Re: [Xen-devel] [PATCH] VMX: fix/adjust trap injection

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.11.15 at 16:01, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 23/11/15 12:49, Jan Beulich wrote:
>> @@ -1525,6 +1516,16 @@ static void vmx_inject_trap(struct hvm_t
>>              __restore_debug_registers(curr);
>>              write_debugreg(6, read_debugreg(6) | DR_STEP);
>>          }
>> +        if ( !nestedhvm_vcpu_in_guestmode(curr) ||
>> +             !nvmx_intercepts_exception(curr, TRAP_debug, _trap.error_code) 
>> )
>> +        {
>> +            unsigned long val;
>> +
>> +            __vmread(GUEST_DR7, &val);
>> +            __vmwrite(GUEST_DR7, val & ~DR_GENERAL_DETECT);
>> +            __vmread(GUEST_IA32_DEBUGCTL, &val);
>> +            __vmwrite(GUEST_IA32_DEBUGCTL, val & ~IA32_DEBUGCTLMSR_LBR);
> 
> Is it worth eliding the __vmwrite's if possible?  It will be fewer
> VMexits if this Xen is running nested, and ISTR it will avoid slowing
> down the vmentry with further consistency checks.

I've considered this, but then decided #DB shouldn't really be
raised on any fast paths (or such paths become non-fast anyway
due to the many exceptions). The code certainly is easier to read
without further conditionals.

Jan


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