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[iobuf] Ensure I/O buffer data sits within unshared cachelines

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On platforms where DMA devices are not in the same coherency domain as
the CPU cache, we must ensure that DMA I/O buffers do not share
cachelines with other data.

Align the start and end of I/O buffers to IOB_ZLEN, which is larger
than any cacheline size we expect to encounter.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2025-07-07 13:21:24 +01:00
parent c21443f0b9
commit 19f1407ad9
3 changed files with 31 additions and 21 deletions
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38
src/core/iobuf.c
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@@ -47,22 +47,26 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
*/
struct io_buffer * alloc_iob_raw ( size_t len, size_t align, size_t offset ) {
struct io_buffer *iobuf;
size_t headroom;
size_t tailroom;
size_t padding;
size_t threshold;
unsigned int align_log2;
void *data;
/* Calculate padding required below alignment boundary to
* ensure that a correctly aligned inline struct io_buffer
* could fit (regardless of the requested offset).
/* Round up requested alignment and calculate initial headroom
* and tailroom to ensure that no cachelines are shared
* between I/O buffer data and other data structures.
*/
padding = ( sizeof ( *iobuf ) + __alignof__ ( *iobuf ) - 1 );
/* Round up requested alignment to at least the size of the
* padding, to simplify subsequent calculations.
*/
if ( align < padding )
align = padding;
if ( align < IOB_ZLEN )
align = IOB_ZLEN;
headroom = ( offset & ( IOB_ZLEN - 1 ) );
tailroom = ( ( - len - offset ) & ( IOB_ZLEN - 1 ) );
padding = ( headroom + tailroom );
offset -= headroom;
len += padding;
if ( len < padding )
return NULL;
/* Round up alignment to the nearest power of two, avoiding
* a potentially undefined shift operation.
@@ -73,8 +77,8 @@ struct io_buffer * alloc_iob_raw ( size_t len, size_t align, size_t offset ) {
align = ( 1UL << align_log2 );
/* Calculate length threshold */
assert ( align >= padding );
threshold = ( align - padding );
assert ( align >= sizeof ( *iobuf ) );
threshold = ( align - sizeof ( *iobuf ) );
/* Allocate buffer plus an inline descriptor as a single unit,
* unless doing so would push the total size over the
@@ -82,11 +86,6 @@ struct io_buffer * alloc_iob_raw ( size_t len, size_t align, size_t offset ) {
*/
if ( len <= threshold ) {
/* Round up buffer length to ensure that struct
* io_buffer is aligned.
*/
len += ( ( - len - offset ) & ( __alignof__ ( *iobuf ) - 1 ) );
/* Allocate memory for buffer plus descriptor */
data = malloc_phys_offset ( len + sizeof ( *iobuf ), align,
offset );
@@ -111,7 +110,8 @@ struct io_buffer * alloc_iob_raw ( size_t len, size_t align, size_t offset ) {
/* Populate descriptor */
memset ( &iobuf->map, 0, sizeof ( iobuf->map ) );
iobuf->head = iobuf->data = iobuf->tail = data;
iobuf->head = data;
iobuf->data = iobuf->tail = ( data + headroom );
iobuf->end = ( data + len );
return iobuf;
@@ -266,7 +266,7 @@ struct io_buffer * iob_concatenate ( struct list_head *list ) {
len += iob_len ( iobuf );
/* Allocate new I/O buffer */
concatenated = alloc_iob_raw ( len, __alignof__ ( *iobuf ), 0 );
concatenated = alloc_iob_raw ( len, IOB_ZLEN, 0 );
if ( ! concatenated )
return NULL;