Re: [Qemu-devel] [PATCH 4/8] hw: arm: Carry RSDP specific data through AcpiRsdpData

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 28 Nov 2018 10:46:41 +0100
Samuel Ortiz <sameo@linux.intel.com> wrote:

> On Tue, Nov 27, 2018 at 05:27:49PM +0100, Igor Mammedov wrote:
> > On Tue, 27 Nov 2018 16:42:18 +0100
> > Samuel Ortiz <sameo@linux.intel.com> wrote:
> >   
> > > Hi Igor,
> > > 
> > > On Tue, Nov 27, 2018 at 04:25:51PM +0100, Igor Mammedov wrote:  
> > > > On Mon, 26 Nov 2018 17:29:37 +0100
> > > > Samuel Ortiz <sameo@linux.intel.com> wrote:
> > > >     
> > > > > That will allow us to generalize the ARM build_rsdp() routine to support
> > > > > both legacy RSDP (The current i386 implementation) and extended RSDP
> > > > > (The ARM implementation).
> > > > > 
> > > > > Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
> > > > > ---
> > > > >  include/hw/acpi/acpi-defs.h | 11 +++++++++++
> > > > >  hw/arm/virt-acpi-build.c    | 27 ++++++++++++++++++++++-----
> > > > >  2 files changed, 33 insertions(+), 5 deletions(-)
> > > > > 
> > > > > diff --git a/include/hw/acpi/acpi-defs.h b/include/hw/acpi/acpi-defs.h
> > > > > index af8e023968..e7fd24c6c5 100644
> > > > > --- a/include/hw/acpi/acpi-defs.h
> > > > > +++ b/include/hw/acpi/acpi-defs.h
> > > > > @@ -53,6 +53,17 @@ struct AcpiRsdpDescriptor {        /* Root System Descriptor Pointer */
> > > > >  } QEMU_PACKED;
> > > > >  typedef struct AcpiRsdpDescriptor AcpiRsdpDescriptor;
> > > > >  
> > > > > +typedef struct AcpiRsdpData {
> > > > > +    uint8_t oem_id[6]; /* OEM identification */
> > > > > +    uint8_t revision;  /* Must be 0 for 1.0, 2 for 2.0 */
> > > > > +
> > > > > +    unsigned *rsdt_tbl_offset;
> > > > > +    unsigned *xsdt_tbl_offset;
> > > > > +} AcpiRsdpData;
> > > > > +    
> > > >     
> > > > > +#define ACPI_RSDP_REV_1 0
> > > > > +#define ACPI_RSDP_REV_2 2    
> > > > it's one time used spec defined values so just use values directly
> > > > in place with a comment, so reader won't have to jump around code
> > > > when comparing to spec.    
> > > It's also used in the ACPI tests fix patch.  
> > it's better to use in test it's own version (we just opencode them there)
> > see fadt_fetch_facs_and_dsdt_ptrs()/sanitize_fadt_ptrs()
> > same applies for length.  
> 
> I think you're trying to explain to me that this:
> 
> 	/* sdt->aml field offset := spec offset - header size */
>         memset(sdt->aml + 0, 0, 4); /* sanitize FIRMWARE_CTRL(36) ptr */
>         memset(sdt->aml + 4, 0, 4); /* sanitize DSDT(40) ptr */
>         if (sdt->header.revision >= 3) {
>             memset(sdt->aml + 96, 0, 8); /* sanitize X_FIRMWARE_CTRL(132) ptr */
>             memset(sdt->aml + 104, 0, 8); /* sanitize X_DSDT(140) ptr */
>         }
> 
> is good coding practice. I'm having a hard time internalizing that
> hard coded constants and comments not directly mapping the code (How do
> I map "sanitize X_FIRMWARE_CTRL(132) ptr" to "sdt->aml + 96, 0, 8"?) is
> indeed good practice. But I'll take the pragmatic route and follow what
> you guys advice for.
Even though it's ugly, constants directly comparable to spec table
definitions and comments are supposed to hep locate relevant field/table,
macro isn't making it better in this case (just a bit more obscure).

At least for ACPI spec bound code, it's consensus that we reached
after discussing how to better to handle this usecase (from maintainability pov).

