linux-kernel - Re: [PATCH] PCI: vmd: Do not disable MSI-X remapping in VMD 28C0 controller

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Date:   Tue, 7 Feb 2023 13:32:20 -0700
From:   "Patel, Nirmal" <nirmal.patel@...ux.intel.com>
To:     Xinghui Li <korantwork@...il.com>, Keith Busch <kbusch@...nel.org>
Cc:     Jonathan Derrick <jonathan.derrick@...ux.dev>,
        lpieralisi@...nel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xinghui Li <korantli@...cent.com>
Subject: Re: [PATCH] PCI: vmd: Do not disable MSI-X remapping in VMD 28C0
 controller

On 2/6/2023 8:18 PM, Xinghui Li wrote:
> Keith Busch <kbusch@...nel.org> 于2023年2月7日周二 02:28写道：
>> On Mon, Feb 06, 2023 at 11:11:36AM -0700, Patel, Nirmal wrote:
>>> I like the idea of module parameter to allow switching between the modes
>>> but keep MSI remapping enabled (non-bypass) by default.
>> Isn't there a more programatic way to go about selecting the best option at
>> runtime?
> Do you mean that the operating mode is automatically selected by
> detecting the number of devices and CPUs instead of being set
> manually?
>> I suspect bypass is the better choice if "num_active_cpus() > pci_msix_vec_count(vmd->dev)".
> For this situation, My speculation is that the PCIE nodes are
> over-mounted and not just because of the CPU to Drive ratio.
> We considered designing online nodes, because we were concerned that
> the IO of different chunk sizes would adapt to different MSI-X modes.
> I privately think that it may be logically complicated if programmatic
> judgments are made.

Also newer CPUs have more MSIx (128) which means we can still have
better performance without bypass. It would be better if user have
can chose module parameter based on their requirements. Thanks.