linux-kernel - Re: [PATCH v4 RESEND 3/5] KVM: x86: hyperv: use get_vcpu_by_vpidx() in kvm_hv_flush

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Date:   Tue, 25 Sep 2018 11:57:52 +0300
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 RESEND 3/5] KVM: x86: hyperv: use get_vcpu_by_vpidx()
 in kvm_hv_flush_tlb()

On Mon, Sep 24, 2018 at 06:55:28PM +0200, Paolo Bonzini wrote:
> On 24/09/2018 18:24, Paolo Bonzini wrote:
> > Hi Paolo,
> > 
> > could you please clarify what needs to be done to get this merged? In
> > particular, are you OK with my comment above or do we need to do
> > something with it (e.g. get back to the 'logarythmic search' from v2)?
> > 
> > In kvm/queue I can see only 'x86/hyper-v: rename ipi_arg_{ex,non_ex}
> > structures' patch from this series applied.
> 
> Hi,
> 
> my plan was to apply only 1/2/5 for now.  I singled out the rename patch
> because that one could be included in 4.19-rc kernels as a cleanup.

Is this supposed to mean you're not happy with the approach taken in
Vitaly's patch?  Can you explain why?  I take my part of guilt for it so
I'd like to know, too.

Speaking of the options we have, the choice depends on the assumptions
we take. (And I guess when you spoke of quadratic complexity you
referred to the algorithm to convert the vp_index mask into the KVM cpu
mask.)

If we can assume that in all relevant cases vp_index coincides with the
cpu index (which I think we can) then Vitaly's approach is the most
efficient.

If, on the opposite, we want to optimize for random mapping between
vp_index and cpu index, then it's probably better instead to iterate
over vcpus and test if their vp_index belongs to the requested mask.

Neither of the above is quadratic.

Dunno if we need to specifically consider intermediate situations.

Anyway using a havier vp_index -> cpu index translation looks like an
overkill to me.

What do you think?

Thanks,
Roman.