linux-kernel - Re: [PATCH v8 6/6] PCI: update device mps when doing pci hotplug

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Message-ID: <CAE9FiQX9gU-S9X_eeB=JwqS8130CONKgrv+mGj1cEgShmxgS-Q@mail.gmail.com>
Date:	Thu, 29 Aug 2013 14:47:08 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Yijing Wang <wangyijing@...wei.com>, Jon Mason <jdmason@...zu.us>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Hanjun Guo <guohanjun@...wei.com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Jin Feng <joe.jin@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 6/6] PCI: update device mps when doing pci hotplug

On Thu, Aug 29, 2013 at 2:09 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> [+cc linux-kernel, since more folks might be interested]

> I don't know what the BIOS "auto" setting means, but it must mean
> something in case 3, because that's the only case where OS support is
> required.  But if the OS is smart enough to manage MPS for hot-added
> devices, why can't the OS also program MPS for the whole system at
> boot-time?
>
> That's why I don't understand what BIOS wants to do.  It sounds like
> they want the performance benefit of larger MPS for devices present in
> hot-plug slots at boot-time, even if the OS doesn't actively manage
> MPS and things blow up if that device is replaced with one that
> supports a smaller MPS.  That choice doesn't make sense.
>
> In case 3, with a non-MPS-aware OS, you get better performance for a
> while, but blow up if a card is replaced.  And with an MPS-aware OS,
> there should be no advantage to case 3: the OS should be able to get
> good performance by programming MPS itself, even without help from the
> BIOS.

With OS default setting on case 3, other two OS are ok with hotplug,
but Linux does not.

Yinghai
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