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

[Michael S. Tsirkin]

On Thu, Mar 12, 2020 at 01:30:01PM +0200, Liran Alon wrote:
> 
> On 12/03/2020 8:12, Michael S. Tsirkin wrote:
> > On Thu, Mar 12, 2020 at 01:20:02AM +0200, Liran Alon wrote:
> > > 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.
> > Sounds excellent. Pls include this info in the commit log.
> Ok. Will do in v2.
> > As with any
> > performance optimization, pls add a bit of info about how you tested
> > and what kind of speedup was seen.
> This is a quite an old patch of ours that I upstream now to contribute to
> community.
> I will need to re-setup such environment for gathering exact performance
> numbers.
> 
> Having said that, note that there isn't really a trade-off here between
> better performance or something else.

Well some guests are known to make crazy assumptions. E.g. they would
see this bit and say "oh so I know this hyperv" or something to
that end.

> We just expose a bit to guest that says to it: "You don't need to do this
> useless thing that cause unnecessary VMExits. You can just do this simple
> operation which is always better because we support it".
> Therefore, as long as other guests just ignore this ACPI table (Which they
> do as far as I've seen from the vast variety of instances we have run on
> production for over 5 years), exposing this just have positive effect.
> 
> Also note that besides VMware which expose it by default, you can also see
> this exposed by default by some cloud hypervisors, such as GCP:
> [    0.000000] ACPI: WAET 0x00000000BFFF5CE0 000028 (v01 Google GOOGWAET
> 00000001 GOOG 00000001)
> 
> > > > > 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).
> > I don't think there's something like this taken into account, no.
> Windows measures at boot-time if the hardware have "changed too much" since
> activation.
> The way it does so, is calculating a "weighted diff score" based on a number
> of hardware properties.
> 
> It is at least documented internally in Ravello that some guests have been
> witnessed to broke their license activation because of ACPI/SMBIOS changes.

Any data on which changes exactly?
All I know about is this list, though it's pretty old.
https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-xp/bb457054(v=technet.10)?redirectedfrom=MSDN

Any chance it was actually to do with the EOM table that bypasses
activation?

> > > 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.
> > I think a flag to disable this might make sense though. For example,
> > some guests might behave differently and get broken.
> Right. That's why I think it's a good practice to have this flag and tie it
> to machine-type.

Tying things to the machine type is not what I had in mind.
A separate flag would also be helpful so users can tweak this
for new machine types, too.

> Guest-visible changes shouldn't be exposed to old machine-types.

Well almost any change in qemu is guest visible to some level.
Even optimizations are guest visible.
We made changes in ACPI without versioning in the past but I'm not
opposed to versioning here. However in that case pls do add a bit
of documentation about why this is done here.
What I am asking about is whether we need a flag to disable
this as part of the stable interface.

> > 
> > 
> > > > > 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)
> > Add documentation that it's a Windows Emulated Device Flags table,
> > helpful to speed up windows guests, and ignored by others.
> Ok. Will do in v2.
> > 
> > > > > +{
> > > > > +    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 
> > > > > https://urldefense.com/v3/__http://support.amd.com/TechDocs/48882_IOMMU.pdf__;!!GqivPVa7Brio!OAQpLo9QhdHiNDa_aRLR_ma1nWLZU1aQhDozYgUlrqBZiz1vKdZgg-lTDMIj_5g$
> > > > > @@ -2859,6 +2872,11 @@ void acpi_build(AcpiBuildTables *tables, 
> > > > > MachineState *machine)
> > > > >                              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);
> > > > >        pc_i440fx_5_0_machine_options(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);
> > > > >        pc_q35_5_0_machine_options(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 {
> > > > >    } QEMU_PACKED;
> > > > >    typedef struct AcpiIortRC AcpiIortRC;
> > > > > +/*
> > > > > + * Windows ACPI Emulated Devices Table.
> > > > > + * Specification:
> > > > > + * 
> > > > > https://urldefense.com/v3/__http://download.microsoft.com/download/7/E/7/7E7662CF-CBEA-470B-A97E-CE7CE0D98DC2/WAET.docx____;!!GqivPVa7Brio!MX1Hzr4X9NtS4pcT1Kb4VlDoV-pobn4n6YYQCkU3U-7imIaxXmu_ZsQzPB0e2Tc$
> > Please include
> > - name of the spec
> The name of the spec is "Windows ACPI Emulated Devices Table".
> You can see this by entering above link...

Links go stale. Then someone will have to dig to find the new location.
Name of the document will be helpful for that.

> > - earliest revision that includes the relevant bits
> The above link is to version 1.0 of the document (Which as far as I know, is
> the only version ever released).
> So the bits exists in all revisions. Which documentation do you want me to
> add then?

1. name of the document
2. revision of the document that includes the bit (if multiple,
include the earliest revision)

-- 
MST