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Subject: Re: [PATCH v3 0/3] KVM: Yield to IPI target if necessary
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <CANRm+Cwv5jqxBW=Ss5nkX7kZM3_Y-Ucs66yx5+wN09=W4pUdzA@mail.gmail.com>
Date:   Mon, 10 Jun 2019 18:48:15 -0700
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        =?utf-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>
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References: <1559178307-6835-1-git-send-email-wanpengli@tencent.com>
 <20190610143420.GA6594@flask> <20190611011100.GB24835@linux.intel.com>
 <CANRm+Cwv5jqxBW=Ss5nkX7kZM3_Y-Ucs66yx5+wN09=W4pUdzA@mail.gmail.com>
To:     Wanpeng Li <kernellwp@gmail.com>
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> On Jun 10, 2019, at 6:45 PM, Wanpeng Li <kernellwp@gmail.com> wrote:
>=20
> On Tue, 11 Jun 2019 at 09:11, Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
>> On Mon, Jun 10, 2019 at 04:34:20PM +0200, Radim Kr=C4=8Dm=C3=A1=C5=99 =
wrote:
>>> 2019-05-30 09:05+0800, Wanpeng Li:
>>>> The idea is from Xen, when sending a call-function IPI-many to =
vCPUs,
>>>> yield if any of the IPI target vCPUs was preempted. 17% performance
>>>> increasement of ebizzy benchmark can be observed in an =
over-subscribe
>>>> environment. (w/ kvm-pv-tlb disabled, testing TLB flush =
call-function
>>>> IPI-many since call-function is not easy to be trigged by userspace
>>>> workload).
>>>=20
>>> Have you checked if we could gain performance by having the yield as =
an
>>> extension to our PV IPI call?
>>>=20
>>> It would allow us to skip the VM entry/exit overhead on the caller.
>>> (The benefit of that might be negligible and it also poses a
>>> complication when splitting the target mask into several PV IPI
>>> hypercalls.)
>>=20
>> Tangetially related to splitting PV IPI hypercalls, are there any =
major
>> hurdles to supporting shorthand?  Not having to generate the mask for
>> ->send_IPI_allbutself and ->kvm_send_ipi_all seems like an easy to =
way
>> shave cycles for affected flows.
>=20
> Not sure why shorthand is not used for native x2apic mode.

Why do you say so? native_send_call_func_ipi() checks if allbutself
shorthand should be used and does so (even though the check can be more
efficient - I=E2=80=99m looking at that code right now=E2=80=A6)=