Re: [PATCH v3 30/41] x86/paravirt: Don't use a PV sched_clock in CoCo guests with trusted TSC

[David Woodhouse]

On Fri, 2026-05-15 at 12:19 -0700, Sean Christopherson wrote:
> Silently ignore attempts to switch to a paravirt sched_clock when running
> as a CoCo guest with trusted TSC.  In hand-wavy theory, a misbehaving
> hypervisor could attack the guest by manipulating the PV clock to affect
> guest scheduling in some weird and/or predictable way.  More importantly,
> reading TSC on such platforms is faster than any PV clock, and sched_clock
> is all about speed.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

And kvmclock. And Xen.

Are there *any* reasons we'd use a PV sched_clock when the TSC is
usable?

Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

> ---
>  arch/x86/kernel/tsc.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 3c15fc10e501..ac4abfec1f05 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -283,6 +283,15 @@ bool using_native_sched_clock(void)
>  int __init __paravirt_set_sched_clock(u64 (*func)(void), bool stable,
>  				      void (*save)(void), void (*restore)(void))
>  {
> +	/*
> +	 * Don't replace TSC with a PV clock when running as a CoCo guest and
> +	 * the TSC is secure/trusted; PV clocks are emulated by the hypervisor,
> +	 * which isn't in the guest's TCB.
> +	 */
> +	if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC) ||
> +	    boot_cpu_has(X86_FEATURE_TDX_GUEST))
> +		return -EPERM;
> +
>  	if (!stable)
>  		clear_sched_clock_stable();
>

Attachment: smime.p7s
Description: S/MIME cryptographic signature