Re: [Xen-devel] [PATCH 02/11] x86/vmx: Don't clobber %dr6 while debugging state is lazy

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.06.18 at 16:16, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 06/06/18 14:50, Jan Beulich wrote:
>>>>> On 04.06.18 at 15:59, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> --- a/xen/arch/x86/hvm/vmx/vmx.c
>>> +++ b/xen/arch/x86/hvm/vmx/vmx.c
>>> @@ -3696,6 +3696,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
>>>               */
>>>              __vmread(EXIT_QUALIFICATION, &exit_qualification);
>>>              HVMTRACE_1D(TRAP_DEBUG, exit_qualification);
>>> +            __restore_debug_registers(v);
>>>              write_debugreg(6, exit_qualification | DR_STATUS_RESERVED_ONE);
>> The change is certainly correct as is, but I'd still like to put out for
>> discussion the alternative option:
>>
>>     if ( v->arch.hvm_vcpu.flag_dr_dirty )
>>         write_debugreg(6, exit_qualification | DR_STATUS_RESERVED_ONE);
>>     else
>>         v->arch.debugreg[6] = exit_qualification | DR_STATUS_RESERVED_ONE;
>>
>> After all the guest may know it's single stepping, and may not care to
>> read DR6 at all.
> 
> All of this code changes across the series (so this specific suggestion
> is incorrect), but to the recommendation in general...

While I've not made it through the second half of the series yet,
another consideration: To avoid the double DR6 write, yet still avoid
an immediate further exit to restore debug registers, why not

    if ( v->arch.hvm_vcpu.flag_dr_dirty )
        write_debugreg(6, exit_qualification | DR_STATUS_RESERVED_ONE);
    else
    {
        v->arch.debugreg[6] = exit_qualification | DR_STATUS_RESERVED_ONE;
        __restore_debug_registers(v);
    }

Jan



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