Re: [Qemu-devel] [PATCH block v1 2/2] block: m25p80: Support read only bdrvs.

[Paolo Bonzini <pbonzini@redhat.com>]

> static void bdrv_sync_complete(void *opaque, int ret)
> {
>     /* do nothing. Masters do not directly interact with the backing store,
>      * only the working copy so no mutexing required.
>      */
> }
>
> static void flash_sync_page(Flash *s, int page)
> {
>     if (s->bdrv) {
>         int bdrv_sector, nb_sectors;
>         QEMUIOVector iov;
>
>         bdrv_sector = (page * s->pi->page_size) / BDRV_SECTOR_SIZE;
>         nb_sectors = DIV_ROUND_UP(s->pi->page_size, BDRV_SECTOR_SIZE);
>         qemu_iovec_init(&iov, 1);
>         qemu_iovec_add(&iov, s->storage + bdrv_sector * BDRV_SECTOR_SIZE,
>                                                 nb_sectors * BDRV_SECTOR_SIZE);
>         bdrv_aio_writev(s->bdrv, bdrv_sector, &iov, nb_sectors,
>                                                 bdrv_sync_complete, NULL);
>     }
> }

Using AIO is a good idea, but you could have overlapping writes here if 
you get close calls to flash_sync_page.  It can be bad.

Serializing can be done in fancy manners, but it can be as easy as 
adding bdrv_drain(s->bdrv) before the bdrv_aio_writev.

Paolo