222 lines
8.5 KiB
Plaintext
222 lines
8.5 KiB
Plaintext
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.
|