On Fri, Aug 11, 2017 at 11:23:10AM +0200, Vitaly Kuznetsov wrote: > 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? No idea on how Xen works, I always just hope it goes away :-) But lets ask some Xen folks. -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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