Re: [win-pv-devel] [PATCH 04/10] Separate checking upcall_pending in shared info from EvtchnPoll

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Matt Wilson [mailto:mswilson@xxxxxxxxx] On Behalf Of Matt Wilson
> Sent: 10 December 2014 06:16
> To: Paul Durrant
> Cc: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH 04/10] Separate checking upcall_pending in shared info
> from EvtchnPoll
> 
> On Thu, Nov 06, 2014 at 02:24:15PM +0000, Paul Durrant wrote:
> > The current EvtchnPoll routine in in the shared info code is tied to
> > the two level event channel ABI, wheras checking upcall_pending is ABI
> > independent. Hence the check should be separated into its own method.
> >
> > This patch therefore introduces version 2 of the SHARED_INFO interface,
> > which has a new EvtchnUpcallPending method. Version 1 is still
> > supported by having its EvtchnPoll variant call EvtchnUpcallPending
> > method internally.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > ---
> >  include/shared_info_interface.h |  43 +++++++++-
> >  include/xen.h                   |   8 ++
> >  src/xen/system.c                |  20 ++++-
> >  src/xenbus/evtchn.c             |  16 +++-
> >  src/xenbus/shared_info.c        | 174 +++++++++++++++++++++++++++++-
> ----------
> >  5 files changed, 207 insertions(+), 54 deletions(-)
> >
> 
> [...]
> 
> > -extern VOID
> > +XEN_API
> > +ULONG
> > +SystemVirtualCpuIndex(
> > +    IN  ULONG           Index
> > +    )
> > +{
> > +    PSYSTEM_CONTEXT     Context = &SystemContext;
> > +    PSYSTEM_CPU         Cpu = &Context->Cpu[Index];
> > +
> > +    ASSERT3U(Index, <, MAXIMUM_PROCESSORS);
> > +
> > +    return Cpu->ApicID / 2;
> 
> Nack. This will break on EC2 instances that expose CPU topology by
> setting the initial APIC ID appropriately (i.e., not according to the
> *2 formula.

That's a shame. The code really doesn't have much option but to use this 
mechanism as Xen provides no explicit way for an HVM guest to get vcpu ids 
until my patch "x86/hvm: Extend HVM cpuid leaf with vcpu id" goes in and use of 
the FIFO ABI requires knowledge of vcpu id.

  Paul

> 
> --msw

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