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

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

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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