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

[Eugene Fedotov <e.fedotov@samsung.com>]

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.