Re: [PATCH v2 3/8] x86/emul: Add pending_dbg field to x86_event

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.08.2023 17:26, Jinoh Kang wrote:
> From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> 
> All #DB exceptions result in an update of %dr6, but this isn't captured in
> Xen's handling.
> 
> PV guests generally work by modifying %dr6 before raising #DB, whereas HVM
> guests do nothing and have a single-step special case in the lowest levels of
> {vmx,svm}_inject_event().  All of this is buggy, but in particular, task
> switches with the trace flag never end up signalling BT in %dr6.
> 
> To begin resolving this issue, add a new pending_dbg field to x86_event
> (unioned with cr2 to avoid taking any extra space), and introduce
> {pv,hvm}_inject_debug_exn() helpers to replace the current callers using
> {pv,hvm}_inject_hw_exception().
> 
> A key property is that pending_dbg is taken with positive polarity to deal
> with RTM sensibly.  Most callers pass in a constant, but callers passing in a
> hardware %dr6 value need to xor the value with X86_DR6_DEFAULT to flip the
> polarity of RTM and reserved fields.
> 
> For PV guests, move the ad-hoc updating of %dr6 into pv_inject_event().  This
> in principle breaks the handing of RTM in do_debug(), but PV guests can't
> actually enable MSR_DEBUGCTL.RTM yet, so this doesn't matter in practice.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> [ jinoh: Rebase onto staging, forward declare struct vcpu ]
> Signed-off-by: Jinoh Kang <jinoh.kang.kr@xxxxxxxxx>
> ---
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> CC: Tim Deegan <tim@xxxxxxx>
> CC: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
> CC: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
> CC: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>
> 
> v1 -> v2: [S-o-b fixes. More details below.]
> 
> - Update DR6 for gdbsx when trapped in PV guest kernel mode

I take it that this refers to ...

> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -1989,17 +1989,17 @@ void do_debug(struct cpu_user_regs *regs)
>          return;
>      }
>  
> -    /* Save debug status register where guest OS can peek at it */
> -    v->arch.dr6 |= (dr6 & ~X86_DR6_DEFAULT);
> -    v->arch.dr6 &= (dr6 | ~X86_DR6_DEFAULT);
> -
>      if ( guest_kernel_mode(v, regs) && v->domain->debugger_attached )
>      {
> +        /* Save debug status register where gdbsx can peek at it */
> +        v->arch.dr6 |= (dr6 & ~X86_DR6_DEFAULT);
> +        v->arch.dr6 &= (dr6 | ~X86_DR6_DEFAULT);
> +
>          domain_pause_for_debugger();
>          return;
>      }

... this code movement. I'm afraid this should have resulted in you
dropping the earlier R-b, and I'm further afraid I'm not convinced 
this is correct, despite seeing why you would want to do this. The
issue is that this way you also alter guest-visible state, when the
intention is that such now happen via ...

> -    pv_inject_hw_exception(X86_EXC_DB, X86_EVENT_NO_EC);
> +    pv_inject_debug_exn(dr6 ^ X86_DR6_DEFAULT);
>  }

... this call alone. I fear I can't currently see how to get both
aspects right, other than by breaking up 

Jan