Re: [Xen-devel] Single step in HVM domU on Intel machine may see wrong DB6

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.02.14 at 06:15, "Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx> wrote:
> @@ -2690,9 +2688,13 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
>              __vmread(EXIT_QUALIFICATION, &exit_qualification);
>              HVMTRACE_1D(TRAP_DEBUG, exit_qualification);
>              write_debugreg(6, exit_qualification | 0xffff0ff0);
> -            if ( !v->domain->debugger_attached || cpu_has_monitor_trap_flag )
> -                goto exit_and_crash;
> -            domain_pause_for_debugger();
> +            if ( v->domain->debugger_attached )
> +                domain_pause_for_debugger();
> +            else 
> +            {
> +                __restore_debug_registers(v);
> +                hvm_inject_hw_exception(TRAP_debug, 
> HVM_DELIVER_NO_ERROR_CODE);
> +            }

I suppose you need to set DR6.BS after restoring the reigsters?

Also, the change looks rather simple - is that really correct for both
cpu_has_monitor_trap_flag and !cpu_has_monitor_trap_flag cases?

> BTW: I also think we should clear the CPU_BASED_MOV_DR_EXITING bit in 
> __restore_debug_registers(). After restore the debug register, we should not 
> trap any DR access unless the VCPU is scheduled out again. Not sure whether I 
> am wrong.
> 
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index b128e81..56a3140 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -394,6 +394,8 @@ static void __restore_debug_registers(struct vcpu *v)
>      write_debugreg(3, v->arch.debugreg[3]);
>      write_debugreg(6, v->arch.debugreg[6]);
>      /* DR7 is loaded from the VMCS. */
> +    v->arch.hvm_vmx.exec_control &= ~CPU_BASED_MOV_DR_EXITING;
> +    vmx_update_cpu_exec_control(v);
>  }
>  
>  /*

That's being done by at least one of its callers (vmx_dr_access())
already, and I think it was purposefully not done in other cases.

Jan


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