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

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Date:   Tue, 15 Nov 2022 13:06:17 +1100
From:   "Oliver O'Halloran" <oohall@...il.com>
To:     "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.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

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
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
would look something like:

echo 2048 > sriov_hw_numvfs
echo 100 > sriov_numvfs

# 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.