Re: [PATCH v8 3/7] crypto/ccp: Disable CPU hotplug while SNP is active

[K Prateek Nayak]

Hello Ashish,

On 6/16/2026 1:19 AM, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
> 
> The SEV firmware enumerates the CPUs at SNP initialization and is not
> aware of the OS bringing CPUs online or offline afterwards, so OS CPU
> hotplug can diverge from the firmware's expectations and break SNP.
> Disable CPU hotplug while SNP is active.

Dumb question: Is this specific to RMPOPT? Otherwise ...

> 
> SNP is fully torn down only on the SNP_SHUTDOWN_EX x86_snp_shutdown
> path; the legacy path leaves SNP enabled in hardware while clearing
> snp_initialized, so __sev_snp_init_locked() can run again.  Track the
> disable with a flag so it is balanced by a matching enable rather than
> stacked, and re-enable hotplug only on the x86_snp_shutdown path, after
> snp_shutdown() has cleared the per-core RMPOPT_BASE MSRs with hotplug
> still disabled.
> 
> This also keeps the CPU set stable for the asynchronous RMPOPT scan
> added later in this series, and ensures cpus_read_lock() in the scan
> is uncontended.
> 
> Suggested-by: Thomas Lendacky <thomas.lendacky@xxxxxxx>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> ---
>  drivers/crypto/ccp/sev-dev.c | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> index 217b6b19802e..c8c3c577463c 100644
> --- a/drivers/crypto/ccp/sev-dev.c
> +++ b/drivers/crypto/ccp/sev-dev.c
> @@ -106,6 +106,9 @@ struct snp_hv_fixed_pages_entry {
>  
>  static LIST_HEAD(snp_hv_fixed_pages);
>  
> +/* Set while SNP has CPU hotplug disabled. */
> +static bool snp_cpu_hotplug_disabled;
> +
>  /* Trusted Memory Region (TMR):
>   *   The TMR is a 1MB area that must be 1MB aligned.  Use the page allocator
>   *   to allocate the memory, which will return aligned memory for the specified
> @@ -1479,6 +1482,17 @@ static int __sev_snp_init_locked(int *error, unsigned int max_snp_asid)
>  
>  	snp_hv_fixed_pages_state_update(sev, HV_FIXED);
>  
> +	/*
> +	 * Disable CPU hotplug while SNP is active.  Guard against stacking
> +	 * the disable count: the legacy SNP_SHUTDOWN_EX path clears
> +	 * snp_initialized without re-enabling hotplug, so this can run
> +	 * again while hotplug is already disabled.
> +	 */
> +	if (!snp_cpu_hotplug_disabled) {
> +		cpu_hotplug_disable();
> +		snp_cpu_hotplug_disabled = true;
> +	}
> +

... should this be done before __sev_do_cmd_locked(SEV_CMD_SNP_INIT_EX)
is issued?

I'm assuming that is when the firmware enumerates the CPUs during SNP
initialization and any hotplug after that should be disallowed?

>  	snp_setup_rmpopt();
>  
>  	sev->snp_initialized = true;
-- 
Thanks and Regards,
Prateek