[SeaBIOS] [Qemu-devel] [PATCH RFC 0/3] seabios: move acpi table formatting out of bios

[Michael S. Tsirkin]

On Wed, May 08, 2013 at 01:59:12PM +0300, Gleb Natapov wrote:
> On Wed, May 08, 2013 at 01:43:25PM +0300, Michael S. Tsirkin wrote:
> > On Wed, May 08, 2013 at 01:34:59PM +0300, Gleb Natapov wrote:
> > > On Wed, May 08, 2013 at 01:29:12PM +0300, Michael S. Tsirkin wrote:
> > > > On Wed, May 08, 2013 at 12:31:50PM +0300, Gleb Natapov wrote:
> > > > > On Tue, May 07, 2013 at 07:01:13PM -0400, Kevin O'Connor wrote:
> > > > > > On Tue, May 07, 2013 at 09:00:48PM +0300, Michael S. Tsirkin wrote:
> > > > > > > On Thu, Apr 25, 2013 at 12:02:20PM +0300, Michael S. Tsirkin wrote:
> > > > > > > > Untested yet, but I thought I'd share the
> > > > > > > > BIOS bits so we can agree on direction.
> > > > > > > > 
> > > > > > > > In particular check out ROM sizes:
> > > > > > > > - Before patchset with DSDT enabled
> > > > > > > >     Total size: 127880  Fixed: 59060  Free: 3192 (used 97.6% of 128KiB rom)
> > > > > > > > - Before patchset with DSDT disabled
> > > > > > > >     Total size: 122844  Fixed: 58884  Free: 8228 (used 93.7% of 128KiB rom)
> > > > > > > > - After patchset:
> > > > > > > >     Total size: 128776  Fixed: 59100  Free: 2296 (used 98.2% of 128KiB rom)
> > > > > > > > - Legacy disabled at build time:
> > > > > > > >     Total size: 119836  Fixed: 58996  Free: 11236 (used 91.4% of 128KiB rom)
> > > > > > > > 
> > > > > > > > As can be seen from this, most size savings come
> > > > > > > > from dropping DSDT, but we do save a bit by removing
> > > > > > > > other tables. Of course the real reason to move tables to QEMU
> > > > > > > > is so that ACPI can better match hardware.
> > > > > > > > 
> > > > > > > > This patchset adds an option to move all code for formatting acpi tables
> > > > > > > > out of BIOS. With this, QEMU has full control over the table layout.
> > > > > > > > All tables are loaded from the new "/etc/acpi/" directory.
> > > > > > > > Any entries in this directory cause BIOS to disable
> > > > > > > > ACPI table generation completely.
> > > > > > > > A generic linker script, controlled by QEMU, is
> > > > > > > > loaded from "/etc/linker-script". It is used to
> > > > > > > > patch in table pointers and checksums.
> > > > > > > 
> > > > > > > After some thought, there are two additional
> > > > > > > options worth considering, in that they simplify
> > > > > > > bios code somewhat:
> > > > > > > 
> > > > > > > - bios could get size from qemu, allocate a buffer
> > > > > > >   (e.g. could be one buffer for all tables)
> > > > > > >   and pass the address to qemu.
> > > > > > >   qemu does all the patching
> > > > > > > 
> > > > > > > - further, qemu could do the copy of tables into
> > > > > > >   that address directly
> > > > > > 
> > > > > > This seems more complex than necessary to me.
> > > > > > 
> > > > > > The important task is to get the tables generated in QEMU - I'd focus
> > > > > > on getting the tables generated in QEMU (one table per fw_cfg "file").
> > > > > > Once that is done, the SeaBIOS side can be easily implemented, and we
> > > > > > can add any enhancements on top if we feel it is necessary.
> > > > > > 
> > > > > +1. This "copy of tables into that address directly" is just an ad-hoc PV
> > > > > isa DMA device in disguise. Such device was refused when libguestfs
> > > > > asked for it, and they wanted it for much better reason - performance.
> > > > > There is existing mechanism to pass data into firmware. Use it please.
> > > > 
> > > > Yes I can code it up using FW_CFG for now.
> > > > 
> > > > One issue with QEMU_CFG_FILE_DIR is that it's broken wrt migration,
> > > > unless we pass in very small bits of data which we
> > > > can guarantee never changes across qemu versions.
> > > > 
> > > Shouldn't we guaranty that ACPI tables do not change for the same
> > > machine type anyway?
> > 
> > That's not practical. They are too big to stay completely unchanged.
> > 
> I will not be surprised if this will cause us problem somehow. Guest
> will see new tables only after reboot/resume from S4 so damage is
> limited, but one thing that comes to mind is table's size change. If
> they grow from one version to the other after resuming a guest from S4
> on new QEMU version part of the tables may be corrupted.

Why would it be corrupted?

In any case, FACS has a hardware signature value for
just such a case. If we know VM can not be resumed on new QEMU,
we can change the signature and it will cold-boot instead.

> > > > Off-list, I suggested fixing it and migrating file
> > > > content, but Anthony thinks it's a bad idea.
> > > > 
> > > Why is this a bad idea to fix device migration?
> > 
> > You misunderstand I think.
> > Question is whether we should be putting so much info in fw_cfg.
> > If we keep fw_cfg for small things we don't need to
> > migrate it. In that case ACPI tables have to be passed in
> > using some other mechanism.
> > 
> Where this notion that fw_cfg is only for a small things is coming
> from? I can assure you this was not the case when the device was
> introduced. In fact it is used today for not so small things like
> bootindex splash screen bitmaps, option rom loading and kernel/initrd
> loading. Some of those are bigger then ACPI tables will ever be.
> And they all should be migrated, so fw_cfg should be fixed anyway.
> 
> --
> 			Gleb.

I'm not arguing with that. Convince Anthony please.

-- 
MST