On 28.06.2010, at 10:13, Avi Kivity wrote:

On 06/28/2010 10:49 AM, Alexander Graf wrote:

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I don't believe we support the kernel actually doing a syscall to =

itself

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anymore, at least on powerpc.  The callers call the underlying =

system

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call function, or kernel_thread.
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That said, I would suggest we allocate a syscall number for this, as =

it

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would document the usage.  (In additon to 0..nr_syscalls - 1 we have
0x1ebe in use).
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That's actually a pretty good idea.
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Since the syscall register is not architectual (or rather it is =

architectural but Linux ignores it) I don't see the point.  It would =
work for Linux but may alias some random parameter for a different =
guest.  We need a reliable method of distinguishing between syscalls and =
hypercalls.  Matching pc would work (but is defeated by inlining) so =
long as we find some other way of identifying the hc pc to the =
hypervisor.

The other alternative I'd see is to reuse an instruction that is not sc. =
We could for example pull the mfpvr trick again, but pass a different =
magic value in the register this time that tells the hypervisor "this is =
a hypercall".

Or we could reserve a different SPR. But from what I've seen there are =
already quite a lot of SPRs out there. More than available numbers :).

The hypercall technique I used here is actually inspired by MOL. They =
use magic constants in r3 and r4 for their "OSI" identification. I'm =
frankly not sure what the best approach is, but considering that =
syscalls from the kernel lie in the guest kernel's hand, we could just =
declare any breakage a guest kernel bug.


Alex