Re: [Qemu-devel] [RFC QEMU PATCH v3 00/10] Implement vNVDIMM for Xen HVM guest

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 13, 2017 at 03:53:26PM +0800, Haozhong Zhang wrote:
> On 10/12/17 17:45 +0200, Paolo Bonzini wrote:
> > On 12/10/2017 14:45, Haozhong Zhang wrote:
> > > Basically, QEMU builds two ROMs for guest, /rom@etc/acpi/tables and
> > > /rom@etc/table-loader. The former is unstructured to guest, and
> > > contains all data of guest ACPI. The latter is a BIOSLinkerLoader
> > > organized as a set of commands, which direct the guest (e.g., SeaBIOS
> > > on KVM/QEMU) to relocate data in the former file, recalculate checksum
> > > of specified area, and fill guest address in specified ACPI field.
> > > 
> > > One part of my patches is to implement a mechanism to tell Xen which
> > > part of ACPI data is a table (NFIT), and which part defines a
> > > namespace device and what the device name is. I can add two new loader
> > > commands for them respectively.
> > > 
> > > Because they just provide information and SeaBIOS in non-xen
> > > environment ignores unrecognized commands, they will not break SeaBIOS
> > > in non-xen environment.
> > > 
> > > On QEMU side, most Xen-specific hacks in ACPI builder could be
> > > dropped, and replaced by adding the new loader commands (though they
> > > may be used only by Xen).
> > > 
> > > On Xen side, a fw_cfg driver and a BIOSLinkerLoader command executor
> > > are needed in, perhaps, hvmloader.
> > 
> > If Xen has to parse BIOSLinkerLoader, it can use the existing commands
> > to process a reduced set of ACPI tables.  In other words,
> > etc/acpi/tables would only include the NFIT, the SSDT with namespace
> > devices, and the XSDT.  etc/acpi/rsdp would include the RSDP table as usual.
> >
> > hvmloader can then:
> > 
> > 1) allocate some memory for where the XSDT will go
> > 
> > 2) process the BIOSLinkerLoader like SeaBIOS would do
> > 
> > 3) find the RSDP in low memory, since the loader script must have placed
> > it there.  If it cannot find it, allocate some low memory, fill it with
> > the RSDP header and revision, and and jump to step 6
> > 
> > 4) If it found QEMU's RSDP, use it to find QEMU's XSDT
> > 
> > 5) Copy ACPI table pointers from QEMU to hvmloader's RSDT and/or XSDT.
> > 
> > 6) build hvmloader tables and link them into the RSDT and/or XSDT as usual.
> > 
> > 7) overwrite the RSDP in low memory with a pointer to hvmloader's own
> > RSDT and/or XSDT, and updated the checksums
> > 
> > QEMU's XSDT remains there somewhere in memory, unused but harmless.
> > 
> 
> It can work for plan tables which do not contain AML.
> 
> However, for a namespace device, Xen needs to know its name in order
> to detect the potential name conflict with those used in Xen built
> ACPI. Xen does not (and is not going to) introduce an AML parser, so
> it cannot get those device names from QEMU built ACPI by its own.
> 
> The idea of either this patch series or the new BIOSLinkerLoader
> command is to let QEMU tell Xen where the definition body of a
> namespace device (i.e. that part within the outmost "Device(NAME)") is
> and what the device name is. Xen, after the name conflict check, can
> re-package the definition body in a namespace device (w/ minimal AML
> builder code added in Xen) and then in SSDT.
> 
> 
> Haozhong

You most likely can do this without a new command.
You can use something similiar to build_append_named_dword
in combination with BIOS_LINKER_LOADER_COMMAND_ADD_POINTER
like vm gen id does.

-- 
MST