[PATCH] Changing the AMD IOMMU API path to work in an atomic context which is necessary for any custom drivers using the IOMMU API while holding a spinlock.

[PATCH] Changing the AMD IOMMU API path to work in an atomic context which is necessary for any custom drivers using the IOMMU API while holding a spinlock.

[murphyt7]

From: Tom Murphy <murphyt7@tcd.ie>

---

This patch allows the IOMMU API path in the AMD driver to be called from an atomic context.
This is useful for anyone building a driver which needs to call the IOMMU API while holding a spinlock.

 drivers/iommu/amd_iommu.c       | 14 ++++++++------
 drivers/iommu/amd_iommu_types.h |  2 +-
 2 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
index 0c910a8..642afc9 100644
--- a/drivers/iommu/amd_iommu.c
+++ b/drivers/iommu/amd_iommu.c
@@ -2904,7 +2904,7 @@ static void protection_domain_free(struct protection_domain *domain)
 static int protection_domain_init(struct protection_domain *domain)
 {
 	spin_lock_init(&domain->lock);
-	mutex_init(&domain->api_lock);
+	spin_lock_init(&domain->api_lock);
 	domain->id = domain_id_alloc();
 	if (!domain->id)
 		return -ENOMEM;
@@ -3088,6 +3088,7 @@ static int amd_iommu_map(struct iommu_domain *dom, unsigned long iova,
 			 phys_addr_t paddr, size_t page_size, int iommu_prot)
 {
 	struct protection_domain *domain = to_pdomain(dom);
+	unsigned long flags;
 	int prot = 0;
 	int ret;
 
@@ -3099,9 +3100,9 @@ static int amd_iommu_map(struct iommu_domain *dom, unsigned long iova,
 	if (iommu_prot & IOMMU_WRITE)
 		prot |= IOMMU_PROT_IW;
 
-	mutex_lock(&domain->api_lock);
-	ret = iommu_map_page(domain, iova, paddr, page_size, prot, GFP_KERNEL);
-	mutex_unlock(&domain->api_lock);
+	spin_lock_irqsave(&domain->api_lock, flags);
+	ret = iommu_map_page(domain, iova, paddr, page_size, prot, GFP_ATOMIC);
+	spin_unlock_irqrestore(&domain->api_lock, flags);
 
 	return ret;
 }
@@ -3110,14 +3111,15 @@ static size_t amd_iommu_unmap(struct iommu_domain *dom, unsigned long iova,
 			   size_t page_size)
 {
 	struct protection_domain *domain = to_pdomain(dom);
+	unsigned long flags;
 	size_t unmap_size;
 
 	if (domain->mode == PAGE_MODE_NONE)
 		return -EINVAL;
 
-	mutex_lock(&domain->api_lock);
+	spin_lock_irqsave(&domain->api_lock, flags);
 	unmap_size = iommu_unmap_page(domain, iova, page_size);
-	mutex_unlock(&domain->api_lock);
+	spin_unlock_irqrestore(&domain->api_lock, flags);
 
 	domain_flush_tlb_pde(domain);
 	domain_flush_complete(domain);
diff --git a/drivers/iommu/amd_iommu_types.h b/drivers/iommu/amd_iommu_types.h
index 0d91785..a9077a5 100644
--- a/drivers/iommu/amd_iommu_types.h
+++ b/drivers/iommu/amd_iommu_types.h
@@ -444,7 +444,7 @@ struct protection_domain {
 	struct iommu_domain domain; /* generic domain handle used by
 				       iommu core code */
 	spinlock_t lock;	/* mostly used to lock the page table*/
-	struct mutex api_lock;	/* protect page tables in the iommu-api path */
+	spinlock_t api_lock;	/* protect page tables in the iommu-api path */
 	u16 id;			/* the domain id written to the device table */
 	int mode;		/* paging mode (0-6 levels) */
 	u64 *pt_root;		/* page table root pointer */
-- 
2.7.4

Re: [PATCH] Changing the AMD IOMMU API path to work in an atomic context which is necessary for any custom drivers using the IOMMU API while holding a spinlock.

