linux-kernel - Re: [PATCH v1] kvm: x86: implement PV send

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Message-ID: <aHopXN73dHW/uKaT@intel.com>
Date: Fri, 18 Jul 2025 19:00:44 +0800
From: Chao Gao <chao.gao@...el.com>
To: Jason Wang <jasowang@...hat.com>
CC: Cindy Lu <lulu@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>, "Vitaly
 Kuznetsov" <vkuznets@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, "Ingo
 Molnar" <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, "maintainer:X86 ARCHITECTURE (32-BIT AND
 64-BIT)" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, "Peter Zijlstra
 (Intel)" <peterz@...radead.org>, "Kirill A. Shutemov" <kas@...nel.org>, "Xin
 Li (Intel)" <xin@...or.com>, Rik van Riel <riel@...riel.com>, "Ahmed S.
 Darwish" <darwi@...utronix.de>, "open list:KVM PARAVIRT (KVM/paravirt)"
	<kvm@...r.kernel.org>, "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] kvm: x86: implement PV send_IPI method

On Fri, Jul 18, 2025 at 03:52:30PM +0800, Jason Wang wrote:
>On Fri, Jul 18, 2025 at 2:25 PM Cindy Lu <lulu@...hat.com> wrote:
>>
>> From: Jason Wang <jasowang@...hat.com>
>>
>> We used to have PV version of send_IPI_mask and
>> send_IPI_mask_allbutself. This patch implements PV send_IPI method to
>> reduce the number of vmexits.

It won't reduce the number of VM-exits; in fact, it may increase them on CPUs
that support IPI virtualization.

With IPI virtualization enabled, *unicast* and physical-addressing IPIs won't
cause a VM-exit. Instead, the microcode posts interrupts directly to the target
vCPU. The PV version always causes a VM-exit.

>>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> Tested-by: Cindy Lu <lulu@...hat.com>
>
>I think a question here is are we able to see performance improvement
>in any kind of setup?

It may result in a negative performance impact.

>
>Thanks
>
>