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

[Shameerali Kolothum Thodi]

Hi Robin,

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy@xxxxxxx]
> Sent: Friday, March 02, 2018 5:17 PM
> To: Alex Williamson <alex.williamson@xxxxxxxxxx>; Shameerali Kolothum
> Thodi <shameerali.kolothum.thodi@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
> 
> On 02/03/18 16:04, Alex Williamson wrote:
> [...]
> >>> 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)

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;