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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Mar 13, 2020 at 10:36:56AM +0100, Igor Mammedov wrote:
> On Thu, 12 Mar 2020 13:09:51 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Thu, Mar 12, 2020 at 05:27:45PM +0100, Igor Mammedov wrote:
> > > On Wed, 11 Mar 2020 19:08:26 +0200
> > > Liran Alon <liran.alon@oracle.com> wrote:
> > >   
> > > > From: Elad Gabay <elad.gabay@oracle.com>
> > > > 
> > > > 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. It also makes sure to introduce
> > > > the new ACPI table only for new machine-types.  
> > > 
> > > in addition to comments made by Michael ...
> > >   
> > > > 
> > > > Signed-off-by: Elad Gabay <elad.gabay@oracle.com>
> > > > Co-developed-by: Liran Alon <liran.alon@oracle.com>
> > > > Signed-off-by: Liran Alon <liran.alon@oracle.com>
> > > > ---
> > > >  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)  
> > > see build_hmat_lb() for example how to doc comment for such function
> > > should look like. Use earliest spec version where table was introduced.
> > >   
> > > > +{
> > > > +    AcpiTableWaet *waet;
> > > > +
> > > > +    waet = acpi_data_push(table_data, sizeof(*waet));
> > > > +    waet->emulated_device_flags = cpu_to_le32(ACPI_WAET_PM_TIMER_GOOD);  
> > > 
> > > we don't use packed structures for building ACPI tables anymore (there is
> > > old code that still does but that's being converted when we touch it)
> > > 
> > > pls use build_append_int_noprefix() api instead, see build_amd_iommu() as
> > > an example how to build binary tables using it and how to use comments
> > > to document fields.
> > > Basic idea is that api makes function building a table match table's
> > > description in spec (each call represents a row in spec) and comment
> > > belonging to a row should contain verbatim field name as used by spec
> > > so reader could copy/past and grep it easily.  
> > 
> > 
> > BTW how about a better name for this function?
> 
> how about [aml|acpi]_int_raw 
> [...]

I'm not sure how this helps.  I think the main problems are
1- very long name
2- only makes sense if you know that ACPI has a special integer prefix
3- easy to confuse which is the value which is the length
4- length is in bytes (typical documentation is in bits)

Your suggestion only fixes issue 1.

Having listed it all out, I suggest the following for the purpose of
building structures:

	acpi/aml/build_append_u8
	acpi/aml/build_append_u16
	acpi/aml/build_append_u32
	acpi/aml/build_append_u64

and maybe
	acpi/aml/build_append_pad( length)

I'm not sure what the best prefix is. I guess we can have them all
with the slightly different arguments.


-- 
MST