Re: [PATCH] Fixed KVM problems with old DOS programs. Compatibility can be forced by module parameter.

[Gerhard Wiesinger <lists@wiesinger.com>]

On 04.11.2015 22:27, Paolo Bonzini wrote:
>
> On 04/11/2015 20:33, Gerhard Wiesinger wrote:
>> Signed-off-by: Gerhard Wiesinger <lists@wiesinger.com>
>> ---
>>   arch/x86/kvm/svm.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index 2f9ed1f..e0b00fc 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -198,6 +198,10 @@ static bool npt_enabled;
>>   static int npt = true;
>>   module_param(npt, int, S_IRUGO);
>>   +/* allow backward compatibility with e.g. old DOS application */
>> +static int npt_task_switch_emulation = true;
>> +module_param(npt_task_switch_emulation, int, S_IRUGO);
>> +
>>   /* allow nested virtualization in KVM/SVM */
>>   static int nested = true;
>>   module_param(nested, int, S_IRUGO);
>> @@ -1177,6 +1181,9 @@ static void init_vmcb(struct vcpu_svm *svm, bool
>> init_event)
>>       if (npt_enabled) {
>>           /* Setup VMCB for Nested Paging */
>>           control->nested_ctl = 1;
>> +        if (!npt_task_switch_emulation) {
>> +            clr_intercept(svm, INTERCEPT_TASK_SWITCH);
>> +        }
>>           clr_intercept(svm, INTERCEPT_INVLPG);
>>           clr_exception_intercept(svm, PF_VECTOR);
>>           clr_cr_intercept(svm, INTERCEPT_CR3_READ);
> What is the problem you are seeing?  KVM can emulate task switches; the
> intercept is set here because of a processor erratum that can mess them
> up even though, in theory, AMD supports task switching from guest mode.

See old thread:
https://lists.nongnu.org/archive/html/qemu-devel/2012-04/msg01506.html

Ciao,
Gerhard