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.
[1] https://patchwork.kernel.org/patch/10919849/
