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diff --git a/Documentation/ia64/aliasing.rst b/Documentation/ia64/aliasing.rst deleted file mode 100644 index a08b36aba015..000000000000 --- a/Documentation/ia64/aliasing.rst +++ /dev/null @@ -1,246 +0,0 @@ -================================== -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 Identify 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. |