[Qemu-devel] [PATCH v3 29/40] ivshmem: Simplify how we cope with short reads from server

[Markus Armbruster <armbru@redhat.com>]

Short reads from a UNIX domain sockets are exceedingly unlikely when
the other side always sends eight bytes and we always read eight
bytes.  We cope with them anyway.  However, the code doing that is
rather convoluted.  Dumb it down radically.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 hw/misc/ivshmem.c | 75 ++++++++++++-------------------------------------------
 1 file changed, 16 insertions(+), 59 deletions(-)

diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
index c1a75db..7b9e769 100644
--- a/hw/misc/ivshmem.c
+++ b/hw/misc/ivshmem.c
@@ -26,7 +26,6 @@
 #include "migration/migration.h"
 #include "qemu/error-report.h"
 #include "qemu/event_notifier.h"
-#include "qemu/fifo8.h"
 #include "sysemu/char.h"
 #include "sysemu/hostmem.h"
 #include "qapi/visitor.h"
@@ -80,7 +79,6 @@ typedef struct IVShmemState {
     uint32_t intrstatus;
 
     CharDriverState *server_chr;
-    Fifo8 incoming_fifo;
     MemoryRegion ivshmem_mmio;
 
     /* We might need to register the BAR before we actually have the memory.
@@ -99,6 +97,8 @@ typedef struct IVShmemState {
     uint32_t vectors;
     uint32_t features;
     MSIVector *msi_vectors;
+    uint64_t msg_buf;           /* buffer for receiving server messages */
+    int msg_buffered_bytes;     /* #bytes in @msg_buf */
 
     Error *migration_blocker;
 
@@ -255,11 +255,6 @@ static const MemoryRegionOps ivshmem_mmio_ops = {
     },
 };
 
-static int ivshmem_can_receive(void * opaque)
-{
-    return sizeof(int64_t);
-}
-
 static void ivshmem_vector_notify(void *opaque)
 {
     MSIVector *entry = opaque;
@@ -459,53 +454,6 @@ static void resize_peers(IVShmemState *s, int nb_peers)
     }
 }
 
-static bool fifo_update_and_get(IVShmemState *s, const uint8_t *buf, int size,
-                                void *data, size_t len)
-{
-    const uint8_t *p;
-    uint32_t num;
-
-    assert(len <= sizeof(int64_t)); /* limitation of the fifo */
-    if (fifo8_is_empty(&s->incoming_fifo) && size == len) {
-        memcpy(data, buf, size);
-        return true;
-    }
-
-    IVSHMEM_DPRINTF("short read of %d bytes\n", size);
-
-    num = MIN(size, sizeof(int64_t) - fifo8_num_used(&s->incoming_fifo));
-    fifo8_push_all(&s->incoming_fifo, buf, num);
-
-    if (fifo8_num_used(&s->incoming_fifo) < len) {
-        assert(num == 0);
-        return false;
-    }
-
-    size -= num;
-    buf += num;
-    p = fifo8_pop_buf(&s->incoming_fifo, len, &num);
-    assert(num == len);
-
-    memcpy(data, p, len);
-
-    if (size > 0) {
-        fifo8_push_all(&s->incoming_fifo, buf, size);
-    }
-
-    return true;
-}
-
-static bool fifo_update_and_get_i64(IVShmemState *s,
-                                    const uint8_t *buf, int size, int64_t *i64)
-{
-    if (fifo_update_and_get(s, buf, size, i64, sizeof(*i64))) {
-        *i64 = GINT64_FROM_LE(*i64);
-        return true;
-    }
-
-    return false;
-}
-
 static void ivshmem_add_kvm_msi_virq(IVShmemState *s, int vector,
                                      Error **errp)
 {
@@ -658,6 +606,14 @@ static void process_msg(IVShmemState *s, int64_t msg, int fd, Error **errp)
     }
 }
 
+static int ivshmem_can_receive(void *opaque)
+{
+    IVShmemState *s = opaque;
+
+    assert(s->msg_buffered_bytes < sizeof(s->msg_buf));
+    return sizeof(s->msg_buf) - s->msg_buffered_bytes;
+}
+
 static void ivshmem_read(void *opaque, const uint8_t *buf, int size)
 {
     IVShmemState *s = opaque;
@@ -665,9 +621,14 @@ static void ivshmem_read(void *opaque, const uint8_t *buf, int size)
     int fd;
     int64_t msg;
 
-    if (!fifo_update_and_get_i64(s, buf, size, &msg)) {
+    assert(size >= 0 && s->msg_buffered_bytes + size <= sizeof(s->msg_buf));
+    memcpy((unsigned char *)&s->msg_buf + s->msg_buffered_bytes, buf, size);
+    s->msg_buffered_bytes += size;
+    if (s->msg_buffered_bytes < sizeof(s->msg_buf)) {
         return;
     }
+    msg = le64_to_cpu(s->msg_buf);
+    s->msg_buffered_bytes = 0;
 
     fd = qemu_chr_fe_get_msgfd(s->server_chr);
     IVSHMEM_DPRINTF("posn is %" PRId64 ", fd is %d\n", msg, fd);
@@ -1022,8 +983,6 @@ static void pci_ivshmem_realize(PCIDevice *dev, Error **errp)
         }
     }
 
-    fifo8_create(&s->incoming_fifo, sizeof(int64_t));
-
     if (s->role_val == IVSHMEM_PEER) {
         error_setg(&s->migration_blocker,
                    "Migration is disabled when using feature 'peer mode' in device 'ivshmem'");
@@ -1036,8 +995,6 @@ static void pci_ivshmem_exit(PCIDevice *dev)
     IVShmemState *s = IVSHMEM(dev);
     int i;
 
-    fifo8_destroy(&s->incoming_fifo);
-
     if (s->migration_blocker) {
         migrate_del_blocker(s->migration_blocker);
         error_free(s->migration_blocker);
-- 
2.4.3