Re: [Qemu-devel] [PATCH v3] pc/fwcfg: unbreak migration from qemu-2.5 and qemu-2.6 during firmware boot

[Eduardo Habkost]

On Tue, Apr 25, 2017 at 05:37:50PM +0200, Igor Mammedov wrote:
> Since 2.7 commit (b2a575a Add optionrom compatible with fw_cfg DMA version)
> regressed migration during firmware exection time by
> abusing fwcfg.dma_enabled property to decide loading
> dma version of option rom AND by mistake disabling DMA
> for 2.6 and earlier globally instead of only for option rom.
> 
> so 2.6 machine type guest is broken when it already runs
> firmware in DMA mode but migrated to qemu-2.7(pc-2.6)
> at that time;
> 
> a) qemu-2.6:pc2.6 (fwcfg.dma=on,firmware=dma,oprom=ioport)
> b) qemu-2.7:pc2.6 (fwcfg.dma=off,firmware=ioport,oprom=ioport)
> 
>   to:   a     b
> from
> a       OK   FAIL
> b       OK   OK
> 
> So we currently have broken forward migration from
> qemu-2.6 to qemu-2.[789] that however could be fixed
> for 2.10 by re-enabling DMA for 2.[56] machine types
> and allowing dma capable option rom only since 2.7.
> As result qemu should end up with:
> 
> c) qemu-2.10:pc2.6 (fwcfg.dma=on,firmware=dma,oprom=ioport)
> 
>    to:  a     b    c
> from
> a      OK   FAIL  OK
> b      OK   OK    OK
> c      OK   FAIL  OK
> 
> where forward migration from qemu-2.6 to qemu-2.10 should
> work again leaving only qemu-2.[789]:pc-2.6 broken.
> 
> Reported-by: Eduardo Habkost <address@hidden>
> Analyzed-by: Laszlo Ersek <address@hidden>
> Signed-off-by: Igor Mammedov <address@hidden>
> Reviewed-by: Laszlo Ersek <address@hidden>

Reviewed-by: Eduardo Habkost <address@hidden>

-- 
Eduardo