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Re: [Qemu-devel] [PATCH v4 1/9] ACPI: Add a function for building named
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v4 1/9] ACPI: Add a function for building named qword entries |
Date: |
Wed, 1 Feb 2017 19:55:36 +0200 |
On Wed, Feb 01, 2017 at 12:46:47PM +0100, Igor Mammedov wrote:
> On Tue, 31 Jan 2017 23:39:44 +0200
> "Michael S. Tsirkin" <address@hidden> wrote:
>
> > 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" <address@hidden> 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.
> I've meant offset inside of writebale blob dest_offset,
> it has to stay the same within a machine type across builds,
> so that if migration happens during linking time, the old
> already shadowed and migrated liker file would match offsets in
> writable fwcfg file on new qemu.
> In other words format of writable fw_cfg file becomes ABI.
We can always add new offsets without versioning though.
So yes an ABI but easier to extend.
> >
> > > However considering that number of files is limited,
> > > offset scales up better.
> > >
> > > > size is to mimick ADD_POINTER.
> > > >