[PATCH 6/7] arm64/kexec: Add core kexec support

[geoff@infradead.org (Geoff Levand)]

Hi Vivek,

On Wed, 2014-10-01 at 10:56 -0400, Vivek Goyal wrote:
> On Tue, Sep 30, 2014 at 12:54:37PM -0700, Geoff Levand wrote:
> 
> [..]
> > > > +{
> > > > +	switch (flag) {
> > > > +	case IND_INDIRECTION:
> > > > +	case IND_SOURCE:
> > > > +		__flush_dcache_area(addr, PAGE_SIZE);
> > > > +		break;
> > > 
> > > So what does __flush_dcache_area() do? Flush data caches. IIUC, addr
> > > is virtual address at this point of time. While copying pages and
> > > walking through the list, I am assuming you have switched off page
> > > tables and you are in some kind of 1:1 physical mode. So how did
> > > flushing data caches related to a virtual address help. I guess we
> > > are not even accessing that virtual address now.
> > 
> > __flush_dcache_area(), and the underling aarch64 civac instruction
> > operate on virtual addresses.  Here we are still running with the
> > MMU on and the identity mapping has not yet been enabled.  This is
> > the sequence:
> > 
> >   flush dcache -> turn off MMU, dcache -> access memory (PoC) directly 
> 
> Sorry, I don't understand that why do we need to flush dcache for source
> and indirection page addresses. Some information here will help.

I think Mark answered this.  The architecture requires us to flush to
the point of coherency (PoC) anything that will be used after the
dcache is disabled.

For more info you can look at the section'D4.4.7 Cache maintenance
operations' in the ARMv8 Reference Manual you can get from here (after
registering):

  http://infocenter.arm.com/help/topic/com.arm.doc.subset.architecture.reference/index.html

-Geoff