Re: [Qemu-devel] [PATCH v4 7/8] hw: acpi: Export and share the ARM RSDP build

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 17 Dec 2018 14:49:59 +0100
Samuel Ortiz <sameo@linux.intel.com> wrote:

> On Mon, Dec 17, 2018 at 01:20:28PM +0100, Igor Mammedov wrote:
> > On Mon, 17 Dec 2018 11:48:37 +0100
> > Samuel Ortiz <sameo@linux.intel.com> wrote:
> > 
> > > Now that build_rsdp() supports building both legacy and current RSDP
> > > tables, we can move it to a generic folder (hw/acpi) and have the i386
> > > ACPI code reuse it in order to reduce code duplication.
> > > 
> > > Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
> > > Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> > > Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> > > Reviewed-by: Andrew Jones <drjones@redhat.com>
> > > Reviewed-by: Igor Mammedov <imammedo@redhat.com>
> > for future reference, if one changes patch in a significant way,
> > one is supposed to drop Tested/Reviewed-by tags so that reviewers
> > would look at it again and we by mistake won't merge not actually
> > reviewed changes.
> > 
> > [...]
> > 
> > > diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> > > index fb877648ac..846cb6d755 100644
> > > --- a/hw/i386/acpi-build.c
> > > +++ b/hw/i386/acpi-build.c
> > > @@ -2547,35 +2547,6 @@ build_amd_iommu(GArray *table_data, BIOSLinker *linker)
> > >                   "IVRS", table_data->len - iommu_start, 1, NULL, NULL);
> > >  }
> > >  
> > > -static void
> > > -build_rsdp(GArray *rsdp_table, BIOSLinker *linker, unsigned rsdt_tbl_offset)
> > > -{
> > > -    /* AcpiRsdpDescriptor describes revision 2 RSDP table and as result we
> > > -     * allocate extra 16 bytes for pc/q35 RSDP rev1 as well. Keep extra 16 bytes
> > > -     * wasted to make sure we won't breake migration for machine types older
> > > -     * than 2.3 due to size mismatch.
> > > -     */
> > > -    AcpiRsdpDescriptor *rsdp = acpi_data_push(rsdp_table, sizeof *rsdp);
> > > -    unsigned rsdt_pa_size = sizeof(rsdp->rsdt_physical_address);
> > > -    unsigned rsdt_pa_offset =
> > > -        (char *)&rsdp->rsdt_physical_address - rsdp_table->data;
> > > -
> > > -    bios_linker_loader_alloc(linker, ACPI_BUILD_RSDP_FILE, rsdp_table, 16,
> > > -                             true /* fseg memory */);
> > > -
> > > -    memcpy(&rsdp->signature, "RSD PTR ", 8);
> > > -    memcpy(rsdp->oem_id, ACPI_BUILD_APPNAME6, 6);
> > > -    /* Address to be filled by Guest linker */
> > > -    bios_linker_loader_add_pointer(linker,
> > > -        ACPI_BUILD_RSDP_FILE, rsdt_pa_offset, rsdt_pa_size,
> > > -        ACPI_BUILD_TABLE_FILE, rsdt_tbl_offset);
> > > -
> > > -    /* Checksum to be filled by Guest linker */
> > > -    bios_linker_loader_add_checksum(linker, ACPI_BUILD_RSDP_FILE,
> > > -        (char *)rsdp - rsdp_table->data, 20 /* ACPI rev 1.0 RSDP size */,
> > > -        (char *)&rsdp->checksum - rsdp_table->data);
> > > -}
> > > -
> > >  typedef
> > >  struct AcpiBuildState {
> > >      /* Copy of table in RAM (for patching). */
> > > @@ -2732,7 +2703,25 @@ void acpi_build(AcpiBuildTables *tables, MachineState *machine)
> > >                 slic_oem.id, slic_oem.table_id);
> > >  
> > >      /* RSDP is in FSEG memory, so allocate it separately */
> > > -    build_rsdp(tables->rsdp, tables->linker, rsdt);
> > > +    {
> > > +        AcpiRsdpData rsdp_data = {
> > > +            .revision = 0,
> > > +            .oem_id = ACPI_BUILD_APPNAME6,
> > > +            .xsdt_tbl_offset = NULL,
> > > +            .rsdt_tbl_offset = &rsdt,
> > > +        };
> > > +        build_rsdp(tables->rsdp, tables->linker, &rsdp_data);
> > > +        if (!pcmc->rsdp_in_ram) {
> > > +            /*
> > > +             * Legacy machine types (2.2 and older) expect to get a complete
> > > +             * revision 2 RSDP table, even though they only look at the
> > not true, rev is set to 0 for pc machines,
> Rev is set to 0 but they effectively expect to get a structure which
> size is the rev 2 one. That's what I meant.
> 
> > the point of the original comment was
> > that we allocate extra 16 bytes but not actually using them and why it's bad
> > to drop it suddenly.
> >
> > > +             * revision 0 fields (xsdt pointer is not set). So in order to
> > > +             * not break migration to those machine types we waste 16 bytes
> > > +             * that we amend to the RSDP revision 0 structure.
> >                           ^^^ added
> > > +             */
> > Perhaps amended original comment would be clearer:
> > 
> >    /* We used to allocate extra space for RSDP rev 2 but used only space for
> >     * legacy RSDP and extra bytes were zeroed out. Keep wasting extra 16 bytes
> >     * to make sure we won't breake migration for machine types 2.2 and older
> >     * due to RSDP blob size mismatch.
> >     */
> > 
> > > +            build_append_int_noprefix(tables->rsdp, 0, 16);
Looks like I've haven't noticed, it might work but are you sure?
0 here is uint64_t and then it's shifted 16 times which is sort of gray area (undefined behavior)

using g_array_append_val[s]() here might be better

> > > +        }
> > > +    }
> > >  
> > >      /* We'll expose it all to Guest so we want to reduce
> > >       * chance of size changes.
> > 
> > With comment fixed:
> > Reviewed-by: Igor Mammedov <imammedo@redhat.com>
> > 
> > since it's minor fixup and doesn't affect applying remaining patches you can post
> > v5 7/8 as reply to v4 7/8
> I'll do that.
> Since I assume the 16 bytes addition is a significant change, I'll also remove all
> reviewed-by/tested-by tags from this patch, except yours.
> 
> Cheers,
> Samuel.
>