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util/iov: add iov_discard_undo()

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The iov_discard_front/back() operations are useful for parsing iovecs
but they modify the array elements. If the original array is needed
after parsing finishes there is currently no way to restore it.

Although g_memdup() can be used before performing destructive
iov_discard_front/back() operations, this is inefficient.

Introduce iov_discard_undo() to restore the array to the state prior to
an iov_discard_front/back() operation.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-Id: <20200917094455.822379-2-stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2020-09-17 10:44:53 +01:00
parent bd0bbb9aba
commit 9dd6f7c28e
3 changed files with 234 additions and 4 deletions
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23
include/qemu/iov.h
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@ -130,6 +130,29 @@ size_t iov_discard_front(struct iovec **iov, unsigned int *iov_cnt,
size_t iov_discard_back(struct iovec *iov, unsigned int *iov_cnt,
size_t bytes);
/* Information needed to undo an iov_discard_*() operation */
typedef struct {
struct iovec *modified_iov;
struct iovec orig;
} IOVDiscardUndo;
/*
* Undo an iov_discard_front_undoable() or iov_discard_back_undoable()
* operation. If multiple operations are made then each one needs a separate
* IOVDiscardUndo and iov_discard_undo() must be called in the reverse order
* that the operations were made.
*/
void iov_discard_undo(IOVDiscardUndo *undo);
/*
* Undoable versions of iov_discard_front() and iov_discard_back(). Use
* iov_discard_undo() to reset to the state before the discard operations.
*/
size_t iov_discard_front_undoable(struct iovec **iov, unsigned int *iov_cnt,
size_t bytes, IOVDiscardUndo *undo);
size_t iov_discard_back_undoable(struct iovec *iov, unsigned int *iov_cnt,
size_t bytes, IOVDiscardUndo *undo);
typedef struct QEMUIOVector {
struct iovec *iov;
int niov;