[Christoph Hellwig]

On Fri, Aug 24, 2018 at 02:28:49PM +0000, murphyt7@tcd.ie wrote:
> From: Tom Murphy <murphyt7@tcd.ie>
> 
> ---
> 
> This patch allows the IOMMU API path in the AMD driver to be called from an atomic context.
> This is useful for anyone building a driver which needs to call the IOMMU API while holding a spinlock.

I don't think that is a good idea.  Please point to the code in the
driver and we can't probably find a better solution.

Re: [PATCH] Changing the AMD IOMMU API path to work in an atomic context which is necessary for any custom drivers using the IOMMU API while holding a spinlock.

[Robin Murphy]

On 24/08/18 15:53, Christoph Hellwig wrote:
> On Fri, Aug 24, 2018 at 02:28:49PM +0000, murphyt7@tcd.ie wrote:
>> From: Tom Murphy <murphyt7@tcd.ie>
>>
>> ---
>>
>> This patch allows the IOMMU API path in the AMD driver to be called from an atomic context.
>> This is useful for anyone building a driver which needs to call the IOMMU API while holding a spinlock.
> 
> I don't think that is a good idea.  Please point to the code in the
> driver and we can't probably find a better solution.

Although IIRC the AMD driver is in fact the only one whose map/unmap 
callbacks aren't already spinlock-safe (or at least it was last time I 
was looking). Stuff like iommu-dma is already relying on this in order 
to implement streaming DMA API calls (which may be in atomic context) on 
top of the corresponding IOMMU API operations.

Robin.

Re: [PATCH] Changing the AMD IOMMU API path to work in an atomic context which is necessary for any custom drivers using the IOMMU API while holding a spinlock.

[Christoph Hellwig]

On Fri, Aug 24, 2018 at 04:24:49PM +0100, Robin Murphy wrote:
> Although IIRC the AMD driver is in fact the only one whose map/unmap
> callbacks aren't already spinlock-safe (or at least it was last time I was
> looking). Stuff like iommu-dma is already relying on this in order to
> implement streaming DMA API calls (which may be in atomic context) on top of
> the corresponding IOMMU API operations.

True.  That should be mentioned in the spinlock, though - and once
it is done the calling conventions should be documented.

Re: [PATCH] Changing the AMD IOMMU API path to work in an atomic context which is necessary for any custom drivers using the IOMMU API while holding a spinlock.

[Christoph Hellwig]

On Fri, Aug 24, 2018 at 11:55:53PM +0100, Tom Murphy wrote:
> We were going to do this by using the iommu-dma api and replacing all the
> existing calls to the DMA api functions in the amd driver with their
> iommu-dma equivalent like in this driver:
> https://elixir.bootlin.com/linux/latest/source/arch/arm64/mm/dma-mapping.c#L810

FYI, I have a a wip branch to move the arm64 wrappers for both
swiotlb and dma-iommu here:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-maybe-coherent

Maybe you could try to base on top of that.

> To do this we need the map/unmap callbacks to be spinlock-safe.

You probably want to send the patch together with the one(s) making use
of it.

Re: [PATCH] Changing the AMD IOMMU API path to work in an atomic context which is necessary for any custom drivers using the IOMMU API while holding a spinlock.

[Joerg Roedel]

On Fri, Aug 24, 2018 at 02:28:49PM +0000, murphyt7@tcd.ie wrote:
> From: Tom Murphy <murphyt7@tcd.ie>
> 
> ---
> 
> This patch allows the IOMMU API path in the AMD driver to be called
> from an atomic context.  This is useful for anyone building a driver
> which needs to call the IOMMU API while holding a spinlock.

A bit more context would be helpful here. This is a pretty intrusive
change since we will drain the memory allocators atomic pools now while
building page-tables. This needs some more justification than just 'some
drivers might need it'.

Further, if we really address this problem it needs to happen through
the IOMMU-API, because the functions you are patching are called from
iommu_map()/iommu_unmap(), and these functions are allowed to sleep.

So the required approach here would be to introduce something like
iommu_map_atomic()/iommu_unmap_atomic() and implement call-backs for
that in more than just one driver.

Regards,

	Joerg