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

[Borislav Petkov]

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.

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