Re: [Xen-devel] [Xen-users] ARM: Xen on Vexpress

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-06-17 at 18:50 +0100, Julien Grall wrote:
> (CCing Xen-devel)
> 
> On 06/16/2014 05:32 PM, Jeenu Viswambharan wrote:
> > On Thu, Jun 12, 2014 at 15:50:02, Jeenu Viswambharan wrote:
> >>>>   - What does DOM0 see as its PC at entry?
> >>>
> >>> The load address of the kernel image, since the zImage protocol
> >>> requires us to enter the kernel at that offset.
> >>
> >> I managed to break at eret before guest entry, and the PC is
> >> 0xafc00000 itself. It does fail to boot bare metal too from that
> >> address. I'll see if I can find what's going wrong.
> > 
> > I've been told that loading Linux at that high an offset from the memory
> > base wouldn't work after all, even on bare metal. I therefore set
> > kernel_addr_r to 0x80008000, which makes Xen load DOM0 at 0x8fc00000. I
> > can see Linux starting to boot from this address on bare metal, but With
> > Xen I'm seeing "uncompression error" (not printed on to the console, but
> > through debugger inspection).
> > 
> > I haven't pinned down why exactly it's throwing this. While I'm at it,
> > I'd appreciate any helpful pointers.
> 
> Did you try to use CONFIG_VMSPLIT_3G rather than CONFIG_VMSPLIT_2G as I
> suggested last week?
> 
> Using CONFIG_VMSPLIT_3G works correctly with 3.15. It looks like another
> issue in the pa - virt delta when it's positive.
> 
> IIRC, it's because linux is assuming that delta is always negative (see
> comment on __PV_BITS_31_24 in arch/arm/include/asm/memory.h).
> 
> At first glance, handling positive offset will be hard because, their
> are rely on the compiler to provide the right layout for the
> instruction. Positive number is not encoded the same way. I will have to
> spend a bit more time to fully understand this code.
> 
> With Ian multiple bank support patch for DOM0, I think we can safely
> assume that the delta will (nearly?) always be the same.

That would be a coincidence of the implementation, not a guarantee which
we would make.

If there is an issue with the pa - virt delta handling then we should
raise it with Linux upstream.

Ian.


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