From a1ca3ed5ece383de86994b04e2fef01dad7e7e95 Mon Sep 17 00:00:00 2001 From: Vladimir Sementsov-Ogievskiy Date: Mon, 18 Feb 2019 17:09:10 +0300 Subject: [PATCH] block: enhance QEMUIOVector structure Add a possibility of embedded iovec, for cases when we need only one local iov. Signed-off-by: Vladimir Sementsov-Ogievskiy Reviewed-by: Eric Blake Message-id: 20190218140926.333779-2-vsementsov@virtuozzo.com Message-Id: <20190218140926.333779-2-vsementsov@virtuozzo.com> Signed-off-by: Stefan Hajnoczi --- include/qemu/iov.h | 64 ++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 62 insertions(+), 2 deletions(-) diff --git a/include/qemu/iov.h b/include/qemu/iov.h index 5f433c7768..48b45987b7 100644 --- a/include/qemu/iov.h +++ b/include/qemu/iov.h @@ -133,10 +133,70 @@ size_t iov_discard_back(struct iovec *iov, unsigned int *iov_cnt, typedef struct QEMUIOVector { struct iovec *iov; int niov; - int nalloc; - size_t size; + + /* + * For external @iov (qemu_iovec_init_external()) or allocated @iov + * (qemu_iovec_init()), @size is the cumulative size of iovecs and + * @local_iov is invalid and unused. + * + * For embedded @iov (QEMU_IOVEC_INIT_BUF() or qemu_iovec_init_buf()), + * @iov is equal to &@local_iov, and @size is valid, as it has same + * offset and type as @local_iov.iov_len, which is guaranteed by + * static assertion below. + * + * @nalloc is always valid and is -1 both for embedded and external + * cases. It is included in the union only to ensure the padding prior + * to the @size field will not result in a 0-length array. + */ + union { + struct { + int nalloc; + struct iovec local_iov; + }; + struct { + char __pad[sizeof(int) + offsetof(struct iovec, iov_len)]; + size_t size; + }; + }; } QEMUIOVector; +QEMU_BUILD_BUG_ON(offsetof(QEMUIOVector, size) != + offsetof(QEMUIOVector, local_iov.iov_len)); + +#define QEMU_IOVEC_INIT_BUF(self, buf, len) \ +{ \ + .iov = &(self).local_iov, \ + .niov = 1, \ + .nalloc = -1, \ + .local_iov = { \ + .iov_base = (void *)(buf), /* cast away const */ \ + .iov_len = (len), \ + }, \ +} + +/* + * qemu_iovec_init_buf + * + * Initialize embedded QEMUIOVector. + * + * Note: "const" is used over @buf pointer to make it simple to pass + * const pointers, appearing in read functions. Then this "const" is + * cast away by QEMU_IOVEC_INIT_BUF(). + */ +static inline void qemu_iovec_init_buf(QEMUIOVector *qiov, + const void *buf, size_t len) +{ + *qiov = (QEMUIOVector) QEMU_IOVEC_INIT_BUF(*qiov, buf, len); +} + +static inline void *qemu_iovec_buf(QEMUIOVector *qiov) +{ + /* Only supports embedded iov */ + assert(qiov->nalloc == -1 && qiov->iov == &qiov->local_iov); + + return qiov->local_iov.iov_base; +} + void qemu_iovec_init(QEMUIOVector *qiov, int alloc_hint); void qemu_iovec_init_external(QEMUIOVector *qiov, struct iovec *iov, int niov); void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len);