Re: [PATCH v11 10/10] genirq/msi: use the MSI doorbell's IOVA when requested

[Auger Eric]

Hi Thomas,

On 26/07/2016 11:04, Thomas Gleixner wrote:
> Eric,
> 
> On Mon, 25 Jul 2016, Auger Eric wrote:
>> On 20/07/2016 11:09, Thomas Gleixner wrote:
>>> On Tue, 19 Jul 2016, Eric Auger wrote:
>>>> @@ -63,10 +63,18 @@ static int msi_compose(struct irq_data *irq_data,
>>>>  {
>>>>  	int ret = 0;
>>>>  
>>>> -	if (erase)
>>>> +	if (erase) {
>>>>  		memset(msg, 0, sizeof(*msg));
>>>> -	else
>>>> +	} else {
>>>> +		struct device *dev;
>>>> +
>>>>  		ret = irq_chip_compose_msi_msg(irq_data, msg);
>>>> +		if (ret)
>>>> +			return ret;
>>>> +
>>>> +		dev = msi_desc_to_dev(irq_data_get_msi_desc(irq_data));
>>>> +		WARN_ON(iommu_msi_msg_pa_to_va(dev, msg));
>>>
>>> What the heck is this call doing? And why is there only a WARN_ON and not a
>>> proper error return code handling?
>>
>> iommu_msi_msg_pa_to_va is part of the new iommu-msi API introduced in PART I
>> of this series. This helper function detects the physical address found in
>> the MSI message has a corresponding allocated IOVA. This happens if the MSI
>> doorbell is accessed through an IOMMU and this IOMMU do not bypass the MSI
>> addresses (ARM case). Allocation of this IOVA was performed in the previous
>> patch.
>>
>> So, if this is the case, the physical address is swapped with the IOVA
>> address. That way the PCIe device will send the MSI with this IOVA and
>> the address will be translated by the IOMMU into the target MSI doorbell PA.
>>
>> Hope this clarifies
> 
> No, it does not. You are explaining in great length what that function is
> doing, but you are not explaining WHY your don't do a proper return code
> handling and just do a WARN_ON() and happily proceed. If that function fails
> then the interrupt will not be functional, so WHY on earth are you continuing?
Oh sorry I focused on the function's goal. Originally I could not return an
error since there is a BUG_ON(ret) afterwards. And typically the userspace can
willingly omit to pass IPA range that map MSIs. But now we have this 2 phases where
we first map the MSIs on pci_enable_msi_range and use the IOVA at compose time
I need to analyze again if the userspace can induce a BUG_ON.

Thanks

Eric
> 
> Thanks,
> 
> 	tglx
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