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.
