Re: [RFC PATCH 0/3] generic hypercall support

Hollis Blanchard wrote:
On Sun, 2009-05-10 at 13:38 -0500, Anthony Liguori wrote:
Gregory Haskins wrote:
Can you back up your claim that PPC has no difference in performance
with an MMIO exit and a "hypercall" (yes, I understand PPC has no "VT"
like instructions, but clearly there are ways to cause a trap, so
presumably we can measure the difference between a PF exit and something
more explicit).
First, the PPC that KVM supports performs very poorly relatively speaking because it receives no hardware assistance this is not the right place to focus wrt optimizations.

And because there's no hardware assistance, there simply isn't a hypercall instruction. Are PFs the fastest type of exits? Probably not but I honestly have no idea. I'm sure Hollis does though.

Memory load from the guest context (for instruction decoding) is a
*very* poorly performing path on most PowerPC, even considering server
PowerPC with hardware virtualization support. No, I don't have any data
for you, but switching the hardware MMU contexts requires some
heavyweight synchronization instructions.

For current ppcemb, you would have to do a memory load no matter what, right? I guess you could have a dedicated interrupt vector or something...

For future ppcemb's, do you know if there is an equivalent of a PF exit type? Does the hardware squirrel away the faulting address somewhere and set PC to the start of the instruction? If so, no guest memory load should be required.

Regards,

Anthony Liguori
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