Re: [Qemu-devel] [PATCH 4/6] i386: acpi: add IVHD device entry for IOAPIC

[Michael S. Tsirkin]

On Wed, Sep 12, 2018 at 02:24:52PM -0500, Brijesh Singh wrote:
> 
> 
> On 09/12/2018 11:35 AM, Igor Mammedov wrote:
> ...
> 
> > > +    /*
> > > +     * When interrupt remapping is enabled, Linux IOMMU driver also 
> > > checks
> > > +     * for special IVHD device (type IO-APIC), which is typically 
> > > presented
> > > +     * as PCI device 14:00.0.
> > Probably it shouldn't be a 'typically' device from somewhere but rather 
> > address
> > fetched from corresponding device model QEMU implements.
> > 
> 
> IOAPIC is not presented as a true PCI device to guest OS. When IOMMU is
> enabled a pseudo address space to added under root PCI bus. PCI 14:0.0
> presents to this pseudo device.
> 
> > 
> > > +     */
> > > +    if (s->intr_enabled) {
> > > +        build_append_int_noprefix(table_data, 0x0100a00000000048, 8);
> >                                                   ^^ this is 
> > incomprehensible,
> > where does this magic number comes from and how was it calculated?
> > 
> 
> In order to provide interrupt remap support, a special IVHD device need
> to be added,  the magic number uses the format defined in Table 95 (IVHD
> device entry type codes).
> 
> 0x01 00a0 00 00 0000 48
> 
> Byte 0: 0x48 (special device)
> Byte 1 & 2: must be zero
> Byte 3: 0 (dte setting)
> Byte 4: 0 (handle)
> Byte 5 & 6: IOAPIC devfn (14:0.0)

Do you mean *bus* devfn? devfn is 0.0.

> Byte 7: 0x1 (IOAPIC) - See Table 97 in spec


Above should go into code comment, along with
first (oldest) version of spec that has this table.
Additionally the number is IMHO more readable as:
        (0x1ull << 56) | (PCI_BUILD_BDF(14, 0) << 40) | 0x48

(assuming I got what it should be).

> 
> > > +    }
> > > +
> > >       build_header(linker, table_data, (void *)(table_data->data + 
> > > iommu_start),
> > >                    "IVRS", table_data->len - iommu_start, 1, NULL, NULL);
> > >   }
> >