Re: [Xen-ia64-devel] Re: ia64 kexec: xen -> linux

[Bjorn Helgaas <bjorn.helgaas@hp.com>]

On Wednesday 04 October 2006 20:53, Horms wrote:
> On Fri, Sep 29, 2006 at 12:48:19PM +0900, Horms wrote:
> > On Wed, Sep 27, 2006 at 11:52:12AM +0200, Tristan Gingold wrote:
> > > Linux and xen call efi in real mode if set_virtual_address_map fails.
> > > You may add an option in both xen and linux to force calling efi in real mode.
> > > This should be really simple and you will be able to make progress.
> > 
> > I took a stab at forcing the call to stay in real mode, 
> > by simply replacing the call to set_virtual_address_map with
> > a return. This should both prevent the mapping from occuring,
> > and force the kernel to use the real mode variants of the calls.
> > 
> > Unfortunately, the boot fails with the following log.
> > A bit of investigation has found that is it dying in
> > 
> >    SAL_CALL()
> >    called by: ia64_sal_cache_flush()
> >    called by: ia64_sal_cache_flush()
> >    called by: ia64_sal_init()
> > 
> > Well, when I say dying in, its probably more accurate to say,
> > never returning from.

I haven't been following this thread and don't know whether
this is related, but someone here is seeing an MCA in this
early SAL_CACHE_FLUSH call.  I haven't looked at it in detail
yet, but apparently it happens because we're making the call
in virtual mode before we have completely set up the TLB and VHPT.

> > I'm a bit of a loss to know why this is occuring. Though
> > I do wonder if SAL (and in this case indrectly PAL) calls are
> > are running into trouble by runing in real mode without
> > set_virtual_address_map() having being called.

SAL_CACHE_FLUSH is one of a few SAL calls that invoke PAL
procedures, and they can't be used in virtual mode until the
OS registers the virtual address of PAL (see SAL spec section
9.1.1).

> It turns out that this was indeed the case, and running SAL calls
> in physical mode (regardless of if EFI is making physical or virtual
> mode calls) seems to fix this boot failure. Though I am yet to determin
> it it solves the xen->linux, linux->xen kexec problem.

In general (I'm not speaking to xen or kexec), I think we want
to run SAL calls in virtual mode.  It looks like we have a problem
with the specific SAL_CACHE_FLUSH call above, but we should be able
to fix that specific problem without changing how we do other SAL
calls.

Bjorn