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Re: [Qemu-devel] [PATCH v2 6/8] hw: arm: Support both legacy and current


From: Andrew Jones
Subject: Re: [Qemu-devel] [PATCH v2 6/8] hw: arm: Support both legacy and current RSDP build
Date: Thu, 29 Nov 2018 16:09:47 +0100
User-agent: NeoMutt/20180716

On Thu, Nov 29, 2018 at 03:59:42PM +0100, Samuel Ortiz wrote:
> On Thu, Nov 29, 2018 at 03:42:43PM +0100, Andrew Jones wrote:
> > On Thu, Nov 29, 2018 at 02:24:26PM +0100, Samuel Ortiz wrote:
> > > We add the ability to build legacy or current RSDP tables, based on the
> > > AcpiRsdpData revision field passed to build_rsdp().
> > > Although arm/virt only uses RSDP v2, adding that capability to
> > > build_rsdp will allow us to share the RSDP build code between ARM and x86.
> > > 
> > > Signed-off-by: Samuel Ortiz <address@hidden>
> > > ---
> > >  hw/arm/virt-acpi-build.c | 37 ++++++++++++++++++++++++++-----------
> > >  1 file changed, 26 insertions(+), 11 deletions(-)
> > > 
> > > diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
> > > index 4782aea4fe..e1338b6f5a 100644
> > > --- a/hw/arm/virt-acpi-build.c
> > > +++ b/hw/arm/virt-acpi-build.c
> > > @@ -378,23 +378,38 @@ build_rsdp(GArray *tbl, BIOSLinker *linker, 
> > > AcpiRsdpData *rsdp_data)
> > >      g_array_append_vals(tbl, rsdp_data->oem_id, 6); /* OEMID */
> > >      build_append_int_noprefix(tbl, rsdp_data->revision, 1); /* Revision 
> > > */
> > >      build_append_int_noprefix(tbl, 0, 4); /* RsdtAddress */
> > > -    build_append_int_noprefix(tbl, 36, 4); /* Length */
> > > -
> > > -    /* XSDT address to be filled by guest linker */
> > > -    build_append_int_noprefix(tbl, 0, 8); /* XsdtAddress */
> > > -    bios_linker_loader_add_pointer(linker, ACPI_BUILD_RSDP_FILE,
> > > -                                   24, 8,
> > > -                                   ACPI_BUILD_TABLE_FILE,
> > > -                                   *rsdp_data->xsdt_tbl_offset);
> > > -
> > > -    build_append_int_noprefix(tbl, 0, 1); /* Extended Checksum */
> > > -    build_append_int_noprefix(tbl, 0, 3); /* Reserved */
> > > +    if (rsdp_data->rsdt_tbl_offset) {
> > 
> > I see why a pointer was used now. Using a pointer ensures a zero
> > offset won't fail this test. However the test could be replaced with
> > rsdp_data->revision == 0.
> > 
> > > +        /* RSDT address to be filled by guest linker */
> > > +        bios_linker_loader_add_pointer(linker,
> > > +                                       ACPI_BUILD_RSDP_FILE, 16, 4,
> > > +                                       ACPI_BUILD_TABLE_FILE,
> > > +                                       *rsdp_data->rsdt_tbl_offset);
> > > +    }
> > >  
> > >      /* Checksum to be filled by guest linker */
> > >      bios_linker_loader_add_checksum(linker, ACPI_BUILD_RSDP_FILE,
> > >                                      0, 20, /* ACPI rev 1.0 RSDP size */
> > >                                      8);
> > >  
> > > +    if (rsdp_data->revision == 0) {
> > > +        /* ACPI 1.0 RSDP, we're done */
> > > +        return;
> > > +    }
> > > +
> > > +    /* The RSDP revision is 2 and later, we must have an XSDT pointer */
> > > +    g_assert(rsdp_data->xsdt_tbl_offset != NULL);
> > 
> > So here's the justification for the pointers. We sanity check the callers.
> We could sanity check the callers without pointers as well, I don't
> think there's a strong advantage for pointers here, except consistence.
> 
> 
> > We're missing the (rsdp_data->revision == 0 && rsdp_data->rsdt_tbl_offset)
> > sanity check though.
> I think there's nothing preventing a caller to include both rsdt and
> xsdt if it wants to be able to run on both < 2.0 and 2.0+ platforms with
> the same table. So if rsdt is set we should add it, regardless of the 
> revision.

True, but if revision is zero, then we *must* have it. Otherwise we'll
silently return from this function with neither an rsdt nor xsdt linked.

Thanks,
drew

> 
> Cheers,
> Samuel.



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