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Message-Id: <201012031448.04382.sheng@linux.intel.com>
Date: Fri, 3 Dec 2010 14:48:04 +0800
From: Sheng Yang <sheng@...ux.intel.com>
To: Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@...p.org>,
Keir Fraser <Keir.Fraser@...citrix.com>,
"xen-devel" <xen-devel@...ts.xensource.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen: HVM X2APIC support
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@...ux.intel.com>
> > > > ---
> > > >
> > > > 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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