Messages in this thread | | | Date | Fri, 11 Aug 2017 14:35:34 +0200 | From | Peter Zijlstra <> | Subject | Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush |
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On Fri, Aug 11, 2017 at 02:22:25PM +0200, Juergen Gross wrote: > Wait - the TLB can be cleared at any time, as Andrew was pointing out. > No cpu can rely on an address being accessible just because IF is being > cleared. All that matters is the existing and valid page table entry. > > So clearing IF on a cpu isn't meant to secure the TLB from being > cleared, but just to avoid interrupts (as the name of the flag is > suggesting).
Yes, but by holding off the TLB invalidate IPI, we hold off the freeing of the concurrently unhooked page-table.
> In the Xen case the hypervisor does the following: > > - it checks whether any of the vcpus specified in the cpumask of the > flush request is running on any physical cpu > - if any running vcpu is found an IPI will be sent to the physical cpu > and the hypervisor will do the TLB flush there
And this will preempt a vcpu which could have IF cleared, right?
> - any vcpu addressed by the flush and not running will be flagged to > flush its TLB when being scheduled the next time > > This ensures no TLB entry to be flushed can be used after return of > xen_flush_tlb_others().
But that is not a sufficient guarantee. We need the IF to hold off the TLB invalidate and thereby hold off the freeing of our page-table pages.
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