Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 14 Nov 2013 09:40:12 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Nov 13, 2013 at 01:04:10PM +0100, Igor Mammedov wrote:
> > On Wed, 13 Nov 2013 00:10:39 +0200
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 
> > > On Tue, Nov 12, 2013 at 07:26:02PM +0100, Paolo Bonzini wrote:
> > > > Il 12/11/2013 14:58, Igor Mammedov ha scritto:
> > > > > 'etc/reserved-memory-end' will allow QEMU to tell BIOS where PCI
> > > > > BARs mapping could safely start in high memory.
> > > > > 
> > > > > Allowing BIOS to start mapping 64-bit PCI BARs at address where it
> > > > > wouldn't conflict with other mappings QEMU might place before it.
> > > > > 
> > > > > That permits QEMU to reserve extra address space before
> > > > > 64-bit PCI hole for memory hotplug.
> > > > 
> > > > I may be royally wrong, but I think the new file should only be added to
> > > > new machine types.  Otherwise, after migrating old machine types from
> > > > new QEMU to old QEMU, you may end up with PCI BARs mapped outside the
> > > > "PCI windows" that exist until before patch 1/2 of this series.
> > > > 
> > > > Does this make sense?
> > > 
> > > Yes.
> > > Generally FW CFG must not be added/removed for a given machine types,
> > > otherwise guest that is migrated while reading it will
> > > get a corrupted result: half old and half new.
> > 
> > Is it true for a file 'etc/reserved-memory-end' though?
> 
> It's true for any FW CFG entry.
> 
> > I've debugged SeaBIOS to learn more about it, and new->old migration with
> > following reboot, showed that file is not found by SeaBIOS (well since old
> > QEMU doesn't have it), as result SeaBIOS fallbacks to the old behavior 
> > placing 64-PCI bars right above ram_over_4G as it was intended.
> > 
> > And with 'etc/reserved-memory-end' == ram_over_4G_end as it is in this
> > patch, there isn't issue whatsoever.
> > 
> > Looks like there is no migrations issues with files, provided SeaBIOS knows
> > how to deal with a missing file.
> 
> Here's an example of the issue:
> 
>     qemu_cfg_read_entry(&count, QEMU_CFG_FILE_DIR, sizeof(count));
> 
> 1. <<<<<<
> 
>     count = be32_to_cpu(count);
> 
>     u32 e;
>     for (e = 0; e < count; e++) {
>         struct QemuCfgFile qfile;
>         qemu_cfg_read(&qfile, sizeof(qfile));
>         qemu_romfile_add(qfile.name, be16_to_cpu(qfile.select)
>                          , 0, be32_to_cpu(qfile.size));
>     }
> 
> 2. <<<<<<<
> 
> 
> If migration happens during qemu_cfg_read_entry before point 1, you can
> get a byte from old count and a byte from new, resulting in a corrupt
> count.
> 
> If migration happens at point 2, you will get incorrect
> mapping between file selector and name. System will likely
> fail to boot.
> 
> There are probably other issues.
> 
> Bottom line: FW CFG is guest visible state. Migration must not change
> it.
Thanks for explanation,
I'll resend this patch with machine compatibility changes.

> 
> > -- 
> > Regards,
> >   Igor


-- 
Regards,
  Igor