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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo