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qemu-file: Add fast ftell code path

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For ftell we flush the output buffer to ensure that we don't have anything
lingering in our internal buffers. This is a very safe thing to do.

However, with the dynamic size measurement that the dynamic vmstate
description will bring this would turn out quite slow.

Instead, we can fast path this specific measurement and just take the
internal buffers into account when telling the kernel our position.

I'm sure I overlooked some corner cases where this doesn't work, so
instead of tuning the safe, existing version, this patch adds a fast
variant of ftell that gets used by the dynamic vmstate description code
which isn't critical when it fails.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
This commit is contained in:
Alexander Graf 2015-01-22 15:01:38 +01:00 committed by Juan Quintela
parent 190c882ce2
commit 9722140011
2 changed files with 17 additions and 0 deletions
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1
include/migration/qemu-file.h
View file

@ -121,6 +121,7 @@ QEMUFile *qemu_bufopen(const char *mode, QEMUSizedBuffer *input);
int qemu_get_fd(QEMUFile *f);
int qemu_fclose(QEMUFile *f);
int64_t qemu_ftell(QEMUFile *f);
int64_t qemu_ftell_fast(QEMUFile *f);
void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, int size);
void qemu_put_byte(QEMUFile *f, int v);
/*

16
migration/qemu-file.c
View file

@ -452,6 +452,22 @@ int qemu_get_byte(QEMUFile *f)
return result;
}
int64_t qemu_ftell_fast(QEMUFile *f)
{
int64_t ret = f->pos;
int i;
if (f->ops->writev_buffer) {
for (i = 0; i < f->iovcnt; i++) {
ret += f->iov[i].iov_len;
}
} else {
ret += f->buf_index;
}
return ret;
}
int64_t qemu_ftell(QEMUFile *f)
{
qemu_fflush(f);