RE: [Xen-ia64-devel] RE: vcpu_translate issue

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

OK, I've checked in a fix incorporating Matt's discovery
and the vcpu_translate region 0 handling code I had earlier
backed out.  ltp-mmap09 now works properly.  Matt can you
confirm that your dom0 boot problem is also fixed?

Thanks,
Dan

> -----Original Message-----
> From: Magenheimer, Dan (HP Labs Fort Collins) 
> Sent: Friday, November 11, 2005 8:48 AM
> To: 'Matt Chapman'; Tian, Kevin
> Cc: xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-ia64-devel] RE: vcpu_translate issue
> 
> Good catch!  I'm guessing the solution will be for
> vcpu_itc (and fast_itc, though it is turned off
> by default) to handle this case by using the
> correct (rr0) rid when in metaphysical mode.
> 
> Dan
> 
> > -----Original Message-----
> > From: Matt Chapman [mailto:matthewc@xxxxxxxxxxxxxxx] 
> > Sent: Thursday, November 10, 2005 11:02 PM
> > To: Tian, Kevin
> > Cc: Magenheimer, Dan (HP Labs Fort Collins); 
> > xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
> > Subject: Re: [Xen-ia64-devel] RE: vcpu_translate issue
> > 
> > On Thu, Nov 10, 2005 at 05:02:52PM +0800, Tian, Kevin wrote:
> > > 
> > > >I think delivering a TLB miss is not sufficient.  Because the
> > > >mapping for the zero page is indeed in Linux's pagetable, the TLB
> > > >miss handler succeeds in finding and inserting it, and the page
> > > >fault handler is not triggered.
> > > 
> > > Actually that depends. Only when short format vhpt entry 
> > exists in TLB
> > > which contains that mapping for zero page, low level TLB 
> > miss handler
> > > can hit it. Or else the flow still goes to C code page 
> > fault handler,
> > > right? 
> > 
> > Yes, the C page fault handler is invoked if it's not in the 
> pagetable.
> > But as far as Linux is concerned, there is a valid read-only VMA at
> > address 0, so the C page fault handler won't raise a fault 
> on read, it
> > will simply insert a read-only mapping in the page table.
> > 
> > So, a write to the zero page will result in an access fault and
> > termination of the process.  A read from the zero page will succeed
> > (incorrectly).  But that doesn't explain why it now hangs there.
> > 
> > The more important problem seems to be that itc inserts 
> into the wrong
> > region when we're metaphysical, so the execution flow is like this:
> > 
> > 1. [rr0=X] guest accesses address 0 -> fault
> > 2. [rr0=X] Xen accesses VHPT at 0x1fffff000000000 -> deliver 
> > IA64_VHPT_FAULT
> > 3. guest switches to metaphysical to walk pagetable
> > 4. [rr0=mprid] guest inserts translation for 0 and 
> 0x1fffff0000000000
> > 5. guest returns to virtual mode (rfi)
> > 6. [rr0=X] goto 1
> > 
> > Matt
> > 
> > 
> 

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