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

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

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.


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