Re: [PATCH 3/3] iommu/virtio: Enable x86 support

[Robin Murphy <robin.murphy@arm.com>]

On 17/02/2020 1:31 pm, Michael S. Tsirkin wrote:
> On Mon, Feb 17, 2020 at 01:22:44PM +0000, Robin Murphy wrote:
>> On 17/02/2020 1:01 pm, Michael S. Tsirkin wrote:
>>> On Mon, Feb 17, 2020 at 10:01:07AM +0100, Jean-Philippe Brucker wrote:
>>>> On Sun, Feb 16, 2020 at 04:50:33AM -0500, Michael S. Tsirkin wrote:
>>>>> On Fri, Feb 14, 2020 at 04:57:11PM +0000, Robin Murphy wrote:
>>>>>> On 14/02/2020 4:04 pm, Jean-Philippe Brucker wrote:
>>>>>>> With the built-in topology description in place, x86 platforms can now
>>>>>>> use the virtio-iommu.
>>>>>>>
>>>>>>> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>>>>>>> ---
>>>>>>>     drivers/iommu/Kconfig | 3 ++-
>>>>>>>     1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
>>>>>>> index 068d4e0e3541..adcbda44d473 100644
>>>>>>> --- a/drivers/iommu/Kconfig
>>>>>>> +++ b/drivers/iommu/Kconfig
>>>>>>> @@ -508,8 +508,9 @@ config HYPERV_IOMMU
>>>>>>>     config VIRTIO_IOMMU
>>>>>>>     	bool "Virtio IOMMU driver"
>>>>>>>     	depends on VIRTIO=y
>>>>>>> -	depends on ARM64
>>>>>>> +	depends on (ARM64 || X86)
>>>>>>>     	select IOMMU_API
>>>>>>> +	select IOMMU_DMA
>>>>>>
>>>>>> Can that have an "if X86" for clarity? AIUI it's not necessary for
>>>>>> virtio-iommu itself (and really shouldn't be), but is merely to satisfy the
>>>>>> x86 arch code's expectation that IOMMU drivers bring their own DMA ops,
>>>>>> right?
>>>>>>
>>>>>> Robin.
>>>>>
>>>>> In fact does not this work on any platform now?
>>>>
>>>> There is ongoing work to use the generic IOMMU_DMA ops on X86. AMD IOMMU
>>>> has been converted recently [1] but VT-d still implements its own DMA ops
>>>> (conversion patches are on the list [2]). On Arm the arch Kconfig selects
>>>> IOMMU_DMA, and I assume we'll have the same on X86 once Tom's work is
>>>> complete. Until then I can add a "if X86" here for clarity.
>>>>
>>>> Thanks,
>>>> Jean
>>>>
>>>> [1] https://lore.kernel.org/linux-iommu/20190613223901.9523-1-murphyt7@tcd.ie/
>>>> [2] https://lore.kernel.org/linux-iommu/20191221150402.13868-1-murphyt7@tcd.ie/
>>>
>>> What about others? E.g. PPC?
>>
>> That was the point I was getting at - while iommu-dma should build just fine
>> for the likes of PPC, s390, 32-bit Arm, etc., they have no architecture code
>> to correctly wire up iommu_dma_ops to devices. Thus there's currently no
>> point pulling it in and pretending it's anything more than a waste of space
>> for architectures other than arm64 and x86. It's merely a historical
>> artefact of the x86 DMA API implementation that when the IOMMU drivers were
>> split out to form drivers/iommu they took some of their relevant arch code
>> with them.
>>
>> Robin.
> 
> 
> Rather than white-listing architectures, how about making the
> architectures in question set some kind of symbol, and depend on it?

Umm, that's basically what we have already? Architectures that use 
iommu_dma_ops select IOMMU_DMA.

The only issue is the oddity of x86 treating IOMMU drivers as part of 
its arch code, which has never come up against a cross-architecture 
driver until now. Hence the options of either maintaining that paradigm 
and having the 'x86 arch code' aspect of this driver "select IOMMU_DMA 
if x86" such that it works out equivalent to AMD_IOMMU, or a more 
involved cleanup to move that responsibility out of 
drivers/iommu/Kconfig entirely and have arch/x86/Kconfig do something 
like "select IOMMU_DMA if IOMMU_API", as Jean suggested up-thread.

In the specific context of IOMMU_DMA we're not talking about any kind of 
white-list, merely a one-off special case for one particular architecture.

Robin.
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