RE: [PATCH v4 4/6] vfio/type1: check dma map request is within a valid iova range

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Shameerali Kolothum Thodi
> Sent: Monday, March 05, 2018 11:44 AM
> To: 'Robin Murphy' <robin.murphy@xxxxxxx>; Alex Williamson
> <alex.williamson@xxxxxxxxxx>
> Cc: Auger Eric <eric.auger@xxxxxxxxxx>; pmorel@xxxxxxxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Linuxarm
> <linuxarm@xxxxxxxxxx>; John Garry <john.garry@xxxxxxxxxx>; xuwei (O)
> <xuwei5@xxxxxxxxxx>
> Subject: RE: [PATCH v4 4/6] vfio/type1: check dma map request is within a valid
> iova range

 [...]

> > >>> I still think you're overstretching the IOMMU reserved region interface
> > >>> by trying to report possible ACS conflicts.  Are we going to update the
> > >>> reserved list on device hotplug?  Are we going to update the list when
> > >>> MMIO is enabled or disabled for each device?  What if the BARs are
> > >>> reprogrammed or bridge apertures changed?  IMO, the IOMMU
> reserved
> > list
> > >>> should account for specific IOVA exclusions that apply to transactions
> > >>> that actually reach the IOMMU, not aliasing that might occur in the
> > >>> downstream topology.  Additionally, the IOMMU group composition must
> > be
> > >>> such that these sorts of aliasing issues can only occur within an IOMMU
> > >>> group.  If a transaction can be affected by the MMIO programming of
> > >>> another group, then the groups are drawn incorrectly for the isolation
> > >>> capabilities of the hardware.  Thanks,
> > >>
> > >> Agree that this is not a perfect thing to do covering all scenarios. As Robin
> > >> pointed out, the current code is playing safe by reserving the full windows.
> > >>
> > >> My suggestion will be to limit this reservation to non-ACS cases only.  This
> > will
> > >> make sure that ACS ARM hardware is not broken by this series and nos-
> ACS
> > >> ones has a chance to run once Qemu is updated to take care of the
> reserved
> > >> regions (at least in some user scenarios).
> > >>
> > >> If you all are fine with this, I can include the ACS check I mentioned earlier
> > in
> > >> iommu_dma_get_resv_regions() and sent out the revised  series.
> > >>
> > >> Please let me know your thoughts.
> > >
> > > IMO, the IOMMU should be concerned with ACS as far as isolation is
> > > concerned and reporting reserved ranges that are imposed at the IOMMU
> > > and leave any aliasing or routing issues in the downstream topology to
> > > other layers, or perhaps to the user.  Unfortunately, enforcing the
> > > iova list in vfio is gated by some movement here since we can't break
> > > existing users.  Thanks,
> >
> > FWIW, given the discussion we've had here I wouldn't object to pushing
> > the PCI window reservation back into the DMA-specific path, such that it
> > doesn't get exposed via the general IOMMU API interface. We *do* want to
> > do it there where we are in total control of our own address space and
> > it just avoids a whole class of problems (even with ACS I'm not sure the
> > root complex can be guaranteed to send a "bad" IOVA out to the SMMU
> > instead of just terminating it for looking like a peer-to-peer attempt).
> >
> > I do agree that it's not scalable for the IOMMU layer to attempt to
> > detect and describe upstream PCI limitations to userspace by itself -
> > they are "reserved regions" rather than "may or may not work regions"
> > after all. If there is a genuine integration issue between an IOMMU and
> > an upstream PCI RC which restricts usable addresses on a given system,
> > that probably needs to be explicitly communicated from firmware to the
> > IOMMU driver anyway, at which point that driver can report the relevant
> > region(s) directly from its own callback.
> >
> > I suppose there's an in-between option of keeping generic window
> > reservations but giving them a new "only reserve this if you're being
> > super-cautious or don't know better" region type which we hide from
> > userspace and ignore in VFIO, but maybe that leaves the lines a but too
> > blurred still.
> 
> Thanks for your reply and details. I have made an attempt to revert the PCI
> window reservation back  into the DMA path. Could you please take a look
> at the below changes and let me know.
> (This is on top of HW MSI reserve patches which is now part of linux-next)

Hi Robin,

Please take a look at the below changes if you get sometime and let me know.
Not sure this is what you have in mind.

