From: Michael Brown <mcb30@ipxe.org>
Date: Fri, 4 Jul 2025 12:29:44 +0000 (+0100)
Subject: [uaccess] Allow for coherent DMA mapping of the 32-bit address space
X-Git-Url: http://git.ipfire.org/gitweb/gitweb.cgi?a=commitdiff_plain;h=c21443f0b9a4dee56ab0f47b096540d6443cda9f;p=thirdparty%2Fipxe.git

[uaccess] Allow for coherent DMA mapping of the 32-bit address space

On platforms where DMA devices are not in the same coherency domain as
the CPU cache, it is necessary to create page table entries where the
translations are marked as uncacheable.

We choose to place iPXE within the low 4GB of memory (since 32-bit DMA
devices are still reasonably common even on systems with 64-bit CPUs).
We therefore need to cover only the low 4GB of memory with these page
table entries.

Update virt_to_phys() to allow for the existence of such a mapping,
assuming that iPXE itself will always reside within the top 4GB of the
64-bit virtual address space (and therefore that the DMA mapping must
lie somewhere below this in the negative virtual address space).

Signed-off-by: Michael Brown <mcb30@ipxe.org>
---

diff --git a/src/include/ipxe/virt_offset.h b/src/include/ipxe/virt_offset.h
index 311bf12fe..2762acb40 100644
--- a/src/include/ipxe/virt_offset.h
+++ b/src/include/ipxe/virt_offset.h
@@ -15,8 +15,11 @@
  *     physical addresses.
  *
  *   - For 64-bit builds: identity-map the required portion of the
- *     physical address space, then map iPXE itself using virtual
- *     addresses in the negative (kernel) address space.
+ *     physical address space, place iPXE within the 32-bit physical
+ *     address space, map iPXE using virtual addresses in the top part
+ *     of the negative (kernel) address space, and optionally map the
+ *     32-bit physical address space with attributes suitable for
+ *     coherent DMA accesses.
  *
  * In both cases, we can define "virt_offset" as "the value to be
  * added to an address within iPXE's own image in order to obtain its
@@ -30,15 +33,17 @@
  *     is a no-op (since all physical addresses are identity-mapped),
  *     and conversion from a virtual address to a physical address
  *     requires an addition of virt_offset if and only if the virtual
- *     address lies in the negative portion of the address space
- *     (i.e. has the MSB set).
+ *     address lies in the high negative portion of the address space
+ *     (i.e. has the MSB set, but has the MSB clear after adding
+ *     virt_offset).
  *
  * For x86_64-pcbios, we identity-map the low 4GB of address space
  * since the only accesses required above 4GB are for MMIO (typically
  * PCI devices with large memory BARs).
  *
  * For riscv64-sbi, we identity-map as much of the physical address
- * space as can be mapped by the paging model (Sv39, Sv48, or Sv57).
+ * space as can be mapped by the paging model (Sv39, Sv48, or Sv57)
+ * and create a coherent DMA mapping of the low 4GB.
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
@@ -88,6 +93,7 @@ UACCESS_INLINE ( offset, phys_to_virt ) ( unsigned long phys ) {
  */
 static inline __always_inline physaddr_t
 UACCESS_INLINE ( offset, virt_to_phys ) ( volatile const void *virt ) {
+	const physaddr_t msb = ( 1ULL << ( 8 * sizeof ( physaddr_t ) - 1 ) );
 	physaddr_t addr = ( ( physaddr_t ) virt );
 
 	/* In a 64-bit build, any valid virtual address with the MSB
@@ -98,14 +104,28 @@ UACCESS_INLINE ( offset, virt_to_phys ) ( volatile const void *virt ) {
 	 * than zero" instruction.
 	 */
 	if ( ( sizeof ( physaddr_t ) > sizeof ( uint32_t ) ) &&
-	     ( ! ( addr & ( 1ULL << ( 8 * sizeof ( physaddr_t ) - 1 ) ) ) ) ) {
+	     ( ! ( addr & msb ) ) ) {
 		return addr;
 	}
 
 	/* In a 32-bit build or in a 64-bit build with a virtual
 	 * address with the MSB set: add virt_offset
 	 */
-	return ( addr + virt_offset );
+	addr += virt_offset;
+
+	/* In a 64-bit build with an address that still has the MSB
+	 * set after adding virt_offset: truncate the original virtual
+	 * address to form a 32-bit physical address.
+	 *
+	 * This test will also typically reduce to a single "branch if
+	 * less than zero" instruction.
+	 */
+	if ( ( sizeof ( physaddr_t ) > sizeof ( uint32_t ) ) &&
+	     ( addr & msb ) ) {
+		return ( ( uint32_t ) ( physaddr_t ) virt );
+	}
+
+	return addr;
 }
 
 #endif /* _IPXE_VIRT_OFFSET_H */