Re: [Xen-devel] [PATCH] iommu: leave IOMMU enabled by default during kexec crash transition

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 22/02/2019 09:52, Jan Beulich wrote:
>>>> On 20.02.19 at 19:19, <igor.druzhinin@xxxxxxxxxx> wrote:
>> On 20/02/2019 08:48, Jan Beulich wrote:
>>>
>>> Some entity needs to decide whether to add the respective command
>>> line option to the crash kernel's command line. It should be this same
>>> entity to tell Xen whether to keep the IOMMU enabled while invoking
>>> the crash kernel. I am merely guessing that this entity is the kexec
>>> tool.
>>>
>>
>> I was just double checking and it seems (assuming the device reset
>> correctly in the crash kernel) everything seem to work even without
>> enabling intel_iommu in the command line - newer kernels handle this
>> case by explicitly disabling translation in any case: they expect it to
>> be enabled by the previous kernel.
> 
> So if they disable translation, how is them doing so any better than us
> doing so, other than theirs occurring slightly later on the time scale and
> hence there being better chances of in-flight (at the time of the crash)
> DMA having completed meanwhile?
> 

There are several reasons why it's better:
a) kernel is able to perform device reset properly as it has bus
specific code that does this. There is even a comment in the code
mentioning that at the moment it disables the translation bus-specific
reset is finished and it's safer (as devices likely stopped DMA at this
point) to do it now.
b) kernel has the drivers that do per-driver-specific reset of the
devices that do not work well with bus-specific reset. It's simply
impossible to implement that in Xen
c) even if a device is uncooperative and keeps sending bus transactions,
an error event that I mentioned earlier will be properly handled as we
have facilities for it in the kernel at that point

Igor

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