linux-kernel - Re: [RFC PATCH v3 1/2] iommu: Add capability IOMMU_CAP

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Message-ID: <eda6ae9f-76eb-3254-ce58-ea355418a4b1@linux.intel.com>
Date:   Sat, 16 Jan 2021 16:47:40 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     baolu.lu@...ux.intel.com, tglx@...utronix.de, ashok.raj@...el.com,
        kevin.tian@...el.com, dave.jiang@...el.com, megha.dey@...el.com,
        dwmw2@...radead.org, alex.williamson@...hat.com,
        bhelgaas@...gle.com, dan.j.williams@...el.com, will@...nel.org,
        joro@...tes.org, dmaengine@...r.kernel.org, eric.auger@...hat.com,
        jacob.jun.pan@...el.com, jgg@...lanox.com, kvm@...r.kernel.org,
        kwankhede@...dia.com, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, iommu@...ts.linux-foundation.org,
        maz@...nel.org, mona.hossain@...el.com, netanelg@...lanox.com,
        parav@...lanox.com, pbonzini@...hat.com, rafael@...nel.org,
        samuel.ortiz@...el.com, sanjay.k.kumar@...el.com,
        shahafs@...lanox.com, tony.luck@...el.com, vkoul@...nel.org,
        yan.y.zhao@...ux.intel.com, yi.l.liu@...el.com
Subject: Re: [RFC PATCH v3 1/2] iommu: Add capability IOMMU_CAP_VIOMMU

Hi Leon,

On 2021/1/16 16:39, Leon Romanovsky wrote:
> On Sat, Jan 16, 2021 at 09:20:16AM +0800, Lu Baolu wrote:
>> Hi,
>>
>> On 2021/1/15 14:31, Leon Romanovsky wrote:
>>> On Fri, Jan 15, 2021 at 07:49:47AM +0800, Lu Baolu wrote:
>>>> Hi Leon,
>>>>
>>>> On 1/14/21 9:26 PM, Leon Romanovsky wrote:
>>>>> On Thu, Jan 14, 2021 at 09:30:02AM +0800, Lu Baolu wrote:
>>>>>> Some vendor IOMMU drivers are able to declare that it is running in a VM
>>>>>> context. This is very valuable for the features that only want to be
>>>>>> supported on bare metal. Add a capability bit so that it could be used.
>>>>>
>>>>> And how is it used? Who and how will set it?
>>>>
>>>> Use the existing iommu_capable(). I should add more descriptions about
>>>> who and how to use it.
>>>
>>> I want to see the code that sets this capability.
>>
>> Currently we have Intel VT-d and the virt-iommu setting this capability.
>>
>>   static bool intel_iommu_capable(enum iommu_cap cap)
>>   {
>>   	if (cap == IOMMU_CAP_CACHE_COHERENCY)
>>   		return domain_update_iommu_snooping(NULL) == 1;
>>   	if (cap == IOMMU_CAP_INTR_REMAP)
>>   		return irq_remapping_enabled == 1;
>> +	if (cap == IOMMU_CAP_VIOMMU)
>> +		return caching_mode_enabled();
>>
>>   	return false;
>>   }
>>
>> And,
>>
>> +static bool viommu_capable(enum iommu_cap cap)
>> +{
>> +	if (cap == IOMMU_CAP_VIOMMU)
>> +		return true;
>> +
>> +	return false;
>> +}
> 
> These two functions are reading this cap and not setting.
> Where can I see code that does "cap = IOMMU_CAP_VIOMMU" and not "=="?

The iommu_capable() is a generic IOMMU interface to query IOMMU
capabilities. It takes @bus and @cap as input, and calls the callback
of vendor iommu. If the vendor iommu driver supports the specific
capability, it returns true. Otherwise, it returns false.

bool iommu_capable(struct bus_type *bus, enum iommu_cap cap)
{
         if (!bus->iommu_ops || !bus->iommu_ops->capable)
                 return false;

         return bus->iommu_ops->capable(cap);
}
EXPORT_SYMBOL_GPL(iommu_capable);

In the vendor iommu's callback, it checks the capability and returns a
value according to its capability, just as showed above.

Best regards,
baolu