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

[Vitaly Kuznetsov]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Thu, Aug 10, 2017 at 07:08:22PM +0000, Jork Loeser wrote:
>
>> > > Subject: Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush
>> 
>> > > Hold on.. if we don't IPI for TLB invalidation. What serializes our
>> > > software page table walkers like fast_gup() ?
>> > 
>> > Hypervisor may implement this functionality via an IPI.
>> > 
>> > K. Y
>> 
>> HvFlushVirtualAddressList() states:
>> This call guarantees that by the time control returns back to the
>> caller, the observable effects of all flushes on the specified virtual
>> processors have occurred.
>> 
>> HvFlushVirtualAddressListEx() refers to HvFlushVirtualAddressList() as adding sparse target VP lists.
>> 
>> Is this enough of a guarantee, or do you see other races?
>
> That's nowhere near enough. We need the remote CPU to have completed any
> guest IF section that was in progress at the time of the call.
>
> So if a host IPI can interrupt a guest while the guest has IF cleared,
> and we then process the host IPI -- clear the TLBs -- before resuming the
> guest, which still has IF cleared, we've got a problem.
>
> Because at that point, our software page-table walker, that relies on IF
> being clear to guarantee the page-tables exist, because it holds off the
> TLB invalidate and thereby the freeing of the pages, gets its pages
> ripped out from under it.

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?

-- 
  Vitaly