linux-kernel - Re: [RFC 0/4] pci/sriov: support VFs dynamic addition

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Message-ID: <726934f2-ded0-317f-2082-bf5ee08cf39d@huawei.com>
Date:   Wed, 16 Nov 2022 08:52:28 +0800
From:   "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>
To:     Oliver O'Halloran <oohall@...il.com>
CC:     Leon Romanovsky <leon@...nel.org>, <bhelgaas@...gle.com>,
        <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <jianjay.zhou@...wei.com>, <zhuangshengen@...wei.com>,
        <arei.gonglei@...wei.com>, <yechuan@...wei.com>,
        <huangzhichao@...wei.com>, <xiehong@...wei.com>
Subject: Re: [RFC 0/4] pci/sriov: support VFs dynamic addition

Hi Oliver,

在 2022/11/15 10:06, Oliver O'Halloran 写道:
> On Tue, Nov 15, 2022 at 1:08 AM Longpeng (Mike, Cloud Infrastructure
> Service Product Dept.) <longpeng2@...wei.com> wrote:
>>
>> *snip*
>>
>> Adding 2K+ VFs to the sysfs need too much time.
>>
>> Look at the bottomhalf of the hypervisor live update:
>> kexec --> add 2K VFs --> restore VMs
>>
>> The downtime can be reduced if the sequence is:
>> kexec --> add 100 VFs（the VMs used） --> resotre VMs --> add 1.9K VFs
> 
> Right, so you want to add VFs in batches rather than all at once.
> Personally I think the bitmap approach is error prone since it renders
> the meaning of pf_dev->sriov->num_VFs unclear and there's some hairy
> code in arch/powerpc/ that approach will likely break. A better

I ran a quick search in the arch/powerpc/, do you mean the 
pseries_call_allow_unfreeze ? Or would you please point the code?

> approach would be to add an attribute to control the number of VFs
> enabled in hardware and allowing sriov_numvfs to accept any number
> between the current value and sriov_hw_numvfs. e.g. your HV setup

Oh, this is a constructive suggestion, I'll try in the next step.

> would look something like:
>
> echo 2048 > sriov_hw_numvfs
> echo 100 > sriov_numvfs
> 
It's fixed to add the first 100 VFs first, maybe providing an interface 
that users can specify which id or which range of ids to be added first 
would be better?

> # time passes
> echo 2048 > sriov_numvfs
> 
> This would be fairly simple to implement and you can make it backwards
> compatible by having writes to sriov_numvfs retain their current
> semantics if sriov_hw_numvfs is zero.
> .