Thanks,
Shameer

> 
> -->8--
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index f05f3cf..ddcbbdb 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -167,40 +167,16 @@ EXPORT_SYMBOL(iommu_put_dma_cookie);
>   * @list: Reserved region list from iommu_get_resv_regions()
>   *
>   * IOMMU drivers can use this to implement their .get_resv_regions callback
> - * for general non-IOMMU-specific reservations. Currently, this covers host
> - * bridge windows for PCI devices and GICv3 ITS region reservation on ACPI
> - * based ARM platforms that may require HW MSI reservation.
> + * for general non-IOMMU-specific reservations. Currently, this covers GICv3
> + * ITS region reservation on ACPI based ARM platforms that may require HW
> MSI
> + * reservation.
>   */
>  void iommu_dma_get_resv_regions(struct device *dev, struct list_head *list)
>  {
> -	struct pci_host_bridge *bridge;
> -	struct resource_entry *window;
> -
> -	if (!is_of_node(dev->iommu_fwspec->iommu_fwnode) &&
> -		iort_iommu_msi_get_resv_regions(dev, list) < 0)
> -		return;
> -
> -	if (!dev_is_pci(dev))
> -		return;
> -
> -	bridge = pci_find_host_bridge(to_pci_dev(dev)->bus);
> -	resource_list_for_each_entry(window, &bridge->windows) {
> -		struct iommu_resv_region *region;
> -		phys_addr_t start;
> -		size_t length;
> -
> -		if (resource_type(window->res) != IORESOURCE_MEM)
> -			continue;
> 
> -		start = window->res->start - window->offset;
> -		length = window->res->end - window->res->start + 1;
> -		region = iommu_alloc_resv_region(start, length, 0,
> -				IOMMU_RESV_RESERVED);
> -		if (!region)
> -			return;
> +	if (!is_of_node(dev->iommu_fwspec->iommu_fwnode))
> +		iort_iommu_msi_get_resv_regions(dev, list);
> 
> -		list_add_tail(&region->list, list);
> -	}
>  }
>  EXPORT_SYMBOL(iommu_dma_get_resv_regions);
> 
> @@ -229,6 +205,23 @@ static int cookie_init_hw_msi_region(struct
> iommu_dma_cookie *cookie,
>  	return 0;
>  }
> 
> +static void iova_reserve_pci_windows(struct pci_dev *dev,
> +		struct iova_domain *iovad)
> +{
> +	struct pci_host_bridge *bridge = pci_find_host_bridge(dev->bus);
> +	struct resource_entry *window;
> +	unsigned long lo, hi;
> +
> +	resource_list_for_each_entry(window, &bridge->windows) {
> +		if (resource_type(window->res) != IORESOURCE_MEM)
> +			continue;
> +
> +		lo = iova_pfn(iovad, window->res->start - window->offset);
> +		hi = iova_pfn(iovad, window->res->end - window->offset);
> +		reserve_iova(iovad, lo, hi);
> +	}
> +}
> +
>  static int iova_reserve_iommu_regions(struct device *dev,
>  		struct iommu_domain *domain)
>  {
> @@ -238,6 +231,9 @@ static int iova_reserve_iommu_regions(struct device
> *dev,
>  	LIST_HEAD(resv_regions);
>  	int ret = 0;
> 
> +	if (dev_is_pci(dev))
> +		iova_reserve_pci_windows(to_pci_dev(dev), iovad);
> +
>  	iommu_get_resv_regions(dev, &resv_regions);
>  	list_for_each_entry(region, &resv_regions, list) {
>  		unsigned long lo, hi;
> 
>