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Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg in
From: |
Igor Mammedov |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS |
Date: |
Thu, 14 Nov 2013 14:37:31 +0100 |
On Thu, 14 Nov 2013 09:40:12 +0200
"Michael S. Tsirkin" <address@hidden> 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" <address@hidden> 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
- [Qemu-devel] [PATCH for-1.8 0/2 v3] pc: inform SeaBIOS where 64-bit PCI hole begins, Igor Mammedov, 2013/11/12
- [Qemu-devel] [PATCH 1/2] pc: map PCI address space as catchall region for not mapped addresses, Igor Mammedov, 2013/11/12
- [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS, Igor Mammedov, 2013/11/12
- Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS, Paolo Bonzini, 2013/11/12
- Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS, Igor Mammedov, 2013/11/12
- Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS, Michael S. Tsirkin, 2013/11/12
- Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS, Paolo Bonzini, 2013/11/12
- Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS, Igor Mammedov, 2013/11/13
- Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS, Michael S. Tsirkin, 2013/11/14
- Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS,
Igor Mammedov <=
- [Qemu-devel] [PATCH 2/2 v4] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS, Igor Mammedov, 2013/11/14