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Re: [Qemu-devel] [PATCH 03/26] Add a hook to allow hypercalls to be emul

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 03/26] Add a hook to allow hypercalls to be emulated on PowerPC
Date: Thu, 17 Mar 2011 08:20:52 -0500
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On 03/16/2011 11:55 PM, David Gibson wrote:
On Wed, Mar 16, 2011 at 03:44:49PM -0500, Anthony Liguori wrote:
On 03/15/2011 11:56 PM, David Gibson wrote:
From: David Gibson<address@hidden>

PowerPC and POWER chips since the POWER4 and 970 have a special
hypervisor mode, and a corresponding form of the system call
instruction which traps to the hypervisor.

qemu currently has stub implementations of hypervisor mode.  That
is, the outline is there to allow qemu to run a PowerPC hypervisor
under emulation.  There are a number of details missing so this
won't actually work at present, but the idea is there.

What there is no provision at all, is for qemu to instead emulate
the hypervisor itself.  That is to have hypercalls trap into qemu
and their result be emulated from qemu, rather than running
hypervisor code within the emulated system.

Hypervisor hardware aware KVM implementations are in the works and
it would  be useful for debugging and development to also allow
full emulation of the same para-virtualized guests as such a KVM.

Therefore, this patch adds a hook which will allow a machine to
set up emulation of hypervisor calls.

Signed-off-by: David Gibson<address@hidden>
  target-ppc/cpu.h    |    2 ++
  target-ppc/helper.c |    4 ++++
  2 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/target-ppc/cpu.h b/target-ppc/cpu.h
index a20c132..eaddc27 100644
--- a/target-ppc/cpu.h
+++ b/target-ppc/cpu.h
@@ -692,6 +692,8 @@ struct CPUPPCState {
      int bfd_mach;
      uint32_t flags;
      uint64_t insns_flags;
+    void (*emulate_hypercall)(CPUState *, void *);
+    void *hcall_opaque;
Is the hypercall handler ever specific to a CPU?
If you mean, "is the hypercall environment ever different from one cpu
to another within the same guest at the same time", then no.  Or at
least, no for any platform that exists now, and anything plausible I
can think of.

Yes, that's what I was asking. So having a function pointer in each CPUState isn't necessary.

If you mean can the hypercall ABI and handling be different for
different CPU models within an architecture, then yes.  It's not there
yet, but BookE CPUs *will* have a quite different hypercall
environment to the PAPR hypercall environment used on IBM servers.

I'd prefer to see this as a generic interface that wasn't specific
to target-ppc.
Basically, add a:

void cpu_hypercall(CPUState *env);

And then implement it within your target.
I'm not exactly sure what you mean by "target" here.  It is *not*
sufficient to make the hypercall function per guest architecture, it
must be per machine.  However, it could be a global hook rather than
in the CPUState.

I'd suggest a totally generic hypercall infrastructure but I know that's not plausible for Power. So I'm suggesting defining cpu_hypercall() in cpu.h, and then somewhere in target-ppc/, you can implement whatever logic you need to support that function.

This fits well with how we dispatch other forms of I/O (cpu_outb, cpu_physical_memory_rw, etc).


Anthony Liguori

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