On 2022/5/31 18:12, Tian, Kevin wrote:
+++ b/include/linux/iommu.h
@@ -105,6 +105,8 @@ struct iommu_domain {
enum iommu_page_response_code (*iopf_handler)(struct
iommu_fault *fault,
void *data);
void *fault_data;
+ ioasid_t pasid; /* Used for DMA requests with PASID */
+ atomic_t pasid_users;
These are poorly named, this is really the DMA API global PASID and
shouldn't be used for other things.
Perhaps I misunderstood, do you mind explaining more?
You still haven't really explained what this is for in this patch,
maybe it just needs a better commit message, or maybe something is
wrong.
I keep saying the DMA API usage is not special, so why do we need to
create a new global pasid and refcount? Realistically this is only
going to be used by IDXD, why can't we just allocate a PASID and
return it to the driver every time a driver asks for DMA API on PASI
mode? Why does the core need to do anything special?
Agree. I guess it was a mistake caused by treating ENQCMD as the
only user although the actual semantics of the invented interfaces
have already evolved to be quite general.
This is very similar to what we have been discussing for iommufd.
a PASID is just an additional routing info when attaching a device
to an I/O address space (DMA API in this context) and by default
it should be a per-device resource except when ENQCMD is
explicitly opt in.
Hence it's right time for us to develop common facility working
for both this DMA API usage and iommufd, i.e.:
for normal PASID attach to a domain, driver:
allocates a local pasid from device local space;
attaches the local pasid to a domain;
for PASID attach in particular for ENQCMD, driver:
allocates a global pasid in system-wide;
attaches the global pasid to a domain;
set the global pasid in PASID_MSR;
In both cases the pasid is stored in the attach data instead of the
domain.
DMA API pasid is no special from above except it needs to allow
one device attached to the same domain twice (one with RID
and the other with RID+PASID).
for iommufd those operations are initiated by userspace via
iommufd uAPI.
My understanding is that device driver owns its PASID policy. If ENQCMD
is supported on the device, the PASIDs should be allocated through
ioasid_alloc(). Otherwise, the whole PASID pool is managed by the device
driver.
For kernel DMA w/ PASID, after the driver has a PASID for this purpose,
it can just set the default domain to the PASID on device. There's no
need for enable/disable() interfaces.
Best regards,
baolu
