> -----Original Message----- > From: Peter Zijlstra [mailto:[email protected]] > Sent: Thursday, August 10, 2017 12:28 > To: Jork Loeser <[email protected]> > Cc: KY Srinivasan <[email protected]>; Simon Xiao <[email protected]>; > Haiyang Zhang <[email protected]>; Stephen Hemminger > <[email protected]>; [email protected]; [email protected]; > [email protected]; [email protected]; [email protected]; > [email protected]; [email protected]; [email protected]; > [email protected]; [email protected] > 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. I see, IF is used as a locking mechanism for the pages. Would CONFIG_HAVE_RCU_TABLE_FREE be an option for x86? There are caveats (statically enabled, RCU for page-free), yet if the resulting perf is still a gain it would be worthwhile for Hyper-V targeted kernels. Regards, Jork
