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+	         MEMORY ATTRIBUTE ALIASING ON IA-64
+
+			   Bjorn Helgaas
+		       <bjorn.helgaas@hp.com>
+			    May 4, 2006
+
+
+MEMORY ATTRIBUTES
+
+    Itanium supports several attributes for virtual memory references.
+    The attribute is part of the virtual translation, i.e., it is
+    contained in the TLB entry.  The ones of most interest to the Linux
+    kernel are:
+
+	WB		Write-back (cacheable)
+	UC		Uncacheable
+	WC		Write-coalescing
+
+    System memory typically uses the WB attribute.  The UC attribute is
+    used for memory-mapped I/O devices.  The WC attribute is uncacheable
+    like UC is, but writes may be delayed and combined to increase
+    performance for things like frame buffers.
+
+    The Itanium architecture requires that we avoid accessing the same
+    page with both a cacheable mapping and an uncacheable mapping[1].
+
+    The design of the chipset determines which attributes are supported
+    on which regions of the address space.  For example, some chipsets
+    support either WB or UC access to main memory, while others support
+    only WB access.
+
+MEMORY MAP
+
+    Platform firmware describes the physical memory map and the
+    supported attributes for each region.  At boot-time, the kernel uses
+    the EFI GetMemoryMap() interface.  ACPI can also describe memory
+    devices and the attributes they support, but Linux/ia64 currently
+    doesn't use this information.
+
+    The kernel uses the efi_memmap table returned from GetMemoryMap() to
+    learn the attributes supported by each region of physical address
+    space.  Unfortunately, this table does not completely describe the
+    address space because some machines omit some or all of the MMIO
+    regions from the map.
+
+    The kernel maintains another table, kern_memmap, which describes the
+    memory Linux is actually using and the attribute for each region.
+    This contains only system memory; it does not contain MMIO space.
+
+    The kern_memmap table typically contains only a subset of the system
+    memory described by the efi_memmap.  Linux/ia64 can't use all memory
+    in the system because of constraints imposed by the identity mapping
+    scheme.
+
+    The efi_memmap table is preserved unmodified because the original
+    boot-time information is required for kexec.
+
+KERNEL IDENTITY MAPPINGS
+
+    Linux/ia64 identity mappings are done with large pages, currently
+    either 16MB or 64MB, referred to as "granules."  Cacheable mappings
+    are speculative[2], so the processor can read any location in the
+    page at any time, independent of the programmer's intentions.  This
+    means that to avoid attribute aliasing, Linux can create a cacheable
+    identity mapping only when the entire granule supports cacheable
+    access.
+
+    Therefore, kern_memmap contains only full granule-sized regions that
+    can referenced safely by an identity mapping.
+
+    Uncacheable mappings are not speculative, so the processor will
+    generate UC accesses only to locations explicitly referenced by
+    software.  This allows UC identity mappings to cover granules that
+    are only partially populated, or populated with a combination of UC
+    and WB regions.
+
+USER MAPPINGS
+
+    User mappings are typically done with 16K or 64K pages.  The smaller
+    page size allows more flexibility because only 16K or 64K has to be
+    homogeneous with respect to memory attributes.
+
+POTENTIAL ATTRIBUTE ALIASING CASES
+
+    There are several ways the kernel creates new mappings:
+
+    mmap of /dev/mem
+
+	This uses remap_pfn_range(), which creates user mappings.  These
+	mappings may be either WB or UC.  If the region being mapped
+	happens to be in kern_memmap, meaning that it may also be mapped
+	by a kernel identity mapping, the user mapping must use the same
+	attribute as the kernel mapping.
+
+	If the region is not in kern_memmap, the user mapping should use
+	an attribute reported as being supported in the EFI memory map.
+
+	Since the EFI memory map does not describe MMIO on some
+	machines, this should use an uncacheable mapping as a fallback.
+
+    mmap of /sys/class/pci_bus/.../legacy_mem
+
+	This is very similar to mmap of /dev/mem, except that legacy_mem
+	only allows mmap of the one megabyte "legacy MMIO" area for a
+	specific PCI bus.  Typically this is the first megabyte of
+	physical address space, but it may be different on machines with
+	several VGA devices.
+
+	"X" uses this to access VGA frame buffers.  Using legacy_mem
+	rather than /dev/mem allows multiple instances of X to talk to
+	different VGA cards.
