cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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dma-attributes.rst (5853B)


      1==============
      2DMA attributes
      3==============
      4
      5This document describes the semantics of the DMA attributes that are
      6defined in linux/dma-mapping.h.
      7
      8DMA_ATTR_WEAK_ORDERING
      9----------------------
     10
     11DMA_ATTR_WEAK_ORDERING specifies that reads and writes to the mapping
     12may be weakly ordered, that is that reads and writes may pass each other.
     13
     14Since it is optional for platforms to implement DMA_ATTR_WEAK_ORDERING,
     15those that do not will simply ignore the attribute and exhibit default
     16behavior.
     17
     18DMA_ATTR_WRITE_COMBINE
     19----------------------
     20
     21DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
     22buffered to improve performance.
     23
     24Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
     25those that do not will simply ignore the attribute and exhibit default
     26behavior.
     27
     28DMA_ATTR_NO_KERNEL_MAPPING
     29--------------------------
     30
     31DMA_ATTR_NO_KERNEL_MAPPING lets the platform to avoid creating a kernel
     32virtual mapping for the allocated buffer. On some architectures creating
     33such mapping is non-trivial task and consumes very limited resources
     34(like kernel virtual address space or dma consistent address space).
     35Buffers allocated with this attribute can be only passed to user space
     36by calling dma_mmap_attrs(). By using this API, you are guaranteeing
     37that you won't dereference the pointer returned by dma_alloc_attr(). You
     38can treat it as a cookie that must be passed to dma_mmap_attrs() and
     39dma_free_attrs(). Make sure that both of these also get this attribute
     40set on each call.
     41
     42Since it is optional for platforms to implement
     43DMA_ATTR_NO_KERNEL_MAPPING, those that do not will simply ignore the
     44attribute and exhibit default behavior.
     45
     46DMA_ATTR_SKIP_CPU_SYNC
     47----------------------
     48
     49By default dma_map_{single,page,sg} functions family transfer a given
     50buffer from CPU domain to device domain. Some advanced use cases might
     51require sharing a buffer between more than one device. This requires
     52having a mapping created separately for each device and is usually
     53performed by calling dma_map_{single,page,sg} function more than once
     54for the given buffer with device pointer to each device taking part in
     55the buffer sharing. The first call transfers a buffer from 'CPU' domain
     56to 'device' domain, what synchronizes CPU caches for the given region
     57(usually it means that the cache has been flushed or invalidated
     58depending on the dma direction). However, next calls to
     59dma_map_{single,page,sg}() for other devices will perform exactly the
     60same synchronization operation on the CPU cache. CPU cache synchronization
     61might be a time consuming operation, especially if the buffers are
     62large, so it is highly recommended to avoid it if possible.
     63DMA_ATTR_SKIP_CPU_SYNC allows platform code to skip synchronization of
     64the CPU cache for the given buffer assuming that it has been already
     65transferred to 'device' domain. This attribute can be also used for
     66dma_unmap_{single,page,sg} functions family to force buffer to stay in
     67device domain after releasing a mapping for it. Use this attribute with
     68care!
     69
     70DMA_ATTR_FORCE_CONTIGUOUS
     71-------------------------
     72
     73By default DMA-mapping subsystem is allowed to assemble the buffer
     74allocated by dma_alloc_attrs() function from individual pages if it can
     75be mapped as contiguous chunk into device dma address space. By
     76specifying this attribute the allocated buffer is forced to be contiguous
     77also in physical memory.
     78
     79DMA_ATTR_ALLOC_SINGLE_PAGES
     80---------------------------
     81
     82This is a hint to the DMA-mapping subsystem that it's probably not worth
     83the time to try to allocate memory to in a way that gives better TLB
     84efficiency (AKA it's not worth trying to build the mapping out of larger
     85pages).  You might want to specify this if:
     86
     87- You know that the accesses to this memory won't thrash the TLB.
     88  You might know that the accesses are likely to be sequential or
     89  that they aren't sequential but it's unlikely you'll ping-pong
     90  between many addresses that are likely to be in different physical
     91  pages.
     92- You know that the penalty of TLB misses while accessing the
     93  memory will be small enough to be inconsequential.  If you are
     94  doing a heavy operation like decryption or decompression this
     95  might be the case.
     96- You know that the DMA mapping is fairly transitory.  If you expect
     97  the mapping to have a short lifetime then it may be worth it to
     98  optimize allocation (avoid coming up with large pages) instead of
     99  getting the slight performance win of larger pages.
    100
    101Setting this hint doesn't guarantee that you won't get huge pages, but it
    102means that we won't try quite as hard to get them.
    103
    104.. note:: At the moment DMA_ATTR_ALLOC_SINGLE_PAGES is only implemented on ARM,
    105	  though ARM64 patches will likely be posted soon.
    106
    107DMA_ATTR_NO_WARN
    108----------------
    109
    110This tells the DMA-mapping subsystem to suppress allocation failure reports
    111(similarly to __GFP_NOWARN).
    112
    113On some architectures allocation failures are reported with error messages
    114to the system logs.  Although this can help to identify and debug problems,
    115drivers which handle failures (eg, retry later) have no problems with them,
    116and can actually flood the system logs with error messages that aren't any
    117problem at all, depending on the implementation of the retry mechanism.
    118
    119So, this provides a way for drivers to avoid those error messages on calls
    120where allocation failures are not a problem, and shouldn't bother the logs.
    121
    122.. note:: At the moment DMA_ATTR_NO_WARN is only implemented on PowerPC.
    123
    124DMA_ATTR_PRIVILEGED
    125-------------------
    126
    127Some advanced peripherals such as remote processors and GPUs perform
    128accesses to DMA buffers in both privileged "supervisor" and unprivileged
    129"user" modes.  This attribute is used to indicate to the DMA-mapping
    130subsystem that the buffer is fully accessible at the elevated privilege
    131level (and ideally inaccessible or at least read-only at the
    132lesser-privileged levels).