Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

[Fenghua Yu]

Hi, Tony,

On Thu, Feb 10, 2022 at 10:31:42AM -0800, Fenghua Yu wrote:
> 
> On Thu, Feb 10, 2022 at 09:24:50AM -0800, Luck, Tony wrote:
> > On Thu, Feb 10, 2022 at 08:27:50AM -0800, Fenghua Yu wrote:
> > > Hi, Jacob,
> > > 
> > > On Wed, Feb 09, 2022 at 07:16:14PM -0800, Jacob Pan wrote:
> > > > Hi Fenghua,
> > > > 
> > > > On Mon,  7 Feb 2022 15:02:48 -0800, Fenghua Yu <fenghua.yu@xxxxxxxxx> wrote:
> > > > 
> > > > > @@ -1047,8 +1040,6 @@ struct iommu_sva *intel_svm_bind(struct device
> > > > > *dev, struct mm_struct *mm, void }
> > > > >  
> > > > >  	sva = intel_svm_bind_mm(iommu, dev, mm, flags);
> > > > > -	if (IS_ERR_OR_NULL(sva))
> > > > > -		intel_svm_free_pasid(mm);
> > > > If bind fails, the PASID has no IOMMU nor CPU context. It should be safe to
> > > > free here.
> > > 
> > > The PASID can not be freed even if bind fails. The PASID allocated earlier
> > > (either in this thread or in another thread) might be populated to other
> > > threads already and being used now.
> > > 
> > > Without freeing the PASID on bind failure, the worst case is the PASID might
> > > not be used in the process (and will be freed on process exit anyway).
> > > 
> > > This all matches with the PASID life time described in the commit message.
> > 
> > But what does this mean for the user that failed that intel_svm_bind_mm()?
> > 
> 
> That means the user may have a PASID but there is no PASID entry for the
> device. Then ENQCMD cannot be executed successfully.
> 
> > Here's a scenario:
> > 
> > Process sets up to use PASID capable device #1. Everything works,
> > so the process has mm->pasid, and the IOMMU has the tables to map
> > virtual addresses coming from device #1 using that PASID.
> > 
> > Now the same process asks to start using PASID capable device #2,
> > but there is a failure at intel_svm_bind_mm().
> > 
> > Fenghua is right that we shouldn't free the PASID. It is in use
> > by at least one thread of the process to access device #1.
> > 
> > But what happens with device #2? Does the caller of intel_svm_bind()
> > do the right thing with the IS_ERR_OR_NULL return value to let the
> > user know that device #2 isn't accessible?
> 
> A driver caller of intel_svm_bind() should handle this failure, i.e. fail
> the binding and let the user know the failure.
> 
> Even if the driver doesn't do the right thing to handle the failure,
> intel_svm_bind() doesn't set up a PASID entry for device #2.
> 
> One example is IDXD driver. User calls open()->IDXD driver idxd_cdev_open()
> ->intel_svm_bind()->intel_svm_bind_mm(). idxd_cdev_open() gets failed "sva"
> and passes the PTR_ERR(sva) to the user and the user cannot open the device.
> Due to the failure, no PASID entry is set up for the device.
> 
> Even if the user ignores the open() failure and tries to run ENQCMD on
> device #2, a PASID table fault will be generated due to no PASID entry
> for the device and the ENQCMD execution will fail.

Plus, the above behavior of handling intel_svm_bind_mm() failure is expected
right behavior and the current series doesn't need to be changed.

Thanks.
 
-Fenghua