linux-kernel - Re: [PATCH 2/7] iommu/dma-iommu: Split iommu_dma_map_msi

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Message-ID: <6b498387-e17a-5901-4d0a-0d44ababba81@arm.com>
Date:   Mon, 29 Apr 2019 14:14:18 +0100
From:   Julien Grall <julien.grall@....com>
To:     Marc Zyngier <marc.zyngier@....com>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org
Cc:     logang@...tatee.com, douliyangs@...il.com,
        miquel.raynal@...tlin.com, jason@...edaemon.net,
        tglx@...utronix.de, joro@...tes.org, robin.murphy@....com,
        bigeasy@...utronix.de, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 2/7] iommu/dma-iommu: Split iommu_dma_map_msi_msg in two
 parts

Hi Marc,

On 23/04/2019 11:54, Marc Zyngier wrote:
> On 18/04/2019 18:26, Julien Grall wrote:
>> On RT, the function iommu_dma_map_msi_msg may be called from
>> non-preemptible context. This will lead to a splat with
>> CONFIG_DEBUG_ATOMIC_SLEEP as the function is using spin_lock
>> (they can sleep on RT).
>>
>> The function iommu_dma_map_msi_msg is used to map the MSI page in the
>> IOMMU PT and update the MSI message with the IOVA.
>>
>> Only the part to lookup for the MSI page requires to be called in
>> preemptible context. As the MSI page cannot change over the lifecycle
>> of the MSI interrupt, the lookup can be cached and re-used later on.
>>
>> This patch split the function iommu_dma_map_msi_msg in two new
>> functions:
>>      - iommu_dma_prepare_msi: This function will prepare the mapping in
>>      the IOMMU and store the cookie in the structure msi_desc. This
>>      function should be called in preemptible context.
>>      - iommu_dma_compose_msi_msg: This function will update the MSI
>>      message with the IOVA when the device is behind an IOMMU.
>>
>> Signed-off-by: Julien Grall <julien.grall@....com>
>> ---
>>   drivers/iommu/dma-iommu.c | 43 ++++++++++++++++++++++++++++++++-----------
>>   include/linux/dma-iommu.h | 21 +++++++++++++++++++++
>>   2 files changed, 53 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
>> index 77aabe637a60..f5c1f1685095 100644
>> --- a/drivers/iommu/dma-iommu.c
>> +++ b/drivers/iommu/dma-iommu.c
>> @@ -888,17 +888,17 @@ static struct iommu_dma_msi_page *iommu_dma_get_msi_page(struct device *dev,
>>   	return NULL;
>>   }
>>   
>> -void iommu_dma_map_msi_msg(int irq, struct msi_msg *msg)
>> +int iommu_dma_prepare_msi(struct msi_desc *desc, phys_addr_t msi_addr)
> 
> I quite like the idea of moving from having an irq to having an msi_desc
> passed to the IOMMU layer...
> 
>>   {
>> -	struct device *dev = msi_desc_to_dev(irq_get_msi_desc(irq));
>> +	struct device *dev = msi_desc_to_dev(desc);
>>   	struct iommu_domain *domain = iommu_get_domain_for_dev(dev);
>>   	struct iommu_dma_cookie *cookie;
>> -	struct iommu_dma_msi_page *msi_page;
>> -	phys_addr_t msi_addr = (u64)msg->address_hi << 32 | msg->address_lo;
>>   	unsigned long flags;
>>   
>> -	if (!domain || !domain->iova_cookie)
>> -		return;
>> +	if (!domain || !domain->iova_cookie) {
>> +		desc->iommu_cookie = NULL;
>> +		return 0;
>> +	}
>>   
>>   	cookie = domain->iova_cookie;
>>   
>> @@ -908,10 +908,33 @@ void iommu_dma_map_msi_msg(int irq, struct msi_msg *msg)
>>   	 * of an MSI from within an IPI handler.
>>   	 */
>>   	spin_lock_irqsave(&cookie->msi_lock, flags);
>> -	msi_page = iommu_dma_get_msi_page(dev, msi_addr, domain);
>> +	desc->iommu_cookie = iommu_dma_get_msi_page(dev, msi_addr, domain);
>>   	spin_unlock_irqrestore(&cookie->msi_lock, flags);
>>   
>> -	if (WARN_ON(!msi_page)) {
>> +	return (desc->iommu_cookie) ? 0 : -ENOMEM;
>> +}
>> +
>> +void iommu_dma_compose_msi_msg(int irq, struct msi_msg *msg)
> 
> ... but I'd like it even better if it was uniform. Can you please move
> the irq_get_msi_desc() to the callers of iommu_dma_compose_msi_msg(),
> and make both functions take a msi_desc?

Make sense. I will modify iommu_dma_compose_msi_msg to take a msi_desc in parameter.

Cheers,

-- 
Julien Grall