Re: [Xen-devel] [PATCH v4 8/9] xen/arm: Implement hypercall for dirty page tracing

[Eugene Fedotov <e.fedotov@xxxxxxxxxxx>]

07.10.2013 17:02, Julien Grall:
> I think checking !page_fault is nearly everywhere is error-prone when
> this function will be modified.
>
> Can you do something like this?
>
> if ( page_fault )
>    // Your code to handle page fault
> else
> {
>    // handle_mmio
> }
>
> It will avoid && !page_fault.
Yes, but I think at page fault condition we should check whether address 
belong MMIO regions (page fault at MMIO addr is possible, but we don't 
need that memory to transfer)
> > >          goto bad_data_abort;
> > >  
> > >      rc = gva_to_ipa(info.gva, &info.gpa);
> > >-    if ( rc == -EFAULT )
> > >+    if ( rc == -EFAULT && !page_fault )
> > >          goto bad_data_abort;
> > >  
> > >      /* XXX: Decode the instruction if ISS is not valid */
> > >-    if ( !dabt.valid )
> > >+    if ( !dabt.valid && !page_fault )
> > >          goto bad_data_abort;
> > >  
> > >      /*
> > >       * Erratum 766422: Thumb store translation fault to Hypervisor may
> > >       * not have correct HSR Rt value.
> > >       */
> > >-    if ( cpu_has_erratum_766422() && (regs->cpsr & PSR_THUMB) && dabt.write )
> > >+    if ( cpu_has_erratum_766422() && (regs->cpsr & PSR_THUMB) && dabt.write
> > >+            && !page_fault)
> > >      {
> > >          rc = decode_instruction(regs, &info.dabt);
> > >          if ( rc )
> > >@@ -1358,6 +1361,16 @@ static void do_trap_data_abort_guest(struct 
> > cpu_user_regs *regs,
> > >          return;
> > >      }
> > >  
> > >+    /* handle permission fault on write */
> > >+    if ( page_fault )
> > >+    {
> > >+        if ( current->domain->arch.dirty.mode == 0 )
> > >+           goto bad_data_abort;
> > >+
> > >+        handle_page_fault(current->domain, info.gpa);
> You must call advance_pc(regs, hsr) here.
Let me explain. I think the difference between handle_page_fault and 
handle_mmio is that the ongoing memory operation (that trapped here) 
should be repeated after handle_page_fault (the page fault handler 
clears read-only bit), but should be stepped out after handle_mmio (to 
do this we call advance_pc to increase the pc register). So, advance_pc 
is intentionally missed.
Best regards,
Evgeny.

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