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

[Anthony Liguori <anthony@codemonkey.ws>]

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<dwg@au1.ibm.com>
>>>
>>> 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<dwg@au1.ibm.com>
>>> ---
>>>   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).

Regards,

Anthony Liguori