Re: [Qemu-devel] [PATCH V11 2/3] i386: Add a Virtual Machine Generation ID device

[Gal Hammer]

On 22/01/2015 15:52, Igor Mammedov wrote:
> On Tue, 16 Dec 2014 17:50:43 +0200
> Gal Hammer <address@hidden> wrote:
>
> > Based on Microsoft's sepecifications (paper can be dowloaded from
> > http://go.microsoft.com/fwlink/?LinkId=260709), add a device
> > description to the SSDT ACPI table and its implementation.
> >
> > The GUID is set using a global "vmgenid.uuid" parameter.
> >
> > Signed-off-by: Gal Hammer <address@hidden>
>
> > --- a/hw/i386/acpi-build.c
> > +++ b/hw/i386/acpi-build.c
> > @@ -257,6 +257,7 @@ static void acpi_get_pci_info(PcPciInfo *info)
> >   #define ACPI_BUILD_TABLE_FILE "etc/acpi/tables"
> >   #define ACPI_BUILD_RSDP_FILE "etc/acpi/rsdp"
> >   #define ACPI_BUILD_TPMLOG_FILE "etc/tpm/log"
> > +#define ACPI_BUILD_VMGENID_FILE "etc/vm-generation-id"
> >
> >   static void
> >   build_header(GArray *linker, GArray *table_data,
> > @@ -1068,6 +1069,8 @@ build_ssdt(GArray *table_data, GArray *linker,
> >   {
> >       MachineState *machine = MACHINE(qdev_get_machine());
> >       uint32_t nr_mem = machine->ram_slots;
> > +    uint32_t vm_gid_physical_address;
> > +    uint32_t vm_gid_offset = 0;
> >       unsigned acpi_cpus = guest_info->apic_id_limit;
> >       int ssdt_start = table_data->len;
> >       uint8_t *ssdt_ptr;
> > @@ -1096,6 +1099,21 @@ build_ssdt(GArray *table_data, GArray *linker,
> >       ACPI_BUILD_SET_LE(ssdt_ptr, sizeof(ssdp_misc_aml),
> >                         ssdt_isa_pest[0], 16, misc->pvpanic_port);
> >
> > +    if (vm_generation_id_set()) {
> > +        vm_gid_physical_address = ssdt_start + ssdt_acpi_vm_gid_addr[0];
> > +        bios_linker_loader_alloc(linker, ACPI_BUILD_VMGENID_FILE, 8, true);
> > +        bios_linker_loader_add_pointer(linker, ACPI_BUILD_VMGENID_FILE,
> > +                                       ACPI_BUILD_TABLE_FILE,
> > +                                       table_data,
> > +                                       &vm_gid_offset,
> > +                                       sizeof(vm_gid_offset));
> could some explain how this pointer magic works,

I can try, but don't you think that a magic is gone once explained? ;-)

>  From my weak understanding it seems broken.
> Lets see:
>
>   [1] &vm_gid_offset - must be pointer inside of dest_file blob 
> (ACPI_BUILD_VMGENID_FILE)
>   [2] vm_gid_offset - should hold offset of the place inside of src_file
>                      (ACPI_BUILD_TABLE_FILE) where to pointer inside of 
> dest_file should point to

The vm_gid_offset should point where in the ACPI_BUILD_VMGENID_FILE the 
VM's GUID is stored. At the moment, it should always be zero because the 
GUID is stored at the begging of the ACPI_BUILD_VMGENID_FILE.

> now:
>    vm_gid_physical_address - holds [2] i.e. offset of VGIA constant in inside 
> SSDT in ACPI_BUILD_TABLE_FILE.
>
> > +    ACPI_BUILD_SET_LE(ssdt_ptr, sizeof(ssdp_misc_aml),
> > +                      ssdt_acpi_vm_gid_addr[0], 32, vm_gid_physical_address);
> Then we write this offset into VGIA in ACPI_BUILD_TABLE_FILE.

Yes. This offset is later patched by the linker to the full physical 
address.

> After BIOS loads tables it's going to patch at
>   [3] ACPI_BUILD_VMGENID_FILE + (&vm_gid_offset - table_data->data) /* only god 
> knows where it will be/
>
> and on top of it write in it value:
>   *(ACPI_BUILD_TABLE_FILE +  *[3])

We know exactly where it is, no need to call for god's help :-).

> This approach in general of patching arbitrary place in AML blob
> to get PHY addr of buffer with UUID, is quite a hack, especially
> in light of that we are trying to hide all direct access to AML
> blobs with related pointer arithmetic and manual patching.
>
> Why not reserve some potion of RAM and pass to BIOS/guest
> a reservation so it won't be part of AddressRangeMemory or
> AddressRangeACPI as MS spec requires? Then you won't need
> jump all above hoops to just get buffer's PHY addr.

I'll be glad to hear a new idea that I didn't already try in one of 
other previous patches. The problem is that the specification requires 
working with a physical address, so it must be allocated from inside the 
guest. Since the OS is not exist in this stage and I also don't want to 
write a special driver just to allocate this buffer I had to choose this 
approach.

> >
> [...]
> >   typedef
> > @@ -1790,6 +1811,11 @@ void acpi_setup(PcGuestInfo *guest_info)
> >       fw_cfg_add_file(guest_info->fw_cfg, ACPI_BUILD_TPMLOG_FILE,
> >                       tables.tcpalog->data, acpi_data_len(tables.tcpalog));
> >
> > +    /* Add a 128-bit fw cfg file which stores the VM generation id. */
> > +    g_array_set_size(tables.vmgenid, 16);
> > +    fw_cfg_add_file(guest_info->fw_cfg, ACPI_BUILD_VMGENID_FILE,
> > +                    tables.vmgenid->data, tables.vmgenid->len);
> shouldn't it be migratable? /i.e. acpi_add_rom_blob(...)/

I'm not too familiar with the migration process, but I assume that this 
memory will be copied as part of the guest memory.

    Gal.