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Re: [Qemu-devel] [PATCH 0/4] net-bridge: rootless bridge support for qem

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 0/4] net-bridge: rootless bridge support for qemu
Date: Thu, 05 Nov 2009 13:58:48 -0600
User-agent: Thunderbird (X11/20090825)

Avi Kivity wrote:
Helpers are really bad. On launch, I find the fragile and hard to do proper error handling with (but that's probably just me). But the real problem is at runtime, if you have a 16GB guest then you have to write-protect 4M ptes and then kvm has to tear down or write protect (not sure which mmu notifier is called) 4M shadow ptes. Once that's done, the guest will have to fault its way back; that's at least 4M exits, around 10 seconds worth of cpu time to execute a couple of syscalls.

If this is such an issue, then it's something that ought to be fixed in the kernel. It's only really applicable to hotplug anyway as you wouldn't have faulted in the memory when initially launching the guest.

I know this has been discussed before, but isn't this why there are things like vfork()? Instead of doing silly things into qemu, if there is concern about this, then it should be fixed in Linux properly.

I'd much prefer a small daemon serving taps on a unix-domain socket. Of course, management should talk to that daemon, not qemu.

I'd rather not have a program running with elevated privileges when it not needed.


Anthony Liguori

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