[Xen-devel] [PATCH v7 5/7] iommu / vtd: introduce a new 'refcount' flag...

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

...to mean 'the page (being) mapped is reference counted'.

An important pre-requisite for PV-IOMMU mapping is being able to tell the
difference between IOMMU entries added at start-of-day by Xen and those
that have been added by a PV-IOMMU map operation. The reason for this is
that the pages for the former do not have an extra reference taken prior to
mapping but the latter will (for safety/security reasons).

This patch therefore introduces a new IOMMF_refcount flag that the
subsequent patch adding the PV-IOMMU map operation will use to mark
entries that it adds. When the VT-d mapping code encounters this flag
it will set a bit in the IOMMU PTE that is ignored by the IOMMU itself,
such that a subsquent lookup operation can determine whether the mapped
page was reference counted or not (and hence forbid a PV-IOMMU unmap
operation in the latter case).

A subsequent patch will implement a similar PTE bit for AMD IOMMUs.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
---
Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>

v7:
 - New in v7.
---
 xen/drivers/passthrough/vtd/iommu.c |  5 +++++
 xen/drivers/passthrough/vtd/iommu.h | 17 +++++++++++------
 xen/include/xen/iommu.h             |  2 ++
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/xen/drivers/passthrough/vtd/iommu.c 
b/xen/drivers/passthrough/vtd/iommu.c
index d8873167e1..cb264d8af4 100644
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -1810,6 +1810,10 @@ static int __must_check intel_iommu_map_page(struct 
domain *d,
     if ( iommu_snoop )
         dma_set_pte_snp(new);
 
+    /* If the page has referenced for mapping then mark it as such */
+    if ( flags & IOMMUF_refcount )
+        dma_set_pte_refcnt(new);
+
     if ( old.val == new.val )
     {
         spin_unlock(&hd->arch.mapping_lock);
@@ -1879,6 +1883,7 @@ static int intel_iommu_lookup_page(struct domain *d, 
dfn_t dfn, mfn_t *mfn,
     *mfn = maddr_to_mfn(dma_pte_addr(val));
     *flags = dma_pte_read(val) ? IOMMUF_readable : 0;
     *flags |= dma_pte_write(val) ? IOMMUF_writable : 0;
+    *flags |= dma_pte_refcnt(val) ? IOMMUF_refcount : 0;
 
     return 0;
 }
diff --git a/xen/drivers/passthrough/vtd/iommu.h 
b/xen/drivers/passthrough/vtd/iommu.h
index 1a992f72d6..880eebaed3 100644
--- a/xen/drivers/passthrough/vtd/iommu.h
+++ b/xen/drivers/passthrough/vtd/iommu.h
@@ -253,21 +253,25 @@ struct context_entry {
  * 1: writable
  * 2-6: reserved
  * 7: super page
- * 8-11: available
+ * 8-9: reserved
+ * 10: ignored by h/w (used for refcount flag)
+ * 11: snoop control
  * 12-63: Host physcial address
  */
 struct dma_pte {
     u64 val;
 };
-#define DMA_PTE_READ (1)
-#define DMA_PTE_WRITE (2)
-#define DMA_PTE_PROT (DMA_PTE_READ | DMA_PTE_WRITE)
-#define DMA_PTE_SP   (1 << 7)
-#define DMA_PTE_SNP  (1 << 11)
+#define DMA_PTE_READ   (1)
+#define DMA_PTE_WRITE  (2)
+#define DMA_PTE_PROT   (DMA_PTE_READ | DMA_PTE_WRITE)
+#define DMA_PTE_SP     (1 << 7)
+#define DMA_PTE_REFCNT (1 << 10)
+#define DMA_PTE_SNP    (1 << 11)
 #define dma_clear_pte(p)    do {(p).val = 0;} while(0)
 #define dma_set_pte_readable(p) do {(p).val |= DMA_PTE_READ;} while(0)
 #define dma_set_pte_writable(p) do {(p).val |= DMA_PTE_WRITE;} while(0)
 #define dma_set_pte_superpage(p) do {(p).val |= DMA_PTE_SP;} while(0)
+#define dma_set_pte_refcnt(p) do {(p).val |= DMA_PTE_REFCNT;} while(0)
 #define dma_set_pte_snp(p)  do {(p).val |= DMA_PTE_SNP;} while(0)
 #define dma_set_pte_prot(p, prot) do { \
         (p).val = ((p).val & ~DMA_PTE_PROT) | ((prot) & DMA_PTE_PROT); \
@@ -280,6 +284,7 @@ struct dma_pte {
             (p).val |= ((addr) & PAGE_MASK_4K); } while (0)
 #define dma_pte_present(p) (((p).val & DMA_PTE_PROT) != 0)
 #define dma_pte_superpage(p) (((p).val & DMA_PTE_SP) != 0)
+#define dma_pte_refcnt(p) (((p).val & DMA_PTE_REFCNT) != 0)
 
 /* interrupt remap entry */
 struct iremap_entry {
diff --git a/xen/include/xen/iommu.h b/xen/include/xen/iommu.h
index 1bf311624c..a56d03b719 100644
--- a/xen/include/xen/iommu.h
+++ b/xen/include/xen/iommu.h
@@ -89,6 +89,8 @@ void iommu_teardown(struct domain *d);
 #define IOMMUF_readable  (1u<<_IOMMUF_readable)
 #define _IOMMUF_writable 1
 #define IOMMUF_writable  (1u<<_IOMMUF_writable)
+#define _IOMMUF_refcount 2
+#define IOMMUF_refcount  (1u<<_IOMMUF_refcount)
 int __must_check iommu_map_page_nocrash(struct domain *d, dfn_t dfn,
                                         mfn_t mfn, unsigned int flags);
 int __must_check iommu_map_page(struct domain *d, dfn_t dfn,
-- 
2.11.0


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