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SubjectRe: [PATCH 6/9] iommu/amd: Implement map_atomic
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On 11/04/2019 19:47, Tom Murphy wrote:
> Instead of using a spin lock I removed the mutex lock from both the
> amd_iommu_map and amd_iommu_unmap path as well. iommu_map doesn’t lock
> while mapping and so if iommu_map is called by two different threads on
> the same iova region it results in a race condition even with the locks.
> So the locking in amd_iommu_map and amd_iommu_unmap doesn't add any real
> protection. The solution to this is for whatever manages the allocated
> iova’s externally to make sure iommu_map isn’t called twice on the same
> region at the same time.

Note that that assumption is not necessarily sufficient - even with
correct address space management you can have cases like two threads
mapping adjacent pages, where even thought they are targeting different
PTEs they can race to install/modify intermediate levels of the
pagetable. I believe AMD is actually OK in that regard, but some drivers
*are* relying on locking for correctness so it can't just be
unequivocally removed everywhere.

Robin.

> Signed-off-by: Tom Murphy <tmurphy@arista.com>
> ---
> drivers/iommu/amd_iommu.c | 25 ++++++++++++++++++-------
> 1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
> index 2d4ee10626b4..b45e0e033adc 100644
> --- a/drivers/iommu/amd_iommu.c
> +++ b/drivers/iommu/amd_iommu.c
> @@ -3089,12 +3089,12 @@ static int amd_iommu_attach_device(struct iommu_domain *dom,
> return ret;
> }
>
> -static int amd_iommu_map(struct iommu_domain *dom, unsigned long iova,
> - phys_addr_t paddr, size_t page_size, int iommu_prot)
> +static int __amd_iommu_map(struct iommu_domain *dom, unsigned long iova,
> + phys_addr_t paddr, size_t page_size, int iommu_prot,
> + gfp_t gfp)
> {
> struct protection_domain *domain = to_pdomain(dom);
> int prot = 0;
> - int ret;
>
> if (domain->mode == PAGE_MODE_NONE)
> return -EINVAL;
> @@ -3104,11 +3104,21 @@ 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);
> + return iommu_map_page(domain, iova, paddr, page_size, prot, gfp);
> +}
>
> - return ret;
> +static int amd_iommu_map(struct iommu_domain *dom, unsigned long iova,
> + phys_addr_t paddr, size_t page_size, int iommu_prot)
> +{
> + return __amd_iommu_map(dom, iova, paddr, page_size, iommu_prot,
> + GFP_KERNEL);
> +}
> +
> +static int amd_iommu_map_atomic(struct iommu_domain *dom, unsigned long iova,
> + phys_addr_t paddr, size_t page_size, int iommu_prot)
> +{
> + return __amd_iommu_map(dom, iova, paddr, page_size, iommu_prot,
> + GFP_ATOMIC);
> }
>
> static size_t amd_iommu_unmap(struct iommu_domain *dom, unsigned long iova,
> @@ -3262,6 +3272,7 @@ const struct iommu_ops amd_iommu_ops = {
> .attach_dev = amd_iommu_attach_device,
> .detach_dev = amd_iommu_detach_device,
> .map = amd_iommu_map,
> + .map_atomic = amd_iommu_map_atomic,
> .unmap = amd_iommu_unmap,
> .iova_to_phys = amd_iommu_iova_to_phys,
> .add_device = amd_iommu_add_device,
>

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