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