[Qemu-devel] [PATCH v2 1/4] ahci: Do not ignore memory access read size

[John Snow <jsnow@redhat.com>]

The only guidance the AHCI specification gives on memory access is:
"Register accesses shall have a maximum size of 64-bits; 64-bit access
must not cross an 8-byte alignment boundary."

I interpret this to mean that aligned or unaligned 1, 2 and 4 byte
accesses should work, as well as aligned 8 byte accesses.

In practice, a real Q35/ICH9 responds to 1, 2, 4 and 8 byte reads
regardless of alignment. Windows 7 can be observed making 1 byte
reads to the middle of 32 bit registers to fetch error codes.

Introduce a wrapper to support unaligned accesses to AHCI.
This wrapper will support aligned 8 byte reads, but will make
no effort to support unaligned 8 byte reads, which although they
will work on real hardware, are not guaranteed to work and do
not appear to be used by either Windows or Linux.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 hw/ide/ahci.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/hw/ide/ahci.c b/hw/ide/ahci.c
index 9e5d862..26df2ca 100644
--- a/hw/ide/ahci.c
+++ b/hw/ide/ahci.c
@@ -331,8 +331,7 @@ static void  ahci_port_write(AHCIState *s, int port, int offset, uint32_t val)
     }
 }
 
-static uint64_t ahci_mem_read(void *opaque, hwaddr addr,
-                              unsigned size)
+static uint64_t ahci_mem_read_32(void *opaque, hwaddr addr)
 {
     AHCIState *s = opaque;
     uint32_t val = 0;
@@ -368,6 +367,30 @@ static uint64_t ahci_mem_read(void *opaque, hwaddr addr,
 }
 
 
+/**
+ * AHCI 1.3 section 3 ("HBA Memory Registers")
+ * Support unaligned 8/16/32 bit reads, and 64 bit aligned reads.
+ * Caller is responsible for masking unwanted higher order bytes.
+ */
+static uint64_t ahci_mem_read(void *opaque, hwaddr addr, unsigned size)
+{
+    hwaddr aligned = addr & ~0x3;
+    int ofst = addr - aligned;
+    uint64_t lo = ahci_mem_read_32(opaque, aligned);
+    uint64_t hi;
+
+    /* if < 8 byte read does not cross 4 byte boundary */
+    if (ofst + size <= 4) {
+        return lo >> (ofst * 8);
+    }
+    g_assert_cmpint(size, >, 1);
+
+    /* If the 64bit read is unaligned, we will produce undefined
+     * results. AHCI does not support unaligned 64bit reads. */
+    hi = ahci_mem_read_32(opaque, aligned + 4);
+    return (hi << 32 | lo) >> (ofst * 8);
+}
+
 
 static void ahci_mem_write(void *opaque, hwaddr addr,
                            uint64_t val, unsigned size)
-- 
2.1.0