Re: [Xen-devel] Xen ARM Dom0less passthrough without IOMMU

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 17/12/2019 18:28, Stefano Stabellini wrote:
> > Then I tried to passthrough the eMMC, but I got the following
> > error:
> > (XEN) DOM1: [    0.879151] sdhci-esdhc-imx 4005d000.usdhc: can't request region 
> > for resource [mem 0x4005d000-0x4005dfff]
> > (XEN) DOM1: [    0.891137] sdhci-esdhc-imx 4005d000.usdhc: sdhci_pltfm_init 
> > failed -16
> > (XEN) DOM1: [    0.900249] sdhci-esdhc-imx: probe of 4005d000.usdhc failed with 
> > error -16
> >
> > Where 0x4005d000 is the physical address of the uSDHC(eMMC) node in the DT.
> > It seems that the DomU1 kernel does not have access to that memory zone.
> It looks like drivers/mmc/host/sdhci-pltfm.c:sdhci_pltfm_init failed,
> but I cannot see a simple reason why it would. As Julien mentioned the
> device tree snippet would be useful. Also the domU config and the full
> device tree would be useful. i.e. did you add "xen,passthrough;" under
> the related uSDHC node on the host device tree?
The only purpose of "xen,passthrough" is to mark the device as disabled 
in Dom0 DT. It will not affect how device will be passthrough to a guest.
In this case, I don't believe the problem is DT related because Linux is 
able to find the regions. If the region were not mapped to the guest, 
then it would be likely result to a data abort later on.
Looking at Andrei's e-mail again, he doesn't mention anything about the 
1:1 mapping. So I assume, he is still using the guest memory layout. The 
physical address 0x4005d000 which is roughly 372KB into the first RAM 
bank for the guest.
> > I'm trying to passthrough the eMMC in order to mount DomU1's root
> > on a SDCard partition, because I couldn't get to DomU1's Linux prompt
> > when I tried to boot with a ramdisk module. I always get this error:
> > (XEN) DOM1: [    1.544199] RAMDISK: Couldn't find valid RAM disk image starting 
> > at 0.
> >
> > Could this be because the ramdisk is too big? The smallest I've tried with
> > Is approximately 60MB in size. What size are the ramdisks that you
> > are using in your dom0less booting demos?
> I don't think so, I could boot with ramdisk 120MB in size or even
> larger. It is probably an address calculation error: it is easy to make
> a small mistake in the addresses so that they end up overlapping.
> Sometimes it is even U-Boot that causes the overlaps.
>
> I would suggest to use ImageBuilder to create the U-Boot boot script to
> load all the binaries and boot the system. Have a look at
> uboot-script-gen in particular:
>
> https://gitlab.com/ViryaOS/imagebuilder/blob/master/scripts/uboot-script-gen
Nice script, but it seems to contain hardcoded value (see offset and 
memaddr override), does not take into account reserved region and assume 
where U-boot/ATF may be loaded. So it may require some work before it 
can be used on NXP board...
Cheers,

--
Julien Grall

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