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    SubjectRe: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for VT-d PI
    2015-01-09 15:56+0100, Paolo Bonzini:
    >
    >
    > On 09/01/2015 15:54, Radim Krčmář wrote:
    > > There are two points relevant to this patch in new KVM's implementation,
    > > ("KVM: x86: amend APIC lowest priority arbitration",
    > > https://lkml.org/lkml/2015/1/9/362)
    > >
    > > 1) lowest priority depends on TPR
    > > 2) there is no need for balancing
    > >
    > > (1) has to be considered with PI as well.
    >
    > The chipset doesn't support it. :(

    I meant that we need to recompute PI entries for lowest priority
    interrupts every time guest's TPR changes.

    Luckily, Linux doesn't use TPR, but other OS might be a reason to drop
    lowest priority from PI optimizations. (Or make it more complicated.)

    > > I kept (2) to avoid whining from people building on that behaviour, but
    > > lowest priority backed by PI could be transparent without it.
    > >
    > > Patch below removes the balancing, but I am not sure this is a price we
    > > allowed ourselves to pay ... what are your opinions?
    >
    > I wouldn't mind, but it requires a lot of benchmarking.

    (I was afraid it would come to that.)


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