> > that way if we break it in qemu's code test would catch the thing
> >   
> > > Also the 0 for revision 1 is a little confusing, I feel the above
> > > definition is clearer.  
> > that's confusion is in the spec, so we just mimic it, no need to add more on top
> >   
> > > 
> > >   
> > > > > +
> > > > >  /* Table structure from Linux kernel (the ACPI tables are under the
> > > > >     BSD license) */
> > > > >  
> > > > > diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> > > > > index 0835900052..2dad465ecf 100644
> > > > > --- a/hw/arm/virt-acpi-build.c
> > > > > +++ b/hw/arm/virt-acpi-build.c
> > > > > @@ -368,7 +368,7 @@ static void acpi_dsdt_add_power_button(Aml *scope)
> > > > >  
> > > > >  /* RSDP */
> > > > >  static void
> > > > > -build_rsdp(GArray *rsdp_table, BIOSLinker *linker, unsigned xsdt_tbl_offset)
> > > > > +build_rsdp(GArray *rsdp_table, BIOSLinker *linker, AcpiRsdpData *rsdp_data)
> > > > >  {
> > > > >      AcpiRsdpDescriptor *rsdp = acpi_data_push(rsdp_table, sizeof *rsdp);
> > > > >      unsigned xsdt_pa_size = sizeof(rsdp->xsdt_physical_address);
> > > > > @@ -379,14 +379,14 @@ build_rsdp(GArray *rsdp_table, BIOSLinker *linker, unsigned xsdt_tbl_offset)
> > > > >                               true /* fseg memory */);
> > > > >  
> > > > >      memcpy(&rsdp->signature, "RSD PTR ", sizeof(rsdp->signature));
> > > > > -    memcpy(rsdp->oem_id, ACPI_BUILD_APPNAME6, sizeof(rsdp->oem_id));
> > > > > +    memcpy(rsdp->oem_id, rsdp_data->oem_id, sizeof(rsdp->oem_id));
> > > > >      rsdp->length = cpu_to_le32(sizeof(*rsdp));
> > > > > -    rsdp->revision = 0x02;
> > > > > +    rsdp->revision = rsdp_data->revision;
> > > > >  
> > > > >      /* Address to be filled by Guest linker */
> > > > >      bios_linker_loader_add_pointer(linker,
> > > > >          ACPI_BUILD_RSDP_FILE, xsdt_pa_offset, xsdt_pa_size,
> > > > > -        ACPI_BUILD_TABLE_FILE, xsdt_tbl_offset);
> > > > > +        ACPI_BUILD_TABLE_FILE, *rsdp_data->xsdt_tbl_offset);
> > > > >  
> > > > >      /* Checksum to be filled by Guest linker */
> > > > >      bios_linker_loader_add_checksum(linker, ACPI_BUILD_RSDP_FILE,
> > > > > @@ -399,6 +399,20 @@ build_rsdp(GArray *rsdp_table, BIOSLinker *linker, unsigned xsdt_tbl_offset)
> > > > >          (char *)&rsdp->extended_checksum - rsdp_table->data);
> > > > >  }
> > > > >  
> > > > > +static void
> > > > > +init_rsdp_data(AcpiRsdpData *data, const char *oem_id, uint8_t revision,
> > > > > +               unsigned *rsdt_offset, unsigned *xsdt_offset)
> > > > > +{
> > > > > +    /* Caller must provide an OEM ID */
> > > > > +    g_assert(oem_id);
> > > > > +    g_assert(strlen(oem_id) >= 6);
> > > > > +
> > > > > +    memcpy(data->oem_id, oem_id, 6);
> > > > > +    data->revision = revision;
> > > > > +    data->rsdt_tbl_offset = rsdt_offset;
> > > > > +    data->xsdt_tbl_offset = xsdt_offset;
> > > > > +}
> > > > > +
> > > > >  static void
> > > > >  build_iort(GArray *table_data, BIOSLinker *linker, VirtMachineState *vms)
> > > > >  {
> > > > > @@ -810,6 +824,7 @@ void virt_acpi_build(VirtMachineState *vms, AcpiBuildTables *tables)
> > > > >      GArray *table_offsets;
> > > > >      unsigned dsdt, xsdt;
> > > > >      GArray *tables_blob = tables->table_data;
> > > > > +    AcpiRsdpData rsdp;    
> > > > s/rsdp/rsdp_info/
> > > >     
> > > > >  
> > > > >      table_offsets = g_array_new(false, true /* clear */,
> > > > >                                          sizeof(uint32_t));
> > > > > @@ -857,7 +872,9 @@ void virt_acpi_build(VirtMachineState *vms, AcpiBuildTables *tables)
> > > > >      build_xsdt(tables_blob, tables->linker, table_offsets, NULL, NULL);
> > > > >  
> > > > >      /* RSDP is in FSEG memory, so allocate it separately */
> > > > > -    build_rsdp(tables->rsdp, tables->linker, xsdt);
> > > > > +    init_rsdp_data(&rsdp, ACPI_BUILD_APPNAME6, ACPI_RSDP_REV_2,
> > > > > +                   NULL, &xsdt);    
> > > > It would be more concise to use declarative style without extra clutter:
> > > > 
> > > > -    init_rsdp_data(&rsdp, ACPI_BUILD_APPNAME6, ACPI_RSDP_REV_2,
> > > > -                   NULL, &xsdt);
> > > > -    build_rsdp(tables->rsdp, tables->linker, &rsdp);
> > > > +    {
> > > > +       AcpiRsdpData rsdp = {
> > > > +           .revision = 2,
> > > > +           .oem_id = ACPI_BUILD_APPNAME6,
> > > > +           .xsdt_tbl_offset = &xsdt,
> > > > +           .rsdt_tbl_offset = NULL,
> > > > +       };
> > > > +       build_rsdp(tables->rsdp, tables->linker, &rsdp);
> > > > +    }    
> > > 2 things here, imo:
> > > 
> > > - This is not more concise.  
> > with function, one have to jump to it's definition/body to find out what
> > each argument is, with declaration + initialization inplace it's clear
> > what values mean as you see fields right there as well.  
> With a structure you need to go and look at the structure definition to
> know which fields you need to initialize and what their names are. And
> no, you can't safely copy paste the above snippet and rest assured your
> code is safe, because C allows you to leave some structure fields
> uninitialized.
true, one has to be more careful with initializing.
I don't have a strong opinion here, I'll leave it up to you.

> > If it's simple structure it is clearer to use initializer, instead of
> > wrapper helper. With complex structure it could be other way around.
> >  
> > > - It's code duplication as almost the same snippet is going to be used
> > >   for i386/acpi-build.c  
> > the same goes for init_rsdp_data(...)  
> I disagree here as well. But I'd like to see this code being merged,
> I'll comply. Do you have any comments on the tests part of that serie,
> besides the fact that it's using defined constants as opposed to hard
> coded ones?
> 
> Cheers,
> Samuel.
> 
>