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    SubjectRe: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set
    On Thu, Nov 09, 2017 at 05:45:23PM +0100, Radim Krcmar wrote:
    > 2017-11-09 17:17+0100, Peter Zijlstra:
    > > On Thu, Nov 09, 2017 at 05:05:36PM +0100, Radim Krcmar wrote:
    > > > 2017-11-09 10:53-0500, Pankaj Gupta:
    > > > > 2] PV TLB should also behave as per option PV_DEDICATED for better performance.
    > > >
    > > > Right,
    > >
    > > Shouldn't KVM do flush_tlb_other() in any case? Not sure how
    > > PV_DEDICATED can help with that.
    >
    > It will, the suggestion was based on recent extension of the
    > flush_tlb_others implementaion, https://lkml.org/lkml/2017/11/8/1146.
    >
    > PV_TLB_FLUSH allows a guest to set a flush bit instead of sending flush
    > IPI if the target VCPU is not running. This would be a waste of time
    > with PV_DEDICATED as all VCPUs are expected to always running.
    >
    > With PV_DEDICATED, the guest should keep using native_flush_tlb_others.

    Is saving that for_each_cpu() really worth the effort compared to the
    cost of actually doing the IPIs and CR3 write?

    Also, you should not put cpumask_t on stack, that's 'broken'.

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