linux-kernel - Re: [PATCH v2 4/7] iommu: Let iommu.strict override ops->def_domain

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Date:   Mon, 28 Nov 2022 21:00:32 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Gerd Bayer <gbayer@...ux.ibm.com>, iommu@...ts.linux.dev,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Wenjia Zhang <wenjia@...ux.ibm.com>
Cc:     baolu.lu@...ux.intel.com, Pierre Morel <pmorel@...ux.ibm.com>,
        linux-s390@...r.kernel.org, borntraeger@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, linux-kernel@...r.kernel.org,
        Julian Ruess <julianr@...ux.ibm.com>
Subject: Re: [PATCH v2 4/7] iommu: Let iommu.strict override
 ops->def_domain_type

On 2022/11/28 19:10, Niklas Schnelle wrote:
> On Thu, 2022-11-17 at 09:55 +0800, Baolu Lu wrote:
>> On 2022/11/17 1:16, Niklas Schnelle wrote:
>>> When iommu.strict=1 is set or iommu_set_dma_strict() was called we
>>> should use IOMMU_DOMAIN_DMA irrespective of ops->def_domain_type.
>>>
>>> Signed-off-by: Niklas Schnelle<schnelle@...ux.ibm.com>
>>> ---
>>>    drivers/iommu/iommu.c | 3 +++
>>>    1 file changed, 3 insertions(+)
>>>
>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>>> index 65a3b3d886dc..d9bf94d198df 100644
>>> --- a/drivers/iommu/iommu.c
>>> +++ b/drivers/iommu/iommu.c
>>> @@ -1562,6 +1562,9 @@ static int iommu_get_def_domain_type(struct device *dev)
>>>    {
>>>    	const struct iommu_ops *ops = dev_iommu_ops(dev);
>>>    
>>> +	if (iommu_dma_strict)
>>> +		return IOMMU_DOMAIN_DMA;
>> If any quirky device must work in IOMMU identity mapping mode, this
>> might introduce functional regression. At least for VT-d platforms, some
>> devices do require IOMMU identity mapping mode for functionality.
> That's a good point. How about instead of unconditionally returning
> IOMMU_DOMAIN_DMA we just do so if the domain type returned by ops-
>> def_domain_type uses a flush queue (i.e. the __IOMMU_DOMAIN_DMA_FQ bit
> is set). That way a device that only supports identity mapping gets to
> set that but iommu_dma_strict at least always prevents use of an IOVA
> flush queue.

def_domain_type returns IOMMU_DOMAIN_DMA, IOMMU_DOMAIN_IDENTIRY or 0
(don't care). From a code perspective, you can force IOMMU_DOMAIN_DMA if
def_domain_type() returns 0.

*But* you need to document the relationship between strict mode and
default domain type somewhere and get that agreed with Robin.

Best regards,
baolu