Re: [PATCH v16 10/13] x86/tsc: Switch Secure TSC guests away from kvm-clock

Nikunj A. Dadhania
2025-01-08T10:45:23Z

On 1/7/2025 8:46 PM, Borislav Petkov wrote:
> On Mon, Jan 06, 2025 at 06:16:30PM +0530, Nikunj A Dadhania wrote:
>> static int kvm_cs_enable(struct clocksource *cs)
>> {
>> + /*
>> + * TSC clocksource should be used for a guest with Secure TSC enabled,
>> + * taint the kernel and warn when the user changes the clocksource to
>> + * kvm-clock.
>> + */
>> + if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC)) {
>> + add_taint(TAINT_WARN, LOCKDEP_STILL_OK);
>> + WARN_ONCE(1, "For Secure TSC guest, changing the clocksource is not allowed!\n");
>
> So this thing is trying to state that changing the clocksource is not allowed
> but it still allows it. Why not simply do this:
>
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index 960260a8d884..d8fef3a65a35 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -151,14 +151,10 @@ bool kvm_check_and_clear_guest_paused(void)
>
> static int kvm_cs_enable(struct clocksource *cs)
> {
> - /*
> - * TSC clocksource should be used for a guest with Secure TSC enabled,
> - * taint the kernel and warn when the user changes the clocksource to
> - * kvm-clock.
> - */
> + /* Only the TSC should be used in a Secure TSC guest. */
> if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC)) {
> - add_taint(TAINT_WARN, LOCKDEP_STILL_OK);
> - WARN_ONCE(1, "For Secure TSC guest, changing the clocksource is not allowed!\n");
> + WARN_ONCE(1, "Secure TSC guest, changing the clocksource is not allowed!\n");
> + return 1;
> }
>
> vclocks_set_used(VDSO_CLOCKMODE_PVCLOCK);
>
> ?

Works as expected:

$ echo 'kvm-clock' > /sys/devices/system/clocksource/clocksource0/current_clocksource
[ 30.333603] ------------[ cut here ]------------
[ 30.334802] Secure TSC guest, changing the clocksource is not allowed!
[ 30.336460] WARNING: CPU: 0 PID: 19 at arch/x86/kernel/kvmclock.c:156 kvm_cs_enable+0x57/0x70

Regards
Nikunj