fw_cfg: amend callback behavior spec to once per select

Currently, the fw_cfg internal API specifies that if an item was set up
with a read callback, the callback must be run each time a byte is read
from the item. This behavior is both wasteful (most items do not have a
read callback set), and impractical for bulk transfers (e.g., DMA read).

At the time of this writing, the only items configured with a callback
are "/etc/table-loader", "/etc/acpi/tables", and "/etc/acpi/rsdp". They
all share the same callback functions: virt_acpi_build_update() on ARM
(in hw/arm/virt-acpi-build.c), and acpi_build_update() on i386 (in
hw/i386/acpi.c). Both of these callbacks are one-shot (i.e. they return
without doing anything at all after the first time they are invoked with
a given build_state; since build_state is also shared across all three
items mentioned above, the callback only ever runs *once*, the first
time either of the listed items is read).

This patch amends the specification for fw_cfg_add_file_callback() to
state that any available read callback will only be invoked once each
time the item is selected. This change has no practical effect on the
current behavior of QEMU, and it enables us to significantly optimize
the behavior of fw_cfg reads during guest firmware setup, eliminating
a large amount of redundant callback checks and invocations.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc Marí <markmb@redhat.com>
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1446733972-1602-3-git-send-email-somlo@cmu.edu
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

1 files changed, 10 insertions, 9 deletions

diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
index 73b0a81..6e6414b 100644
--- a/hw/nvram/fw_cfg.c
+++ b/hw/nvram/fw_cfg.c
@@ -252,7 +252,8 @@ static void fw_cfg_write(FWCfgState *s, uint8_t value)
 
 static int fw_cfg_select(FWCfgState *s, uint16_t key)
 {
-    int ret;
+    int arch, ret;
+    FWCfgEntry *e;
 
     s->cur_offset = 0;
     if ((key & FW_CFG_ENTRY_MASK) >= FW_CFG_MAX_ENTRY) {
@@ -261,6 +262,12 @@ static int fw_cfg_select(FWCfgState *s, uint16_t key)
     } else {
         s->cur_entry = key;
         ret = 1;
+        /* entry successfully selected, now run callback if present */
+        arch = !!(key & FW_CFG_ARCH_LOCAL);
+        e = &s->entries[arch][key & FW_CFG_ENTRY_MASK];
+        if (e->read_callback) {
+            e->read_callback(e->callback_opaque, s->cur_offset);
+        }
     }
 
     trace_fw_cfg_select(s, key, ret);
@@ -276,9 +283,6 @@ static uint8_t fw_cfg_read(FWCfgState *s)
     if (s->cur_entry == FW_CFG_INVALID || !e->data || s->cur_offset >= e->len)
         ret = 0;
     else {
-        if (e->read_callback) {
-            e->read_callback(e->callback_opaque, s->cur_offset);
-        }
         ret = e->data[s->cur_offset++];
     }
 
@@ -371,10 +375,6 @@ static void fw_cfg_dma_transfer(FWCfgState *s)
                 len = (e->len - s->cur_offset);
             }
 
-            if (e->read_callback) {
-                e->read_callback(e->callback_opaque, s->cur_offset);
-            }
-
             /* If the access is not a read access, it will be a skip access,
              * tested before.
              */
@@ -513,7 +513,8 @@ static void fw_cfg_reset(DeviceState *d)
 {
     FWCfgState *s = FW_CFG(d);
 
-    fw_cfg_select(s, 0);
+    /* we never register a read callback for FW_CFG_SIGNATURE */
+    fw_cfg_select(s, FW_CFG_SIGNATURE);
 }
 
 /* Save restore 32 bit int as uint16_t