On Tue, Apr 30, 2019 at 04:32:01PM +0200, Peter Zijlstra wrote:
On Tue, Apr 30, 2019 at 10:24:23PM +0800, Perr Zhang wrote:
In commit 45def77ebf79, the order of function calls in kvm_fast_pio() was changed. This causes that the vm(XP,and also XP's iso img) failed to boot. This doesn't happen with win10 or ubuntu.
After revert the order, the vm(XP) succeedes to boot. In addition, the change of calls's order of kvm_fast_pio() in commit 45def77ebf79 has no obvious reason.
There are three reasons explicitly listed in the changelog:
Updating %rip prior to executing to userspace has several drawbacks:
- Userspace sees the wrong %rip on the exit, e.g. if PIO emulation fails it will likely yell about the wrong address. - Single step exits to userspace for are effectively dropped as KVM_EXIT_DEBUG is overwritten with KVM_EXIT_IO. - Behavior of PIO emulation is different depending on whether it goes down the fast path or the slow path.
This Changelog fails to explain why the order is important and equally fails to inform the future reader of that code. So this very same thing will happen again in 6 months time or thereabout.
There's a more precise fix submitted for this bug[1]. In theory v2 already went out, but I still don't see it posted to the KVM list. Either the KVM list or my mail client is being weird.