OvmfPkg/QemuFwCfgLib: support QEMU's DMA-like fw_cfg access method

The benefits of the DMA-like access method are (a) speed, (b) write
support in QEMU 2.9+.

(IOPort-based write support was discontinued in QEMU 2.4, and the
DMA-based one is being added to QEMU 2.9. Write support needs no separate
feature detection because writeability is governed on the level of
individual fw_cfg files -- if a file meant to be written by the firmware
exists in the directory, then it is writeable with the DMA method.)

We don't enable this feature for the SEC library instance, because:
- the SEC instance remains without clients (I've checked that it builds
  though),
- in SEC, any possible fw_cfg use is expected to be small and read-only.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgLib.c

@@ -98,6 +98,70 @@ QemuFwCfgSelectItem (
 }
 
 
+/**
+  Transfer an array of bytes using the DMA interface.
+
+  @param[in]     Size    Size in bytes to transfer.
+  @param[in,out] Buffer  Buffer to read data into or write data from. May be
+                         NULL if Size is zero.
+  @param[in]     Write   TRUE if writing to fw_cfg from Buffer, FALSE if
+                         reading from fw_cfg into Buffer.
+**/
+VOID
+InternalQemuFwCfgDmaBytes (
+  IN     UINT32   Size,
+  IN OUT VOID     *Buffer OPTIONAL,
+  IN     BOOLEAN  Write
+  )
+{
+  volatile FW_CFG_DMA_ACCESS Access;
+  UINT32                     AccessHigh, AccessLow;
+  UINT32                     Status;
+
+  if (Size == 0) {
+    return;
+  }
+
+  Access.Control = SwapBytes32 (
+                    Write ? FW_CFG_DMA_CTL_WRITE : FW_CFG_DMA_CTL_READ
+                    );
+  Access.Length  = SwapBytes32 (Size);
+  Access.Address = SwapBytes64 ((UINTN)Buffer);
+
+  //
+  // Delimit the transfer from (a) modifications to Access, (b) in case of a
+  // write, from writes to Buffer by the caller.
+  //
+  MemoryFence ();
+
+  //
+  // Start the transfer.
+  //
+  AccessHigh = (UINT32)RShiftU64 ((UINTN)&Access, 32);
+  AccessLow  = (UINT32)(UINTN)&Access;
+  IoWrite32 (0x514, SwapBytes32 (AccessHigh));
+  IoWrite32 (0x518, SwapBytes32 (AccessLow));
+
+  //
+  // Don't look at Access.Control before starting the transfer.
+  //
+  MemoryFence ();
+
+  //
+  // Wait for the transfer to complete.
+  //
+  do {
+    Status = SwapBytes32 (Access.Control);
+    ASSERT ((Status & FW_CFG_DMA_CTL_ERROR) == 0);
+  } while (Status != 0);
+
+  //
+  // After a read, the caller will want to use Buffer.
+  //
+  MemoryFence ();
+}
+
+
 /**
   Reads firmware configuration bytes into a buffer
 
@@ -112,6 +176,10 @@ InternalQemuFwCfgReadBytes (
   IN VOID                   *Buffer  OPTIONAL
   )
 {
+  if (InternalQemuFwCfgDmaIsAvailable () && Size <= MAX_UINT32) {
+    InternalQemuFwCfgDmaBytes ((UINT32)Size, Buffer, FALSE);
+    return;
+  }
   IoReadFifo8 (0x511, Size, Buffer);
 }
 
@@ -160,6 +228,10 @@ QemuFwCfgWriteBytes (
   )
 {
   if (InternalQemuFwCfgIsAvailable ()) {
+    if (InternalQemuFwCfgDmaIsAvailable () && Size <= MAX_UINT32) {
+      InternalQemuFwCfgDmaBytes ((UINT32)Size, Buffer, TRUE);
+      return;
+    }
     IoWriteFifo8 (0x511, Size, Buffer);
   }
 }