On Thu, Jul 20, 2023 at 8:14 PM Stefan Hajnoczi <stefa...@redhat.com> wrote:
>
> On Tue, Jul 04, 2023 at 01:06:26AM -0700, Mattias Nissler wrote:
> > According to old commit messages, this was introduced to retry a DMA
> > operation at a later point in case the single bounce buffer is found to
> > be busy. This was never used widely - only the dma-helpers code made use
> > of it, but there are other device models that use multiple DMA mappings
> > (concurrently) and just failed.
> >
> > After the improvement to support multiple concurrent bounce buffers,
> > the condition the notification callback allowed to work around no
> > longer exists, so we can just remove the logic and simplify the code.
> >
> > Signed-off-by: Mattias Nissler <mniss...@rivosinc.com>
> > ---
> > softmmu/dma-helpers.c | 28 -----------------
> > softmmu/physmem.c | 71 -------------------------------------------
> > 2 files changed, 99 deletions(-)
>
> I'm not sure if it will be possible to remove this once a limit is
> placed bounce buffer space.
I investigated this in detail and concluded that you're right
unfortunately. In particular, after I found that Linux likes to use
megabyte-sided DMA buffers with xhci-attached USB storage, I don't
think it's realistic to set a reasonable fixed limit that accommodates
most workloads in practice.
>
> >
> > diff --git a/softmmu/dma-helpers.c b/softmmu/dma-helpers.c
> > index 2463964805..d05d226f11 100644
> > --- a/softmmu/dma-helpers.c
> > +++ b/softmmu/dma-helpers.c
> > @@ -68,23 +68,10 @@ typedef struct {
> > int sg_cur_index;
> > dma_addr_t sg_cur_byte;
> > QEMUIOVector iov;
> > - QEMUBH *bh;
> > DMAIOFunc *io_func;
> > void *io_func_opaque;
> > } DMAAIOCB;
> >
> > -static void dma_blk_cb(void *opaque, int ret);
> > -
> > -static void reschedule_dma(void *opaque)
> > -{
> > - DMAAIOCB *dbs = (DMAAIOCB *)opaque;
> > -
> > - assert(!dbs->acb && dbs->bh);
> > - qemu_bh_delete(dbs->bh);
> > - dbs->bh = NULL;
> > - dma_blk_cb(dbs, 0);
> > -}
> > -
> > static void dma_blk_unmap(DMAAIOCB *dbs)
> > {
> > int i;
> > @@ -101,7 +88,6 @@ static void dma_complete(DMAAIOCB *dbs, int ret)
> > {
> > trace_dma_complete(dbs, ret, dbs->common.cb);
> >
> > - assert(!dbs->acb && !dbs->bh);
> > dma_blk_unmap(dbs);
> > if (dbs->common.cb) {
> > dbs->common.cb(dbs->common.opaque, ret);
> > @@ -164,13 +150,6 @@ static void dma_blk_cb(void *opaque, int ret)
> > }
> > }
> >
> > - if (dbs->iov.size == 0) {
> > - trace_dma_map_wait(dbs);
> > - dbs->bh = aio_bh_new(ctx, reschedule_dma, dbs);
> > - cpu_register_map_client(dbs->bh);
> > - goto out;
> > - }
> > -
> > if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) {
> > qemu_iovec_discard_back(&dbs->iov,
> > QEMU_ALIGN_DOWN(dbs->iov.size,
> > dbs->align));
> > @@ -189,18 +168,12 @@ static void dma_aio_cancel(BlockAIOCB *acb)
> >
> > trace_dma_aio_cancel(dbs);
> >
> > - assert(!(dbs->acb && dbs->bh));
> > if (dbs->acb) {
> > /* This will invoke dma_blk_cb. */
> > blk_aio_cancel_async(dbs->acb);
> > return;
> > }
> >
> > - if (dbs->bh) {
> > - cpu_unregister_map_client(dbs->bh);
> > - qemu_bh_delete(dbs->bh);
> > - dbs->bh = NULL;
> > - }
> > if (dbs->common.cb) {
> > dbs->common.cb(dbs->common.opaque, -ECANCELED);
> > }
