Re: [Qemu-devel] [PATCH v4 1/9] ACPI: Add a function for building named qword entries

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

On Tue, Jan 31, 2017 at 10:51:02AM +0100, Igor Mammedov wrote:
> On Mon, 30 Jan 2017 22:28:41 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Fri, Jan 27, 2017 at 10:43:13AM -0500, Kevin O'Connor wrote:
> > > On Fri, Jan 27, 2017 at 03:46:33PM +0100, Laszlo Ersek wrote:  
> > > > On 01/27/17 15:18, Kevin O'Connor wrote:  
> > > > > If an offset is going to be added, shouldn't both a source offset and
> > > > > destination offset be used?
> > > > > 
> > > > >         /*
> > > > >          * COMMAND_WRITE_POINTER - update a writeable file named
> > > > >          * @pointer.dest_file at @pointer.dest_offset, by writing pointer
> > > > >          * plus @pointer.src_offset to the blob originating from
> > > > >          * @src_file. 1,2,4 or 8 byte unsigned write is used depending
> > > > >          * on @pointer.size.
> > > > >          */
> > > > >         struct {
> > > > >             char dest_file[BIOS_LINKER_LOADER_FILESZ];
> > > > >             char src_file[BIOS_LINKER_LOADER_FILESZ];
> > > > >             uint32_t src_offset, dest_offset;
> > > > >             uint8_t size;
> > > > >         } pointer;
> > > > > 
> > > > > I doubt the offsets or size is really all that important though.  
> > > > 
> > > > The offset into the fw_cfg file that receives the allocation address is
> > > > important, that allows the same file to receive several different
> > > > addresses (for different downloaded blobs), at different offsets.
> > > > 
> > > > OTOH, asking the firmware to add a constant to the address value before
> > > > writing it to the fw_cfg file is not necessary, in my opinion. The blob
> > > > that the firmware allocated and downloaded originates from QEMU to begin
> > > > with, so QEMU knows its internal structure.  
> > > 
> > > I guess I'm missing why QEMU would want to use the same writable file
> > > for multiple pointers as well as why it would want support for
> > > pointers smaller than 8 bytes in size.  If it's because it may be
> > > easier to support an internal QEMU blob of a particular format, then
> > > adding a src_offset would facilitate that.
> > > 
> > > However, if it was done so that WRITE_POINTER mimicks ADD_POINTER then
> > > that's fine too.  I'm okay with either format.
> > > 
> > > -Kevin  
> > 
> > Both reasons :) offset is because it's easier for QEMU not to have to add
> > more files (e.g. it simplifies cross-version migration if we don't).
> On one hand offset simplifies since one file could be re-used for
> several pointers, on the other hand it doesn't make difference wrt
> migration since offset becomes ABI and has to be maintained in
> cross-version migration scenario (size of file shouldn't be issue
> as they are re-sizable now). So we just end-up with offset vs new file
> versioning.

Not really - offset is migrated automatically since it's in RAM.
No need to version it.

> However considering that number of files is limited,
> offset scales up better.
> 
> > size is to mimick ADD_POINTER.
> >