Date:   Mon, 26 Sep 2022 15:29:49 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>, iommu@...ts.linux.dev,
        linux-s390@...r.kernel.org, borntraeger@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, joro@...tes.org, will@...nel.org,
        robin.murphy@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] iommu/s390: Fix duplicate domain attachments

On Thu, 2022-09-22 at 11:33 -0300, Jason Gunthorpe wrote:
> On Thu, Sep 22, 2022 at 11:52:37AM +0200, Niklas Schnelle wrote:
> > Since commit fa7e9ecc5e1c ("iommu/s390: Tolerate repeat attach_dev
> > calls") we can end up with duplicates in the list of devices attached to
> > a domain. This is inefficient and confusing since only one domain can
> > actually be in control of the IOMMU translations for a device. Fix this
> > by detaching the device from the previous domain, if any, on attach.
> > Add a WARN_ON() in case we still have attached devices on freeing the
> > domain.
> > 
> > Fixes: fa7e9ecc5e1c ("iommu/s390: Tolerate repeat attach_dev calls")
> > Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> > ---
> > Changes since v1:
> > - WARN_ON() non-empty list in s390_domain_free()
> > - Drop the found flag and instead WARN_ON() if we're detaching
> >   from a domain that isn't the active domain for the device
> > 
> >  drivers/iommu/s390-iommu.c | 81 ++++++++++++++++++++++----------------
> >  1 file changed, 46 insertions(+), 35 deletions(-)
> > 
> > diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
> > index c898bcbbce11..187d2c7ba9ff 100644
> > --- a/drivers/iommu/s390-iommu.c
> > +++ b/drivers/iommu/s390-iommu.c
> > @@ -78,19 +78,48 @@ static struct iommu_domain *s390_domain_alloc(unsigned domain_type)
> >  static void s390_domain_free(struct iommu_domain *domain)
> >  {
> >  	struct s390_domain *s390_domain = to_s390_domain(domain);
> > +	unsigned long flags;
> >  
> > +	spin_lock_irqsave(&s390_domain->list_lock, flags);
> > +	WARN_ON(!list_empty(&s390_domain->devices));
> > +	spin_unlock_irqrestore(&s390_domain->list_lock, flags);
> 
> Minor, but, this is about to free the memory holding the lock, we
> don't need to take it to do the WARN_ON.. list_empty() is already
> lockless safe.
> 
> > static int __s390_iommu_detach_device(struct s390_domain *s390_domain,
> >                                      struct zpci_dev *zdev)
> > {
> 
> This doesn't return a failure code anymore, make it void
> 
> >  static int s390_iommu_attach_device(struct iommu_domain *domain,
> >  				    struct device *dev)
> >  {
> >  	struct s390_domain *s390_domain = to_s390_domain(domain);
> >  	struct zpci_dev *zdev = to_zpci_dev(dev);
> >  	struct s390_domain_device *domain_device;
> > +	struct s390_domain *prev_domain = NULL;
> >  	unsigned long flags;
> > -	int cc, rc;
> > +	int cc, rc = 0;
> >  
> >  	if (!zdev)
> >  		return -ENODEV;
> > @@ -99,16 +128,15 @@ static int s390_iommu_attach_device(struct iommu_domain *domain,
> >  	if (!domain_device)
> >  		return -ENOMEM;
> >  
> > -	if (zdev->dma_table && !zdev->s390_domain) {
> > -		cc = zpci_dma_exit_device(zdev);
> > -		if (cc) {
> > +	if (zdev->s390_domain) {
> > +		prev_domain = zdev->s390_domain;
> > +		rc = __s390_iommu_detach_device(zdev->s390_domain, zdev);
> > +	} else if (zdev->dma_table) {
> > +		if (zpci_dma_exit_device(zdev))
> >  			rc = -EIO;
> > -			goto out_free;
> > -		}
> >  	}
> > -
> > -	if (zdev->s390_domain)
> > -		zpci_unregister_ioat(zdev, 0);
> > +	if (rc)
> > +		goto out_free;
> >  
> >  	zdev->dma_table = s390_domain->dma_table;
> >  	cc = zpci_register_ioat(zdev, 0, zdev->start_dma, zdev->end_dma,
> > @@ -129,7 +157,7 @@ static int s390_iommu_attach_device(struct iommu_domain *domain,
> >  		   domain->geometry.aperture_end != zdev->end_dma) {
> >  		rc = -EINVAL;
> >  		spin_unlock_irqrestore(&s390_domain->list_lock, flags);
> > -		goto out_restore;
> > +		goto out_unregister_restore;
> >  	}
> >  	domain_device->zdev = zdev;
> >  	zdev->s390_domain = s390_domain;
> > @@ -138,14 +166,15 @@ static int s390_iommu_attach_device(struct iommu_domain *domain,
> >  
> >  	return 0;
> >  
> > +out_unregister_restore:
> > +	zpci_unregister_ioat(zdev, 0);
> >  out_restore:
> > -	if (!zdev->s390_domain) {
> > +	zdev->dma_table = NULL;
> > +	if (prev_domain)
> > +		s390_iommu_attach_device(&prev_domain->domain,
> > +					 dev);
> 
> Huh. That is a surprising thing
> 
> I think this function needs some re-ordering to avoid this condition
> 
> The checks for aperture should be earlier, and they are not quite
> right. The aperture is only allowed to grow. If it starts out as 0 and
> then is set to something valid on first attach, a later attach cannot
> then shrink it. There could already be mappings in the domain under
> the now invalidated aperture and no caller is prepared to deal with
> this.
> 
> That leaves the only error case as zpci_register_ioat() - which seems
> like it is the actual "attach" operation. Since
> __s390_iommu_detach_device() is just internal accounting (and can't
> fail) it should be moved after

I did miss a problem in my initial answer. While zpci_register_ioat()
is indeed the actual "attach" operation, it assumes that at that point
no DMA address translations are registered. In that state DMA is
blocked of course. With that zpci_register_ioat() needs to come after
the zpci_unregister_ioat() that is part of __s390_iommu_detach_device()
and zpci_dma_exit_device(). If we do call those though we fundamentally
need to restore the previous domain / DMA API state on any subsequent
failure. If we don't restore we would leave the device detached from
any domain with DMA blocked. I wonder if this could be an acceptable
failure state though? It's safe as no DMA is possible and we could get
out of it with a successful attach.

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