Re: [PATCH v3 6/6] KVM: TDX: Explicitly do WBINVD upon reboot notifier

[Binbin Wu]

On 6/26/2025 6:48 PM, Kai Huang wrote:
> On TDX platforms, during kexec, the kernel needs to make sure there's no
> dirty cachelines of TDX private memory before booting to the new kernel
> to avoid silent memory corruption to the new kernel.
>
> During kexec, the kexec-ing CPU firstly invokes native_stop_other_cpus()
> to stop all remote CPUs before booting to the new kernel.  The remote
> CPUs will then execute stop_this_cpu() to stop themselves.
>
> The kernel has a percpu boolean to indicate whether the cache of a CPU
> may be in incoherent state.  In stop_this_cpu(), the kernel does WBINVD
> if that percpu boolean is true.
>
> TDX turns on that percpu boolean on a CPU when the kernel does SEAMCALL.
> This makes sure the caches will be flushed during kexec.
>
> However, the native_stop_other_cpus() and stop_this_cpu() have a "race"
> which is extremely rare to happen but could cause system to hang.
>
> Specifically, the native_stop_other_cpus() firstly sends normal reboot
> IPI to remote CPUs and wait one second for them to stop.  If that times
> out, native_stop_other_cpus() then sends NMIs to remote CPUs to stop
> them.
>
> The aforementioned race happens when NMIs are sent.  Doing WBINVD in
> stop_this_cpu() makes each CPU take longer time to stop and increases
> the chance of the race to happen.
>
> Register reboot notifier in KVM to explicitly flush caches upon
> receiving reboot notifier (e.g., during kexec) for TDX.  This moves the
> WBINVD to an earlier stage than stop_this_cpus(), avoiding a possibly
> lengthy operation at a time where it could cause this race.
Two nits below.

Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

> Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
> Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Tested-by: Farrah Chen <farrah.chen@xxxxxxxxx>
> ---
>
> v2 -> v3:
>   - Update changelog to address Paolo's comments and Add Paolo's Ack:
>     https://lore.kernel.org/lkml/3a7c0856-6e7b-4d3d-b966-6f17f1aca42e@xxxxxxxxxx/
>
> ---
>   arch/x86/include/asm/tdx.h  |  3 +++
>   arch/x86/kvm/vmx/tdx.c      | 45 +++++++++++++++++++++++++++++++++++++
>   arch/x86/virt/vmx/tdx/tdx.c |  9 ++++++++
>   3 files changed, 57 insertions(+)
[...]
> +
> +static int tdx_reboot_notify(struct notifier_block *nb, unsigned long code,
> +			     void *unused)
> +{
> +	/*
> +	 * Flush cache for all CPUs upon the reboot notifier.  This
> +	 * avoids having to do WBINVD in stop_this_cpu() during kexec.
> +	 *
> +	 * Kexec calls native_stop_other_cpus() to stop remote CPUs
> +	 * before booting to new kernel, but that code has a "race"
> +	 * when the normal REBOOT IPI timesout and NMIs are sent to

timesout should be times out or timeouts?

> +	 * remote CPUs to stop them.  Doing WBINVD in stop_this_cpu()
> +	 * could potentially increase the posibility of the "race".
s/posibility/possibility

> +	 */
> +	if (code == SYS_RESTART)
> +		on_each_cpu(smp_func_cpu_flush_cache, NULL, 1);
> +	return NOTIFY_DONE;
> +}
> +
[...]