On 02.10.2013, at 11:11, Alexander Graf wrote: > > On 02.10.2013, at 11:06, Benjamin Herrenschmidt wrote: > >> On Wed, 2013-10-02 at 10:46 +0200, Paolo Bonzini wrote: >> >>> >>> Thanks. Any chance you can give some numbers of a kernel hypercall and >>> a userspace hypercall on Power, so we have actual data? For example a >>> hypercall that returns H_PARAMETER as soon as possible. >> >> I don't have (yet) numbers at hand but we have basically 3 places where >> we can handle hypercalls: >> >> - 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. >> This is the fastest path since we don't take out the other threads nor >> perform any expensive context change. This is where we put the >> "accelerated" H_RANDOM as well. >> >> - 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 >> there, page faults, etc... >> >> - Qemu. This adds the round trip to userspace on top of the above. > > 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. > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
