Re: [Xen-devel] [PATCH v3 11/24] x86/emul: Implement singlestep as a retire flag

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.12.16 at 12:23, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 01/12/16 11:16, Jan Beulich wrote:
>>>>> On 30.11.16 at 14:50, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> @@ -3422,6 +3422,16 @@ static int sh_page_fault(struct vcpu *v,
>>>          v->arch.paging.last_write_emul_ok = 0;
>>>  #endif
>>>  
>>> +    if ( emul_ctxt.ctxt.retire.singlestep )
>>> +    {
>>> +        if ( is_hvm_vcpu(v) )
>>> +            hvm_inject_hw_exception(TRAP_debug, X86_EVENT_NO_EC);
>>> +        else
>>> +            pv_inject_hw_exception(TRAP_debug, X86_EVENT_NO_EC);
>>> +
>>> +        goto emulate_done;
>> This results in skipping the PAE special code (which I think is intended)
> 
> Correct
> 
>> but also the trace_shadow_emulate(), which I don't think is wanted.
> 
> Hmm.  It is only the PAE case we want to skip.  Perhaps changing the PAE
> entry condition to
> 
> diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
> index c45d260..28ff945 100644
> --- a/xen/arch/x86/mm/shadow/multi.c
> +++ b/xen/arch/x86/mm/shadow/multi.c
> @@ -3480,7 +3480,7 @@ static int sh_page_fault(struct vcpu *v,
>      }
>  
>  #if GUEST_PAGING_LEVELS == 3 /* PAE guest */
> -    if ( r == X86EMUL_OKAY ) {
> +    if ( r == X86EMUL_OKAY && !emul_ctxt.ctxt.retire.raw ) {
>          int i, emulation_count=0;
>          this_cpu(trace_emulate_initial_va) = va;
>          /* Emulate up to four extra instructions in the hope of catching
> 
> would be better, along with suitable comments and style fixes?

Yes I think so (and I see Tim has said so too).

>>> @@ -5415,11 +5414,11 @@ x86_emulate(
>>>      if ( !mode_64bit() )
>>>          _regs.eip = (uint32_t)_regs.eip;
>>>  
>>> -    *ctxt->regs = _regs;
>>> +    /* Was singestepping active at the start of this instruction? */
>>> +    if ( (rc == X86EMUL_OKAY) && (ctxt->regs->eflags & EFLG_TF) )
>>> +        ctxt->retire.singlestep = true;
>> Don't we need to avoid doing this when mov_ss is set? Or does the
>> hardware perhaps do the necessary deferring for us?
> 
> I am currently reading up about this in the manual.

Tell me if you find anything. All I have is my memory of good old
DOS days, where I recall single stepping over %ss loads always
surprised me (over time of course with a fading level of surprise)
in taking two steps. The only thing I can't tell for sure is whether
this maybe was a cute feature of the debugger (recognizing the
%ss load instructions).

Jan


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