Re: [Qemu-devel] [PATCH v2 4/6] ide: Update ide_drive_get to be HBA agnostic

[John Snow]

On 09/30/2014 03:38 AM, Markus Armbruster wrote:
John Snow <js...@redhat.com> writes:

Instead of duplicating the logic for the if_ide
(bus,unit) mappings, rely on the blockdev layer
for managing those mappings for us, and use the
drive_get_by_index call instead.

This allows ide_drive_get to work for AHCI HBAs
as well, and can be used in the Q35 initialization.

Lastly, change the nature of the argument to
ide_drive_get so that represents the number of
total drives we can support, and not the total
number of buses. This will prevent array overflows
if the units-per-default-bus property ever needs
to be adjusted for compatibility reasons.

Signed-off-by: John Snow <js...@redhat.com>
---
  blockdev.c                |  9 +++++++++
  hw/alpha/dp264.c          |  2 +-
  hw/i386/pc_piix.c         |  2 +-
  hw/ide/core.c             | 21 +++++++++++++++------
  hw/mips/mips_fulong2e.c   |  2 +-
  hw/mips/mips_malta.c      |  2 +-
  hw/mips/mips_r4k.c        |  2 +-
  hw/ppc/mac_newworld.c     |  2 +-
  hw/ppc/mac_oldworld.c     |  2 +-
  hw/ppc/prep.c             |  2 +-
  hw/sparc64/sun4u.c        |  2 +-
  include/sysemu/blockdev.h |  1 +
  12 files changed, 34 insertions(+), 15 deletions(-)

diff --git a/blockdev.c b/blockdev.c
index 9b05f1b..ffaad39 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -135,6 +135,15 @@ void blockdev_auto_del(BlockDriverState *bs)
      }
  }

+int drive_get_max_devs(BlockInterfaceType type)
+{
+    if (type >= IF_IDE && type < IF_COUNT) {
+        return if_max_devs[type];
+    }
+
+    return 0;
+}
+

drive_get_max_bus() returns -1 for a type without drives.  Includes
invalid types.  When it returns a non-negative number, a drive on that
bus exists.

Your drive_get_max_devs() has a similar name, but different semantics:
it returns a positive number when the implied HBA supports multiple
buses, else zero.  The "else" includes invalid types.  When it returns a
positive number, then the HBA can take that many units per bus.

No big deal, but functions comments would be nice.

Should invalid type be treated as a programming error instead?

  static int drive_index_to_bus_id(BlockInterfaceType type, int index)
  {
      int max_devs = if_max_devs[type];
diff --git a/hw/alpha/dp264.c b/hw/alpha/dp264.c
index b178a03..ab61bb6 100644
--- a/hw/alpha/dp264.c
+++ b/hw/alpha/dp264.c
@@ -97,7 +97,7 @@ static void clipper_init(MachineState *machine)
      /* IDE disk setup.  */
      {
          DriveInfo *hd[MAX_IDE_BUS * MAX_IDE_DEVS];
-        ide_drive_get(hd, MAX_IDE_BUS);
+        ide_drive_get(hd, MAX_IDE_BUS * MAX_IDE_DEVS);

More obviously correct would be

         ide_drive_get(hd, ARRAY_SIZE(hd));

Less so here, because the declaration is right next to the use, more so
elsewhere, where it isn't.


          pci_cmd646_ide_init(pci_bus, hd, 0);
      }
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 103d756..2760c81 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -239,7 +239,7 @@ static void pc_init1(MachineState *machine,

      pc_nic_init(isa_bus, pci_bus);

-    ide_drive_get(hd, MAX_IDE_BUS);
+    ide_drive_get(hd, MAX_IDE_BUS * MAX_IDE_DEVS);
      if (pci_enabled) {
          PCIDevice *dev;
          if (xen_enabled()) {
diff --git a/hw/ide/core.c b/hw/ide/core.c
index 190700a..e7c1050 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -2558,16 +2558,25 @@ const VMStateDescription vmstate_ide_bus = {
      }
  };

-void ide_drive_get(DriveInfo **hd, int max_bus)
+void ide_drive_get(DriveInfo **hd, int n)
  {
      int i;
+    int highest_bus = drive_get_max_bus(IF_IDE) + 1;

Actually, this is the "highest bus" + 1 :)

+    int n_buses = n / drive_get_max_devs(IF_IDE);

What if drive_get_max_devs(IF_IDE) returns 0?

You could side-step the question by using drive_index_to_bus_id(n).


-    if (drive_get_max_bus(IF_IDE) >= max_bus) {
-        fprintf(stderr, "qemu: too many IDE bus: %d\n", max_bus);
-        exit(1);

Before: fatal.

+    /* Note: The number of actual buses available is not known.
+     * We compute this based on the size of the DriveInfo* array, n.
+     * If it is less than (drive_get_max_devs(IF_IDE) * num_real_buses),
+     * We will stop looking for drives prematurely instead of overfilling
+     * the array. */
+

You might want to consider winged comments:

     /*
      * Note: The number of actual buses available is not known.
      * We compute this based on the size of the DriveInfo* array, n.
      * If it is less than (drive_get_max_devs(IF_IDE) * num_real_buses),
      * We will stop looking for drives prematurely instead of overfilling
      * the array.
      */

+    if (highest_bus > n_buses) {
+        error_report("Warning: Too many IDE buses defined (%d > %d)",
+                     highest_bus, n_buses);

After: warning.

Why?

I'll fix the divide by zero and address the other comments. I can adjust 
the semantics to match the other function -- sometimes I don't realize 
I've accidentally created a function that is similar to, but behaves 
differently from, another.

The reasoning behind a warning instead of a hard error was that if we 
provide less slots in the HD table than is necessary for covering the 
full number of buses/units in the board, we're going to stop early. It's 
not necessarily a fatal error, so I replaced the hard exit() with a warning.

If we do want a hard exit, I should probably start baking in the error 
pathways down this far to do it appropriately instead of terminating 
execution.

      }

-    for(i = 0; i < max_bus * MAX_IDE_DEVS; i++) {
-        hd[i] = drive_get(IF_IDE, i / MAX_IDE_DEVS, i % MAX_IDE_DEVS);
+    for (i = 0; i < n; i++) {
+        hd[i] = drive_get_by_index(IF_IDE, i);
      }
+

Stray blank line.

  }

Function is much nicer now, thanks!

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