[PATCH] ioemu: improve DMA transfers

[Samuel Thibault <samuel.thibault@eu.citrix.com>]

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Hello,

I was wondering about the not so good performances of the IDE emulation
even in the case of DMA transfers.  It happens that qemu is actually
limiting the qemu / dom0 kernel reads/writes to 8KB at a time, which is
not very big (the SCSI emulation uses 64KB).  Increasing it provides a
very good improvement, as the attached graph shows.  On my test machine,
55MB/s can be achieved with a buffer of 128KB, to be compared to the
native 65MB/s.  The stubdomain approach is limited to 44KB because of
the limit in the PV blk interface.

Samuel


ioemu: improve DMA transfers
by increasing the size of DMA buffers.

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>

diff -r dc1fe3bd1393 tools/ioemu/hw/ide.c
--- a/tools/ioemu/hw/ide.c	Thu Mar 20 10:53:10 2008 +0000
+++ b/tools/ioemu/hw/ide.c	Thu Mar 20 16:11:03 2008 +0000
@@ -189,6 +189,15 @@
 
 /* set to 1 set disable mult support */
 #define MAX_MULT_SECTORS 16
+#ifdef CONFIG_STUBDOM
+#include <xen/io/blkif.h>
+#define IDE_DMA_BUF_SIZE (BLKIF_MAX_SEGMENTS_PER_REQUEST * TARGET_PAGE_SIZE)
+#else
+#define IDE_DMA_BUF_SIZE 131072
+#endif
+#if (IDE_DMA_BUF_SIZE < MAX_MULT_SECTORS * 512)
+#error "IDE_DMA_BUF_SIZE must be bigger or equal to MAX_MULT_SECTORS * 512"
+#endif
 
 /* ATAPI defines */
 
@@ -932,8 +941,8 @@
 
     /* launch next transfer */
     n = s->nsector;
-    if (n > MAX_MULT_SECTORS)
-        n = MAX_MULT_SECTORS;
+    if (n > IDE_DMA_BUF_SIZE)
+        n = IDE_DMA_BUF_SIZE;
     s->io_buffer_index = 0;
     s->io_buffer_size = n * 512;
 #ifdef DEBUG_AIO
@@ -1041,8 +1050,8 @@
 
     /* launch next transfer */
     n = s->nsector;
-    if (n > MAX_MULT_SECTORS)
-        n = MAX_MULT_SECTORS;
+    if (n > IDE_DMA_BUF_SIZE)
+        n = IDE_DMA_BUF_SIZE;
     s->io_buffer_index = 0;
     s->io_buffer_size = n * 512;
 
@@ -1336,8 +1345,8 @@
         data_offset = 16;
     } else {
         n = s->packet_transfer_size >> 11;
-        if (n > (MAX_MULT_SECTORS / 4))
-            n = (MAX_MULT_SECTORS / 4);
+        if (n > (IDE_DMA_BUF_SIZE / 4))
+            n = (IDE_DMA_BUF_SIZE / 4);
         s->io_buffer_size = n * 2048;
         data_offset = 0;
     }
@@ -2306,7 +2315,7 @@
 
     for(i = 0; i < 2; i++) {
         s = ide_state + i;
-        s->io_buffer = qemu_memalign(getpagesize(), MAX_MULT_SECTORS*512 + 4);
+        s->io_buffer = qemu_memalign(getpagesize(), IDE_DMA_BUF_SIZE*512 + 4);
         if (i == 0)
             s->bs = hd0;
         else
--- a/tools/ioemu/hw/scsi-disk.c	Thu Mar 20 10:53:10 2008 +0000
+++ b/tools/ioemu/hw/scsi-disk.c	Thu Mar 20 16:11:03 2008 +0000
@@ -34,9 +34,10 @@
 #define SENSE_ILLEGAL_REQUEST 5
 
 #ifdef CONFIG_STUBDOM
-#define SCSI_DMA_BUF_SIZE    32768
+#include <xen/io/blkif.h>
+#define SCSI_DMA_BUF_SIZE    (BLKIF_MAX_SEGMENTS_PER_REQUEST * TARGET_PAGE_SIZE)
 #else
-#define SCSI_DMA_BUF_SIZE    65536
+#define SCSI_DMA_BUF_SIZE    131072
 #endif
 
 typedef struct SCSIRequest {

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