From 3dbda6dc52d909d78961533530fbc2e4fcb802e5 Mon Sep 17 00:00:00 2001
From: James Gowans <jgowans@amazon.com>
Date: Mon, 3 May 2021 15:52:43 +0100
Subject: [PATCH] pci: Touch each page when mapped for IO

This functionality is required for memory oversubscription: the instance
needs to make DMA-able pages resident before initiating a DMA
transaction. Once the kernel is booted, the ppIOMMU driver takes care of
this. However, before the kernel has booted, EDK2 will be doing DMA, and
hence it needs to ensure that the target pages are DMA-able.

Much like how the ppIOMMU hooks into dma_map_ops, this patch adds some
instructions to the function which is invoked when UEFI makes memory
region DMA-able to touch all pages in the region.

This patch doesn't attempt to differentiate overcommit vs
non-overcommit: the functionality of touches pages before DMA will be
done for all EC2 instances. The reason just to keep stuff simple:
although non-overcommit instances have no use for this functionality the
overhead is low enough that it's okay to do it always.

Signed-off-by: James Gowans <jgowans@amazon.com>
Reviewed-by: Alan Powell <alpowell@amazon.com>
Reviewed-by: Roland Paterson-Jones <rolandp@amazon.com>

diff --git a/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c b/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c
index 157a0ada80..51aa6986dc 100644
--- a/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c
+++ b/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c
@@ -7,6 +7,7 @@ SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
 
+#include <Library/IoLib.h>
 #include "PciHostBridge.h"
 #include "PciRootBridge.h"
 #include "PciHostResource.h"
@@ -1331,6 +1332,7 @@ RootBridgeIoMap (
   PCI_ROOT_BRIDGE_INSTANCE  *RootBridge;
   EFI_PHYSICAL_ADDRESS      PhysicalAddress;
   MAP_INFO                  *MapInfo;
+  UINTN                     PageTouchOffset;
 
   if ((HostAddress == NULL) || (NumberOfBytes == NULL) || (DeviceAddress == NULL) ||
       (Mapping == NULL))
@@ -1347,6 +1349,9 @@ RootBridgeIoMap (
 
   RootBridge = ROOT_BRIDGE_FROM_THIS (This);
 
+  for (PageTouchOffset = 0; PageTouchOffset < *NumberOfBytes; PageTouchOffset += EFI_PAGE_SIZE)
+    MmioRead8((UINTN) HostAddress + PageTouchOffset);
+
   if (mIoMmu != NULL) {
     if (!RootBridge->DmaAbove4G) {
       //