> > @@ -239,7 +212,6 @@ BlockAIOCB *dma_blk_io(AioContext *ctx,
> > dbs->dir = dir;
> > dbs->io_func = io_func;
> > dbs->io_func_opaque = io_func_opaque;
> > - dbs->bh = NULL;
> > qemu_iovec_init(&dbs->iov, sg->nsg);
> > dma_blk_cb(dbs, 0);
> > return &dbs->common;
> > diff --git a/softmmu/physmem.c b/softmmu/physmem.c
> > index 56130b5a1d..2b4123c127 100644
> > --- a/softmmu/physmem.c
> > +++ b/softmmu/physmem.c
> > @@ -2908,49 +2908,6 @@ typedef struct {
> > uint8_t buffer[];
> > } BounceBuffer;
> >
> > -static size_t bounce_buffers_in_use;
> > -
> > -typedef struct MapClient {
> > - QEMUBH *bh;
> > - QLIST_ENTRY(MapClient) link;
> > -} MapClient;
> > -
> > -QemuMutex map_client_list_lock;
> > -static QLIST_HEAD(, MapClient) map_client_list
> > - = QLIST_HEAD_INITIALIZER(map_client_list);
> > -
> > -static void cpu_unregister_map_client_do(MapClient *client)
> > -{
> > - QLIST_REMOVE(client, link);
> > - g_free(client);
> > -}
> > -
> > -static void cpu_notify_map_clients_locked(void)
> > -{
> > - MapClient *client;
> > -
> > - while (!QLIST_EMPTY(&map_client_list)) {
> > - client = QLIST_FIRST(&map_client_list);
> > - qemu_bh_schedule(client->bh);
> > - cpu_unregister_map_client_do(client);
> > - }
> > -}
> > -
> > -void cpu_register_map_client(QEMUBH *bh)
> > -{
> > - MapClient *client = g_malloc(sizeof(*client));
> > -
> > - qemu_mutex_lock(&map_client_list_lock);
> > - client->bh = bh;
> > - QLIST_INSERT_HEAD(&map_client_list, client, link);
> > - /* Write map_client_list before reading in_use. */
> > - smp_mb();
> > - if (qatomic_read(&bounce_buffers_in_use)) {
> > - cpu_notify_map_clients_locked();
> > - }
> > - qemu_mutex_unlock(&map_client_list_lock);
> > -}
> > -
> > void cpu_exec_init_all(void)
> > {
> > qemu_mutex_init(&ram_list.mutex);
> > @@ -2964,28 +2921,6 @@ void cpu_exec_init_all(void)
> > finalize_target_page_bits();
> > io_mem_init();
> > memory_map_init();
> > - qemu_mutex_init(&map_client_list_lock);
> > -}
> > -
> > -void cpu_unregister_map_client(QEMUBH *bh)
> > -{
> > - MapClient *client;
> > -
> > - qemu_mutex_lock(&map_client_list_lock);
> > - QLIST_FOREACH(client, &map_client_list, link) {
> > - if (client->bh == bh) {
> > - cpu_unregister_map_client_do(client);
> > - break;
> > - }
> > - }
> > - qemu_mutex_unlock(&map_client_list_lock);
> > -}
> > -
> > -static void cpu_notify_map_clients(void)
> > -{
> > - qemu_mutex_lock(&map_client_list_lock);
> > - cpu_notify_map_clients_locked();
> > - qemu_mutex_unlock(&map_client_list_lock);
> > }
> >
> > static bool flatview_access_valid(FlatView *fv, hwaddr addr, hwaddr len,
> > @@ -3077,8 +3012,6 @@ void *address_space_map(AddressSpace *as,
> > memory_region_ref(mr);
> >
> > if (!memory_access_is_direct(mr, is_write)) {
> > - qatomic_inc_fetch(&bounce_buffers_in_use);
> > -
> > BounceBuffer *bounce = g_malloc(l + sizeof(BounceBuffer));
> > bounce->addr = addr;
> > bounce->mr = mr;
> > @@ -3122,10 +3055,6 @@ void address_space_unmap(AddressSpace *as, void
> > *buffer, hwaddr len,
> > }
> > memory_region_unref(bounce->mr);
> > g_free(bounce);
> > -
> > - if (qatomic_dec_fetch(&bounce_buffers_in_use) == 1) {
> > - cpu_notify_map_clients();
> > - }
> > return;
> > }
> >
> > --
> > 2.34.1
> >
