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From:   <Alex_Gagniuc@Dellteam.com>
To:     <gregkh@linuxfoundation.org>, <keith.busch@intel.com>
Subject: Re: [PATCH v2] PCI/MSI: Don't touch MSI bits when the PCI device is
 disconnected
Thread-Topic: [PATCH v2] PCI/MSI: Don't touch MSI bits when the PCI device is
 disconnected
Thread-Index: AQHUT50UQ290XPGvJEGnaSTTjbCF+g==
Date:   Thu, 8 Nov 2018 22:49:08 +0000
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References: <20180918221501.13112-1-mr.nuke.me@gmail.com>
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On 11/08/2018 04:43 PM, Greg Kroah-Hartman wrote:=0A=
> =0A=
> [EXTERNAL EMAIL]=0A=
> Please report any suspicious attachments, links, or requests for sensitiv=
e information.=0A=
> =0A=
> =0A=
> On Thu, Nov 08, 2018 at 03:32:58PM -0700, Keith Busch wrote:=0A=
>> On Thu, Nov 08, 2018 at 02:01:17PM -0800, Greg Kroah-Hartman wrote:=0A=
>>> On Thu, Nov 08, 2018 at 02:09:17PM -0600, Bjorn Helgaas wrote:=0A=
>>>> I'm having second thoughts about this.  One thing I'm uncomfortable=0A=
>>>> with is that sprinkling pci_dev_is_disconnected() around feels ad hoc=
=0A=
>>>> instead of systematic, in the sense that I don't know how we convince=
=0A=
>>>> ourselves that this (and only this) is the correct place to put it.=0A=
>>>=0A=
>>> I think my stance always has been that this call is not good at all=0A=
>>> because once you call it you never really know if it is still true as=
=0A=
>>> the device could have been removed right afterward.=0A=
>>>=0A=
>>> So almost any code that relies on it is broken, there is no locking and=
=0A=
>>> it can and will race and you will loose.=0A=
>>=0A=
>> AIUI, we're not trying to create code to rely on this. This more about=
=0A=
>> reducing reliance on hardware. If the software misses the race once and=
=0A=
>> accesses disconnected device memory, that's usually not a big deal to=0A=
>> let hardware sort it out, but the point is not to push our luck.=0A=
> =0A=
> Then why even care about this call at all?  If you need to really know=0A=
> if the read worked, you have to check the value.  If the value is FF=0A=
> then you have a huge hint that the hardware is now gone.  And you can=0A=
> rely on it being gone, you can never rely on making the call to the=0A=
> function to check if the hardware is there to be still valid any point=0A=
> in time after the call returns.=0A=
=0A=
In the case that we're trying to fix, this code executing is a result of =
=0A=
the device being gone, so we can guarantee race-free operation. I agree =0A=
that there is a race, in the general case. As far as checking the result =
=0A=
for all F's, that's not an option when firmware crashes the system as a =0A=
result of the mmio read/write. It's never pretty when firmware gets =0A=
involved.=0A=
=0A=
Alex=0A=