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Re: [Xen-devel] [PATCH v2] x86: enable RCU based table free

On Thu, Aug 24, 2017 at 11:22:58AM +0200, Vitaly Kuznetsov wrote:
> On x86 software page-table walkers depend on the fact that remote TLB flush
> does an IPI: walk is performed lockless but with interrupts disabled and in
> case the pagetable is freed the freeing CPU will get blocked as remote TLB
> flush is required. On other architecture which don't require an IPI to do
> remote TLB flush we have an RCU-based mechanism (see
> include/asm-generic/tlb.h for more details).
> In virtualized environments we may want to override .flush_tlb_others hook
> in pv_mmu_ops and use a hypercall asking the hypervisor to do remote TLB
> flush for us. This breaks the assumption about IPI. Xen PV does this for
> years and the upcoming remote TLB flush for Hyper-V will do it too. This
> is not safe, software pagetable walkers may step on an already freed page.
> Solve the issue by enabling RCU-based table free mechanism. Testing with
> kernbench and mmap/munmap microbenchmars didn't show any notable
> performance impact.
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Acked-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> Changes since v1:
> - Enable HAVE_RCU_TABLE_FREE unconditionally to avoid different code pathes
>   for no reason [Linus Torvalds]

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

 Kirill A. Shutemov

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