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

Borislav Petkov
2025-01-07T15:17:35Z

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);

?

--
Regards/Gruss,
Boris.