Re: [PATCH v4 4/6] iommu: Add PASID support for DMA mapping API users

Jacob Pan
2022-05-23T15:20:12Z

Hi Kevin,

On Mon, 23 May 2022 08:25:33 +0000, "Tian, Kevin" <kevin.tian@intel.com>
wrote:

> > From: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > Sent: Thursday, May 19, 2022 2:21 AM
> >
> > DMA mapping API is the de facto standard for in-kernel DMA. It operates
> > on a per device/RID basis which is not PASID-aware.
> >
> > Some modern devices such as Intel Data Streaming Accelerator, PASID is
> > required for certain work submissions. To allow such devices use DMA
> > mapping API, we need the following functionalities:
> > 1. Provide device a way to retrieve a PASID for work submission within
> > the kernel
> > 2. Enable the kernel PASID on the IOMMU for the device
> > 3. Attach the kernel PASID to the device's default DMA domain, let it
> > be IOVA or physical address in case of pass-through.
> >
> > This patch introduces a driver facing API that enables DMA API
> > PASID usage. Once enabled, device drivers can continue to use DMA APIs
> > as is. There is no difference in dma_handle between without PASID and
> > with PASID.
> >
> > Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > ---
> > drivers/iommu/dma-iommu.c | 114
> > ++++++++++++++++++++++++++++++++++++++
> > include/linux/dma-iommu.h | 3 +
> > 2 files changed, 117 insertions(+)
> >
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index 1ca85d37eeab..6ad7ba619ef0 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -34,6 +34,8 @@ struct iommu_dma_msi_page {
> > phys_addr_t phys;
> > };
> >
> > +static DECLARE_IOASID_SET(iommu_dma_pasid);
> > +
> > enum iommu_dma_cookie_type {
> > IOMMU_DMA_IOVA_COOKIE,
> > IOMMU_DMA_MSI_COOKIE,
> > @@ -370,6 +372,118 @@ void iommu_put_dma_cookie(struct
> > iommu_domain *domain)
> > domain->iova_cookie = NULL;
> > }
> >
> > +/* Protect iommu_domain DMA PASID data */
> > +static DEFINE_MUTEX(dma_pasid_lock);
> > +/**
> > + * iommu_attach_dma_pasid --Attach a PASID for in-kernel DMA. Use the
> > device's
> > + * DMA domain.
> > + * @dev: Device to be enabled
> > + * @pasid: The returned kernel PASID to be used for DMA
> > + *
> > + * DMA request with PASID will be mapped the same way as the legacy
> > DMA.
> > + * If the device is in pass-through, PASID will also pass-through. If
> > the
> > + * device is in IOVA, the PASID will point to the same IOVA page table.
> > + *
> > + * @return err code or 0 on success
> > + */
> > +int iommu_attach_dma_pasid(struct device *dev, ioasid_t *pasid)
>
> iommu_attach_dma_domain_pasid? 'dma_pasid' is too broad from
> a API p.o.v.
>
I agree dma_pasid is too broad, technically it is dma_api_pasid but seems
too long.
My concern with dma_domain_pasid is that the pasid can also be used for
identity domain.

> > +{
> > + struct iommu_domain *dom;
> > + ioasid_t id, max;
> > + int ret = 0;
> > +
> > + dom = iommu_get_domain_for_dev(dev);
> > + if (!dom || !dom->ops || !dom->ops->attach_dev_pasid)
> > + return -ENODEV;
> > +
> > + /* Only support domain types that DMA API can be used */
> > + if (dom->type == IOMMU_DOMAIN_UNMANAGED ||
> > + dom->type == IOMMU_DOMAIN_BLOCKED) {
> > + dev_warn(dev, "Invalid domain type %d", dom->type);
> > + return -EPERM;
> > + }
>
> WARN_ON.
>
> and probably we can just check whether domain is default domain here.
>
good point, I will just use
struct iommu_domain *def_domain = iommu_get_dma_domain(dev);

> > +
> > + mutex_lock(&dma_pasid_lock);
> > + id = dom->dma_pasid;
> > + if (!id) {
> > + /*
> > + * First device to use PASID in its DMA domain,
> > allocate
> > + * a single PASID per DMA domain is all we need, it is
> > also
> > + * good for performance when it comes down to IOTLB
> > flush.
> > + */
> > + max = 1U << dev->iommu->pasid_bits;
> > + if (!max) {
> > + ret = -EINVAL;
> > + goto done_unlock;
> > + }
> > +
> > + id = ioasid_alloc(&iommu_dma_pasid, 1, max, dev);
> > + if (id == INVALID_IOASID) {
> > + ret = -ENOMEM;
> > + goto done_unlock;
> > + }
> > +
> > + dom->dma_pasid = id;
> > + atomic_set(&dom->dma_pasid_users, 1);
>
> this is always accessed with lock held hence no need to be atomic.
>
good catch, will fix

> > + }
> > +
> > + ret = iommu_attach_device_pasid(dom, dev, id);
> > + if (!ret) {
> > + *pasid = id;
> > + atomic_inc(&dom->dma_pasid_users);
> > + goto done_unlock;
> > + }
> > +
> > + if (atomic_dec_and_test(&dom->dma_pasid_users)) {
> > + ioasid_free(id);
> > + dom->dma_pasid = 0;
> > + }
> > +done_unlock:
> > + mutex_unlock(&dma_pasid_lock);
> > + return ret;
> > +}
> > +EXPORT_SYMBOL(iommu_attach_dma_pasid);
> > +
> > +/**
> > + * iommu_detach_dma_pasid --Disable in-kernel DMA request with PASID
> > + * @dev: Device's PASID DMA to be disabled
> > + *
> > + * It is the device driver's responsibility to ensure no more incoming
> > DMA
> > + * requests with the kernel PASID before calling this function. IOMMU
> > driver
> > + * ensures PASID cache, IOTLBs related to the kernel PASID are cleared
> > and
> > + * drained.
> > + *
> > + */
> > +void iommu_detach_dma_pasid(struct device *dev)
> > +{
> > + struct iommu_domain *dom;
> > + ioasid_t pasid;
> > +
> > + dom = iommu_get_domain_for_dev(dev);
> > + if (WARN_ON(!dom || !dom->ops || !dom->ops->detach_dev_pasid))
> > + return;
> > +
> > + /* Only support DMA API managed domain type */
> > + if (WARN_ON(dom->type == IOMMU_DOMAIN_UNMANAGED ||
> > + dom->type == IOMMU_DOMAIN_BLOCKED))
> > + return;
> > +
> > + mutex_lock(&dma_pasid_lock);
> > + pasid = iommu_get_pasid_from_domain(dev, dom);
> > + if (!pasid || pasid == INVALID_IOASID) {
> > + dev_err(dev, "No valid DMA PASID attached\n");
> > + mutex_unlock(&dma_pasid_lock);
> > + return;
> > + }
>
> here just use dom->dma_pasid and let iommu driver to figure out
> underlying whether this device has been attached to the domain
> with the said pasid.
>
Yeah, I am checking the pasid matching in the iommu driver. My thinking is
that here is a quick sanity check in the common code to rule out invalid value.

Thanks a lot!

Jacob