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

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 18/06/14 09:29, Ian Campbell wrote:
> 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.
I will raise again the issue to Linux. But I start to think this is a 
restriction by Linux but I can't find a clear comment about it. Not much 
(maybe none of) ARM platform has the start physical address very high.
Regards,

--
Julien Grall

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