Re: [Xen-devel] [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush

[Peter Zijlstra]

On Fri, Aug 11, 2017 at 12:05:45PM +0100, Andrew Cooper wrote:
> >> Oh, I see your concern. Hyper-V, however, is not the first x86
> >> hypervisor trying to avoid IPIs on remote TLB flush, Xen does this
> >> too. Briefly looking at xen_flush_tlb_others() I don't see anything
> >> special, do we know how serialization is achieved there?
> > No idea on how Xen works, I always just hope it goes away :-) But lets
> > ask some Xen folks.
> 
> How is the software pagewalker relying on IF being clear safe at all (on
> native, let alone under virtualisation)?  Hardware has no architectural
> requirement to keep entries in the TLB.

No, but it _can_, therefore when we unhook pages we _must_ invalidate.

It goes like:

	CPU0			CPU1

	unhook page
				cli
				traverse page tables
	TLB invalidate --->	<IF clear, therefore CPU0 waits>
				sti
				<IPI>
				 TLB invalidate
			<------	 complete
				</IPI>
	free page

So the CPU1 page-table walker gets an existence guarantee of the
page-tables by clearing IF.

> In the virtualisation case, at any point the vcpu can be scheduled on a
> different pcpu even during a critical region like that, so the TLB
> really can empty itself under your feet.

Not the point.