1 files changed, 132 insertions, 0 deletions
diff --git a/src/include/ipxe/virt_offset.h b/src/include/ipxe/virt_offset.h
new file mode 100644
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+++ b/src/include/ipxe/virt_offset.h
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+#ifndef _IPXE_VIRT_OFFSET_H
+#define _IPXE_VIRT_OFFSET_H
+
+/**
+ * @file
+ *
+ * Virtual offset memory model
+ *
+ * No currently supported machine provides a full 64 bits of physical
+ * address space.  When we have ownership of the page tables (or
+ * segmentation mechanism), we can therefore use the following model:
+ *
+ *   - For 32-bit builds: set up a circular map so that all 32-bit
+ *     virtual addresses are at a fixed offset from the 32-bit
+ *     physical addresses.
+ *
+ *   - For 64-bit builds: identity-map the required portion of the
+ *     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
+ * physical address".  With this definition:
+ *
+ *   - For 32-bit builds: conversion between physical and virtual
+ *     addresses is a straightforward addition or subtraction of
+ *     virt_offset, since the whole 32-bit address space is circular.
+ *
+ *   - For 64-bit builds: conversion from any valid physical address
+ *     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 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)
+ * and create a coherent DMA mapping of the low 4GB.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+FILE_SECBOOT ( PERMITTED );
+
+#ifdef UACCESS_OFFSET
+#define UACCESS_PREFIX_offset
+#else
+#define UACCESS_PREFIX_offset __offset_
+#endif
+
+/** Virtual address offset
+ *
+ * This is defined to be the value to be added to an address within
+ * iPXE's own image in order to obtain its physical address, as
+ * described above.
+ */
+extern const unsigned long virt_offset;
+
+/** Allow for architecture-specific overrides of virt_offset */
+#include <bits/virt_offset.h>
+
+/**
+ * Convert physical address to virtual address
+ *
+ * @v phys		Physical address
+ * @ret virt		Virtual address
+ */
+static inline __always_inline void *
+UACCESS_INLINE ( offset, phys_to_virt ) ( unsigned long phys ) {
+
+	/* In a 64-bit build, any valid physical address is directly
+	 * usable as a virtual address, since physical addresses are
+	 * identity-mapped.
+	 */
+	if ( sizeof ( physaddr_t ) > sizeof ( uint32_t ) )
+		return ( ( void * ) phys );
+
+	/* In a 32-bit build: subtract virt_offset */
+	return ( ( void * ) ( phys - virt_offset ) );
+}
+
+/**
+ * Convert virtual address to physical address
+ *
+ * @v virt		Virtual address
+ * @ret phys		Physical address
+ */
+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
+	 * clear is directly usable as a physical address, since it
+	 * must lie within the identity-mapped portion.
+	 *
+	 * This test will typically reduce to a single "branch if less
+	 * than zero" instruction.
+	 */
+	if ( ( sizeof ( physaddr_t ) > sizeof ( uint32_t ) ) &&
+	     ( ! ( 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
+	 */
+	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 */