+
+	The /dev/mem mmap constraints apply.
+
+    mmap of /proc/bus/pci/.../??.?
+
+    	This is an MMIO mmap of PCI functions, which additionally may or
+	may not be requested as using the WC attribute.
+
+	If WC is requested, and the region in kern_memmap is either WC
+	or UC, and the EFI memory map designates the region as WC, then
+	the WC mapping is allowed.
+
+	Otherwise, the user mapping must use the same attribute as the
+	kernel mapping.
+
+    read/write of /dev/mem
+
+	This uses copy_from_user(), which implicitly uses a kernel
+	identity mapping.  This is obviously safe for things in
+	kern_memmap.
+
+	There may be corner cases of things that are not in kern_memmap,
+	but could be accessed this way.  For example, registers in MMIO
+	space are not in kern_memmap, but could be accessed with a UC
+	mapping.  This would not cause attribute aliasing.  But
+	registers typically can be accessed only with four-byte or
+	eight-byte accesses, and the copy_from_user() path doesn't allow
+	any control over the access size, so this would be dangerous.
+
+    ioremap()
+
+	This returns a mapping for use inside the kernel.
+
+	If the region is in kern_memmap, we should use the attribute
+	specified there.
+
+	If the EFI memory map reports that the entire granule supports
+	WB, we should use that (granules that are partially reserved
+	or occupied by firmware do not appear in kern_memmap).
+
+	If the granule contains non-WB memory, but we can cover the
+	region safely with kernel page table mappings, we can use
+	ioremap_page_range() as most other architectures do.
+
+	Failing all of the above, we have to fall back to a UC mapping.
+
+PAST PROBLEM CASES
+
+    mmap of various MMIO regions from /dev/mem by "X" on Intel platforms
+
+      The EFI memory map may not report these MMIO regions.
+
+      These must be allowed so that X will work.  This means that
+      when the EFI memory map is incomplete, every /dev/mem mmap must
+      succeed.  It may create either WB or UC user mappings, depending
+      on whether the region is in kern_memmap or the EFI memory map.
+
+    mmap of 0x0-0x9FFFF /dev/mem by "hwinfo" on HP sx1000 with VGA enabled
+
+      The EFI memory map reports the following attributes:
+        0x00000-0x9FFFF WB only
+        0xA0000-0xBFFFF UC only (VGA frame buffer)
+        0xC0000-0xFFFFF WB only
+
+      This mmap is done with user pages, not kernel identity mappings,
+      so it is safe to use WB mappings.
+
+      The kernel VGA driver may ioremap the VGA frame buffer at 0xA0000,
+      which uses a granule-sized UC mapping.  This granule will cover some
+      WB-only memory, but since UC is non-speculative, the processor will
+      never generate an uncacheable reference to the WB-only areas unless
+      the driver explicitly touches them.
+
+    mmap of 0x0-0xFFFFF legacy_mem by "X"
+
+      If the EFI memory map reports that the entire range supports the
+      same attributes, we can allow the mmap (and we will prefer WB if
+      supported, as is the case with HP sx[12]000 machines with VGA
+      disabled).
+
+      If EFI reports the range as partly WB and partly UC (as on sx[12]000
+      machines with VGA enabled), we must fail the mmap because there's no
+      safe attribute to use.
+
+      If EFI reports some of the range but not all (as on Intel firmware
+      that doesn't report the VGA frame buffer at all), we should fail the
+      mmap and force the user to map just the specific region of interest.
+
+    mmap of 0xA0000-0xBFFFF legacy_mem by "X" on HP sx1000 with VGA disabled
+
+      The EFI memory map reports the following attributes:
+        0x00000-0xFFFFF WB only (no VGA MMIO hole)
+
+      This is a special case of the previous case, and the mmap should
+      fail for the same reason as above.
+
+    read of /sys/devices/.../rom
+
+      For VGA devices, this may cause an ioremap() of 0xC0000.  This
+      used to be done with a UC mapping, because the VGA frame buffer
+      at 0xA0000 prevents use of a WB granule.  The UC mapping causes
+      an MCA on HP sx[12]000 chipsets.
+
+      We should use WB page table mappings to avoid covering the VGA
+      frame buffer.
+
+NOTES
+
+    [1] SDM rev 2.2, vol 2, sec 4.4.1.
+    [2] SDM rev 2.2, vol 2, sec 4.4.6.