Re: [PATCH 05/10] iommu/ioasid: Create an IOASID set for host SVA use

[Jacob Pan]

Hi Jean,

On Wed, 1 Apr 2020 15:53:16 +0200
Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx> wrote:

> On Wed, Mar 25, 2020 at 10:55:26AM -0700, Jacob Pan wrote:
> > Bare metal SVA allocates IOASIDs for native process addresses. This
> > should be separated from VM allocated IOASIDs thus under its own
> > set.
> > 
> > This patch creates a system IOASID set with its quota set to
> > PID_MAX. This is a reasonable default in that SVM capable devices
> > can only bind to limited user processes.  
> 
> Yes realistically there won't be more than PID_MAX_DEFAULT=0x8000
> bound address spaces. My machine uses a PID_MAX of 4 million though,
> so in theory more than 0x8000 processes may want a bond.
Got it, I assume we can adjust the system set quota as necessary.

> On Arm the
> limit of shared contexts per VM is currently a little less than
> 0x10000 (which is the number of CPU ASIDs).
> 
I guess shared contexts means shared address? then it makes sense
#IOASID < #ASID.

> But quotas are only necessary for VMs, when the host shares the PASID
> space with them (which isn't a use-case for Arm systems as far as I
> know, each VM gets its own PASID space).
Is there a host-guest PASID translation? or the PASID used by the VM is
physical PASID? When a page request comes in to SMMU, how does it know
the owner of the PASID if PASID range can overlap between host and
guest?

> Could we have quota-free IOASID sets for the host?
> 
Yes, perhaps just add a flag such that the set has its own namespace.
You mean have this quota-free IOASID set even co-exist with VMs? I still
don't get how PRQ works.

That is not the use case for VT-d in that we have to have system-wide
allocation for host PASIDs. We have enqcmd which can take a PASID from
the per task MSR and deliver to multiple devices, so even though the
PASID table is per device the PASID name space must be global.

> For the SMMU I'd like to allocate two sets, one SVA and one private
> for auxiliary domains, and I don't think giving either a quota makes
> much sense at the moment.
I agree we don;t need the quota if we don't support guest SVA at the
same time.

So the sva set and aux_domain set PASIDs have their own namespaces?

> There can be systems using only SVA and
> systems using only private PASIDs. I think it should be
> first-come-first-served until admins want a knob to define a policy
> themselves, based on cgroups for example.
> 
> > Signed-off-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/iommu/intel-iommu.c | 8 +++++++-
> >  drivers/iommu/ioasid.c      | 9 +++++++++
> >  include/linux/ioasid.h      | 9 +++++++++
> >  3 files changed, 25 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/iommu/intel-iommu.c
> > b/drivers/iommu/intel-iommu.c index ec3fc121744a..af7a1ef7b31e
> > 100644 --- a/drivers/iommu/intel-iommu.c
> > +++ b/drivers/iommu/intel-iommu.c
> > @@ -3511,8 +3511,14 @@ static int __init init_dmars(void)
> >  		goto free_iommu;
> >  
> >  	/* PASID is needed for scalable mode irrespective to SVM */
> > -	if (intel_iommu_sm)
> > +	if (intel_iommu_sm) {
> >  		ioasid_install_capacity(intel_pasid_max_id);
> > +		/* We should not run out of IOASIDs at boot */
> > +		if (ioasid_alloc_system_set(PID_MAX_DEFAULT)) {
> > +			pr_err("Failed to enable host PASID
> > allocator\n");
> > +			intel_iommu_sm = 0;
> > +		}
> > +	}
> >  
> >  	/*
> >  	 * for each drhd
> > diff --git a/drivers/iommu/ioasid.c b/drivers/iommu/ioasid.c
> > index 6265d2dbbced..9135af171a7c 100644
> > --- a/drivers/iommu/ioasid.c
> > +++ b/drivers/iommu/ioasid.c
> > @@ -39,6 +39,9 @@ struct ioasid_data {
> >  static ioasid_t ioasid_capacity;
> >  static ioasid_t ioasid_capacity_avail;
> >  
> > +int system_ioasid_sid;
> > +static DECLARE_IOASID_SET(system_ioasid);
> > +
> >  /* System capacity can only be set once */
> >  void ioasid_install_capacity(ioasid_t total)
> >  {
> > @@ -51,6 +54,12 @@ void ioasid_install_capacity(ioasid_t total)
> >  }
> >  EXPORT_SYMBOL_GPL(ioasid_install_capacity);
> >  
> > +int ioasid_alloc_system_set(int quota)
> > +{
> > +	return ioasid_alloc_set(&system_ioasid, quota,
> > &system_ioasid_sid); +}
> > +EXPORT_SYMBOL_GPL(ioasid_alloc_system_set);  
> 
> I think this helper could stay in the VT-d driver for the moment. If
> the SMMU driver ever implements auxiliary domains it will use a
> private IOASID set, separate from the shared IOASID set managed by
> iommu-sva. Both could qualify as "system set".
> 
Sounds good. Perhaps remove the special "system set". SVA code,
VFIO, VT-d, or SMMU driver can all allocate their own sets.
So to meet both SMMU and VT-d requirements, we should do:
1. add an IOASID_PRIVATE flag to ioasid_alloc_set(), indicating this is
a private set
2. All APIs operate on the set_id accordingly, e.g. ioasid_find() will
only search within the private set. Private set is excluded from from
global search (VT-d needs this in PRQ).

Since VT-d already needs private PASIDs for guest SVM where
GPASID!=HPASID, I feel we can just reuse the per ioasid_set Xarray for
both quota-free private set and guest set.


Thanks for the feedback!

Jacob

> Thanks,
> Jean
> 
> > +
> >  /*
> >   * struct ioasid_allocator_data - Internal data structure to hold
> > information
> >   * about an allocator. There are two types of allocators:
> > diff --git a/include/linux/ioasid.h b/include/linux/ioasid.h
> > index 8c82d2625671..097b1cc043a3 100644
> > --- a/include/linux/ioasid.h
> > +++ b/include/linux/ioasid.h
> > @@ -29,6 +29,9 @@ struct ioasid_allocator_ops {
> >  	void *pdata;
> >  };
> >  
> > +/* Shared IOASID set for reserved for host system use */
> > +extern int system_ioasid_sid;
> > +
> >  #define DECLARE_IOASID_SET(name) struct ioasid_set name = { 0 }
> >  
> >  #if IS_ENABLED(CONFIG_IOASID)
> > @@ -41,6 +44,7 @@ int ioasid_register_allocator(struct
> > ioasid_allocator_ops *allocator); void
> > ioasid_unregister_allocator(struct ioasid_allocator_ops
> > *allocator); int ioasid_attach_data(ioasid_t ioasid, void *data);
> > void ioasid_install_capacity(ioasid_t total); +int
> > ioasid_alloc_system_set(int quota); int ioasid_alloc_set(struct
> > ioasid_set *token, ioasid_t quota, int *sid); void
> > ioasid_free_set(int sid, bool destroy_set); int
> > ioasid_find_sid(ioasid_t ioasid); @@ -88,5 +92,10 @@ static inline
> > void ioasid_install_capacity(ioasid_t total) {
> >  }
> >  
> > +static inline int ioasid_alloc_system_set(int quota)
> > +{
> > +	return -ENOTSUPP;
> > +}
> > +
> >  #endif /* CONFIG_IOASID */
> >  #endif /* __LINUX_IOASID_H */
> > -- 
> > 2.7.4
> >   

[Jacob Pan]