Re: [PATCH v1 1/8] x86/sev: Make SVSM calls preemption-safe

[Borislav Petkov <bp@alien8.de>]

+ folks who added the vtpm stuff.

On Sat, Aug 29, 2026 at 03:39:39AM +0000, Melody Wang wrote:
> Two functions in the the SVSM vTPM guest implementation do not disable
> preemption when fetching CAA.

"	... when fetching the SVSM Calling Area Address (CAA).

The SVSM CAA..."

This way, you introduce what CAA means and then use the abbreviation.

> The SVSM calling area is a per-CPU structure. When a thread is preempted
> and migrated to a different CPU after fetching the per-CPU Calling Area
> Address (CAA), the SVSM call will execute on the new CPU with the

and here you can use "CAA" directly because you've explained it above.

> original CPU's CAA. Which is wrong.
> 
> Move the CAA fetching operation inside svsm_perform_call_protocol()
> which disables interrupts around the SVSM call and thus runs
> preemption-safe.
> 
> Signed-off-by: Melody Wang <huibo.wang@amd.com>

Should this be CC:stable so that it gets backported?

I think we want to do that and mark it with

Fixes: 770de678bc28 ("x86/sev: Add SVSM vTPM probe/send_command functions")

> ---
>  arch/x86/coco/sev/svsm.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/coco/sev/svsm.c b/arch/x86/coco/sev/svsm.c
> index 916d62cd17dc..92ea93f506c0 100644
> --- a/arch/x86/coco/sev/svsm.c
> +++ b/arch/x86/coco/sev/svsm.c
> @@ -74,6 +74,15 @@ int svsm_perform_call_protocol(struct svsm_call *call)
>  
>  	flags = native_local_irq_save();
>  
> +	/*
> +	 * 'caa' is a per-CPU variable. To avoid using a stale or incorrect
> +	 * 'caa' if the task is preempted or migrates to another CPU after it

"... or migrated to another CPU after the CAA has been fetched..."

> +	 * is fetched, always fetch 'caa' and then issue the SVSM call with
> +	 * interrupts disabled. This ensures the correct 'caa' is used even
> +	 * under preemption or CPU migration.

s/even under preemption or CPU migration//

That's kinda clear.

Leaving in the rest for the newly CCed.

> +	 */
> +	call->caa = svsm_get_caa();
> +
>  	ghcb = __sev_get_ghcb(&state);
>  
>  	do {
> @@ -321,7 +330,6 @@ int snp_svsm_vtpm_send_command(u8 *buffer)
>  {
>  	struct svsm_call call = {};
>  
> -	call.caa = svsm_get_caa();
>  	call.rax = SVSM_VTPM_CALL(SVSM_VTPM_CMD);
>  	call.rcx = __pa(buffer);
>  
> @@ -345,7 +353,6 @@ bool snp_svsm_vtpm_probe(void)
>  	if (!snp_vmpl)
>  		return false;
>  
> -	call.caa = svsm_get_caa();
>  	call.rax = SVSM_VTPM_CALL(SVSM_VTPM_QUERY);
>  
>  	if (svsm_perform_call_protocol(&call))
> -- 
> 2.43.0
> 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette