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Re: [PATCH] acpi: Add Windows ACPI Emulated Device Table (WAET)

From: Liran Alon
Subject: Re: [PATCH] acpi: Add Windows ACPI Emulated Device Table (WAET)
Date: Thu, 12 Mar 2020 01:20:02 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 11/03/2020 22:36, Michael S. Tsirkin wrote:
Thanks for the patch! Some questions/comments:

On Wed, Mar 11, 2020 at 07:08:26PM +0200, Liran Alon wrote:
From: Elad Gabay <address@hidden>

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 hypervisors, such as VMware, by default.

This patch adds WAET ACPI Table to QEMU.
Could you add a bit more info? Why is this so useful we are adding this
by default? How does it change windows behaviour when present?
It changes behavior as documented in the WAET specification linked below (and the comments above the flags definitions). Specifically for ACPI_WAET_PM_TIMER_GOOD (Which is the only bit we set), the guest performs only one read of ACPI PM Timer instead of multiple to obtain it's value.
Which improves performance as it removes unnecessary VMExits.

It also makes sure to introduce
the new ACPI table only for new machine-types.
OK and why is that?
As ACPI tables are guest-visible, we should make sure to not change it between machine-types. For example, a change in ACPI tables may invalidate a Windows guest license activation (As platform have changed). But this is just a good practice in general and in the past it was said by maintainers that this is one of the main reasons that ACPI and SMBIOS generation have moved from SeaBIOS to QEMU.

Signed-off-by: Elad Gabay <address@hidden>
Co-developed-by: Liran Alon <address@hidden>
Signed-off-by: Liran Alon <address@hidden>
  hw/i386/acpi-build.c        | 18 ++++++++++++++++++
  hw/i386/pc_piix.c           |  2 ++
  hw/i386/pc_q35.c            |  2 ++
  include/hw/acpi/acpi-defs.h | 25 +++++++++++++++++++++++++
  include/hw/i386/pc.h        |  1 +
  5 files changed, 48 insertions(+)

diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
index 9c4e46fa7466..29f70741cd96 100644
--- a/hw/i386/acpi-build.c
+++ b/hw/i386/acpi-build.c
@@ -2512,6 +2512,19 @@ 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);
+static void
+build_waet(GArray *table_data, BIOSLinker *linker)
+    AcpiTableWaet *waet;
+    waet = acpi_data_push(table_data, sizeof(*waet));
Can combine with the previous line.
Ok. Will do in v2.

+    waet->emulated_device_flags = cpu_to_le32(ACPI_WAET_PM_TIMER_GOOD);
+    build_header(linker, table_data,
+                 (void *)waet, "WAET", sizeof(*waet), 1, NULL, NULL);
   *   IVRS table as specified in AMD IOMMU Specification v2.62, Section 5.2
   *   accessible here 
@@ -2859,6 +2872,11 @@ void acpi_build(AcpiBuildTables *tables, MachineState 
                            machine->nvdimms_state, machine->ram_slots);
+ if (!pcmc->do_not_add_waet_acpi) {
+        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);
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 9088db8fb601..2d11a8b50a9c 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -432,9 +432,11 @@ DEFINE_I440FX_MACHINE(v5_0, "pc-i440fx-5.0", NULL,
static void pc_i440fx_4_2_machine_options(MachineClass *m)
+    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
      m->alias = NULL;
      m->is_default = false;
+    pcmc->do_not_add_waet_acpi = true;
      compat_props_add(m->compat_props, hw_compat_4_2, hw_compat_4_2_len);
      compat_props_add(m->compat_props, pc_compat_4_2, pc_compat_4_2_len);
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 84cf925cf43a..1e0a726b27a7 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -361,8 +361,10 @@ DEFINE_Q35_MACHINE(v5_0, "pc-q35-5.0", NULL,
static void pc_q35_4_2_machine_options(MachineClass *m)
+    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
      m->alias = NULL;
+    pcmc->do_not_add_waet_acpi = true;
      compat_props_add(m->compat_props, hw_compat_4_2, hw_compat_4_2_len);
      compat_props_add(m->compat_props, pc_compat_4_2, pc_compat_4_2_len);
diff --git a/include/hw/acpi/acpi-defs.h b/include/hw/acpi/acpi-defs.h
index 57a3f58b0c9a..803c904471d5 100644
--- a/include/hw/acpi/acpi-defs.h
+++ b/include/hw/acpi/acpi-defs.h
@@ -634,4 +634,29 @@ struct AcpiIortRC {
  typedef struct AcpiIortRC AcpiIortRC;
+ * Windows ACPI Emulated Devices Table.
+ * Specification:
+ * 
+ */
+ * Indicates whether the RTC has been enhanced not to require acknowledgment
+ * after it asserts an interrupt. With this bit set, an interrupt handler can
+ * bypass reading the RTC register C to unlatch the pending interrupt.
+ */
+#define ACPI_WAET_RTC_GOOD      (1 << 0)
+ * Indicates whether the ACPI PM timer has been enhanced not to require
+ * multiple reads. With this bit set, only one read of the ACPI PM timer is
+ * necessary to obtain a reliable value.
+ */
+#define ACPI_WAET_PM_TIMER_GOOD (1 << 1)
ACPI spec is so huge we really can't add enums for all values,
it just does not scale.

So we switched to a different way to do this: you add e.g. 1 << 1
in the code directly, and put the comments there.

Ok. I will change this as you say in v2.

BTW it seems other code in acpi-build.c still relies on flags definitions in acpi-defs.h (As I have done in this v1). E.g. ACPI_DMAR_TYPE_*, ACPI_APIC_*, ACPI_FADT_F_*.
I assume this is just code that wasn't changed yet to the new convention?

Igor this is becoming a FAQ. Could you write up the way ACPI
generation code should look?


Thanks for the review,

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