvm still have limitation like it can't
> > use MSI/MSI-X assigned device.
> 
> That is not true: upstream Linux kernels can remap MSI/MSI-X into pirqs,
> if it doesn't work is a bug :)
> If you are interested give a look at
> arch/x86/pci/xen.c:xen_hvm_setup_msi_irqs.

That's great!
> 
> > > > Signed-off-by: Sheng Yang <sheng@xxxxxxxxxxxxxxx>
> > > > ---
> > > > 
> > > >  arch/x86/include/asm/xen/hypervisor.h |   33
> > > >  +++++++++++++++++++++++++++++++++ arch/x86/kernel/apic/apic.c
> > > >  
> > > >  |    4 +++-
> > > >  
> > > >  arch/x86/xen/enlighten.c              |   19 -------------------
> > > >  3 files changed, 36 insertions(+), 20 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/include/asm/xen/hypervisor.h
> > > > b/arch/x86/include/asm/xen/hypervisor.h index 396ff4c..e862874 100644
> > > > --- a/arch/x86/include/asm/xen/hypervisor.h
> > > > +++ b/arch/x86/include/asm/xen/hypervisor.h
> > > > @@ -37,4 +37,37 @@
> > > > 
> > > >  extern struct shared_info *HYPERVISOR_shared_info;
> > > >  extern struct start_info *xen_start_info;
> > > > 
> > > > +#include <asm/processor.h>
> > > > +
> > > > +static inline uint32_t xen_cpuid_base(void)
> > > > +{
> > > > +       uint32_t base, eax, ebx, ecx, edx;
> > > > +       char signature[13];
> > > > +
> > > > +       for (base = 0x40000000; base < 0x40010000; base += 0x100) {
> > > > +               cpuid(base, &eax, &ebx, &ecx, &edx);
> > > > +               *(uint32_t *)(signature + 0) = ebx;
> > > > +               *(uint32_t *)(signature + 4) = ecx;
> > > > +               *(uint32_t *)(signature + 8) = edx;
> > > > +               signature[12] = 0;
> > > > +
> > > > +               if (!strcmp("XenVMMXenVMM", signature) && ((eax - base) 
> > > > >= 2))
> > > > +                       return base;
> > > > +       }
> > > > +
> > > > +       return 0;
> > > > +}
> > > > +
> > > > +#ifdef CONFIG_XEN
> > > > +static inline bool xen_para_available(void)
> > > > +{
> > > > +       return 0;
> > > > +}
> > > > +#else
> > > > +static inline bool xen_para_available(void)
> > > > +{
> > > > +       return (xen_cpuid_base() != 0);
> > > > +}
> > > > +#endif
> > > 
> > > So this returns true if you're running a kernel without CONFIG_XEN
> > > under Xen?  Does that assume that all versions of Xen implement x2apic
> > > emulation?  Why wouldn't we also want this for CONFIG_XEN kernels?
> > 
> > Because only the ones that implement the feature would expose x2apic
> > CPUID.
> > 
> > For CONFIG_XEN(pv or pvhvm), they both use evtchn, so no need for x2apic.
> 
> In that case you need to check for CONFIG_XEN_PVHVM and the presence of
> xen_feature(XENFEAT_hvm_pirqs) because only in this case a PV on HVM
> guests are able to remap both GSIs and MSIs into evtchns.
> So I would do something like this:
> 
> 
> #ifdef CONFIG_XEN_PVHVM
> static inline bool xen_para_available(void)
> {
>     if (xen_cpuid_base() != 0 && xen_feature(XENFEAT_hvm_pirqs))
>       return 0;
>     else
>         return 1;

I suppose only HVM guest without XENFEAT_hvm_pirqs may need this. But does this 
code covered PV guest as well? We don't need cover them.

> }
> #else
> static inline bool xen_para_available(void)
> {
>       return (xen_cpuid_base() != 0);
> }
> #endif
> 
> 
> This is assuming that enabling x2apic doesn't prevent Linux from
> receiving evtchns either using the callback vector mechanism or the
> legacy platform-pci interrupt.

I suppose only legacy platform-pci would need this, because it would use lapic. 
Callback vector method would use evtchn so this won't be enabled.

> Finally when running as dom0 would this feature create problems in the
> presence of a real x2apic?

I don't think this can be enabled on dom0.

This one target on HVM domain, maybe also PVHVM domain without 
XENFEAT_hvm_pirqs, 
but not the domains using evtchn.

--
regards
Yang, Sheng

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