[PATCH v2 2/3] acpi: Add Windows ACPI Emulated Device Table (WAET)

[Liran Alon]

Microsoft introduced this ACPI table to avoid Windows guests performing
various workarounds for device erratas. As the virtual device emulated
by VMM may not have the errata.

Currently, WAET allows hypervisor to inform guest about two
specific behaviors: One for RTC and the other for ACPI PM timer.

Support for WAET have been introduced since Windows Vista. This ACPI
table is also exposed by other common hypervisors by default, including:
VMware, GCP and AWS.

This patch adds WAET ACPI Table to QEMU.

We set "ACPI PM timer good" bit in "Emualted Device Flags" field to
indicate that the ACPI PM timer has been enhanced to not require
multiple reads to obtain a reliable value.
This results in improving the performance of Windows guests that use
ACPI PM timer by avoiding unnecessary VMExits caused by these multiple
reads.

Co-developed-by: Elad Gabay <address@hidden>
Signed-off-by: Liran Alon <address@hidden>
---
 hw/i386/acpi-build.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
index 9c4e46fa7466..1c3a2e8fcb3c 100644
--- a/hw/i386/acpi-build.c
+++ b/hw/i386/acpi-build.c
@@ -2512,6 +2512,34 @@ build_dmar_q35(GArray *table_data, BIOSLinker *linker)
     build_header(linker, table_data, (void *)(table_data->data + dmar_start),
                  "DMAR", table_data->len - dmar_start, 1, NULL, NULL);
 }
+
+/*
+ * Windows ACPI Emulated Devices Table
+ * (Version 1.0 - April 6, 2009)
+ * Spec: 
http://download.microsoft.com/download/7/E/7/7E7662CF-CBEA-470B-A97E-CE7CE0D98DC2/WAET.docx
+ *
+ * Helpful to speedup Windows guests and ignored by others.
+ */
+static void
+build_waet(GArray *table_data, BIOSLinker *linker)
+{
+    int waet_start = table_data->len;
+
+    /* WAET header */
+    acpi_data_push(table_data, sizeof(AcpiTableHeader));
+    /*
+     * Set "ACPI PM timer good" flag.
+     *
+     * Tells Windows guests that our ACPI PM timer is reliable in the
+     * sense that guest can read it only once to obtain a reliable value.
+     * Which avoids costly VMExits caused by guest re-reading it unnecessarily.
+     */
+    build_append_int_noprefix(table_data, 1 << 1 /* ACPI PM timer good */, 4);
+
+    build_header(linker, table_data, (void *)(table_data->data + waet_start),
+                 "WAET", table_data->len - waet_start, 1, NULL, NULL);
+}
+
 /*
  *   IVRS table as specified in AMD IOMMU Specification v2.62, Section 5.2
  *   accessible here http://support.amd.com/TechDocs/48882_IOMMU.pdf
@@ -2859,6 +2887,9 @@ void acpi_build(AcpiBuildTables *tables, MachineState 
*machine)
                           machine->nvdimms_state, machine->ram_slots);
     }
 
+    acpi_add_table(table_offsets, tables_blob);
+    build_waet(tables_blob, tables->linker);
+
     /* Add tables supplied by user (if any) */
     for (u = acpi_table_first(); u; u = acpi_table_next(u)) {
         unsigned len = acpi_table_len(u);
-- 
2.20.1