Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems

On 02.10.2013, at 11:11, Alexander Graf wrote:

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On 02.10.2013, at 11:06, Benjamin Herrenschmidt wrote:
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On Wed, 2013-10-02 at 10:46 +0200, Paolo Bonzini wrote:
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Thanks.  Any chance you can give some numbers of a kernel hypercall =

and

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a userspace hypercall on Power, so we have actual data?  For example =

a

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hypercall that returns H_PARAMETER as soon as possible.

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I don't have (yet) numbers at hand but we have basically 3 places =

where

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we can handle hypercalls:
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- Kernel real mode. This is where most of our MMU stuff goes for
example unless it needs to trigger a page fault in Linux. This is
executed with translation disabled and the MMU still in guest =

context.

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This is the fastest path since we don't take out the other threads =

nor

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perform any expensive context change. This is where we put the
"accelerated" H_RANDOM as well.
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- Kernel virtual mode. That's a full exit, so all threads are out and
MMU switched back to host Linux. Things like vhost MMIO emulation =

goes

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there, page faults, etc...
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- Qemu. This adds the round trip to userspace on top of the above.

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Right, and the difference for the patch in question is really whether =

we handle in in kernel virtual mode or in QEMU, so the bulk of the =
overhead (kicking threads out of  guest context, switching MMU context, =
etc) happens either way.

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So the additional overhead when handling it in QEMU here really boils =

down to the user space roundtrip (plus another random number read =
roundtrip).

Ah, sorry, I misread the patch. You're running the handler in real mode =
of course :).

So how do you solve live migration between a kernel that has this patch =
and one that doesn't?


Alex