On Tue, 2022-01-04 at 22:25 +0000, Sean Christopherson wrote:
> On Mon, Dec 13, 2021, Maxim Levitsky wrote:
> > Apparently on some systems AVIC is disabled in CPUID but still usable.
> >
> > Allow the user to override the CPUID if the user is willing to
> > take the risk.
> >
> > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/svm/svm.c | 11 +++++++++--
> > 1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index c9668a3b51011..468cc385c35f0 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -206,6 +206,9 @@ module_param(tsc_scaling, int, 0444);
> > static bool avic;
> > module_param(avic, bool, 0444);
> >
> > +static bool force_avic;
> > +module_param_unsafe(force_avic, bool, 0444);
> > +
> > bool __read_mostly dump_invalid_vmcb;
> > module_param(dump_invalid_vmcb, bool, 0644);
> >
> > @@ -4656,10 +4659,14 @@ static __init int svm_hardware_setup(void)
> > nrips = false;
> > }
> >
> > - enable_apicv = avic = avic && npt_enabled && boot_cpu_has(X86_FEATURE_AVIC);
> > + enable_apicv = avic = avic && npt_enabled && (boot_cpu_has(X86_FEATURE_AVIC) || force_avic);
> >
> > if (enable_apicv) {
> > - pr_info("AVIC enabled\n");
> > + if (!boot_cpu_has(X86_FEATURE_AVIC)) {
> > + pr_warn("AVIC is not supported in CPUID but force enabled");
> > + pr_warn("Your system might crash and burn");
> > + } else
>
> Needs curly braces, though arguably the "AVIC enabled" part should be printed
> regardless of boot_cpu_has(X86_FEATURE_AVIC).
>
> > + pr_info("AVIC enabled\n");
>
> This is all more than a bit terrifying, though I can see the usefuless for a
> developer. At the very least, this should taint the kernel. This should also
> probably be buried behind a Kconfig that is itself buried behind EXPERT.
>
I used 'module_param_unsafe' which does taint the kernel.
Best regards,
Maxim Levitsky
