RE: [PATCH v5 47/51] x86/paravirt: Don't use a PV sched_clock in CoCo guests with trusted TSC

[Michael Kelley <mhklinux@xxxxxxxxxxx>]

From: Sean Christopherson <seanjc@xxxxxxxxxx> Sent: Wednesday, July 1, 2026 
12:32 PM
> 
> 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.
> 
> Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  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 012321fed5e5..a146fc7b5e74 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;

Do a pr_warn() in the error case? Your commit message says to
do the ignore silently, but I wonder if that's a good idea. At least
for Hyper-V, the error case shouldn't happen.

Michael

> +
>       if (!stable)
>               clear_sched_clock_stable();
> 
> --
> 2.55.0.rc0.799.gd6f94ed593-goog
>