Re: [PATCH 01/11] x86/snp: drop support for SNP hotplug

[Borislav Petkov]

On Mon, Mar 02, 2026 at 12:13:24PM -0700, Tycho Andersen wrote:
> From: "Tycho Andersen (AMD)" <tycho@xxxxxxxxxx>
> 
> During an SNP_INIT(_EX), the SEV firmware checks that all CPUs have SNPEn

Please write those in a human-readable form - not as code in text. Commit
messages are still predominantly read by humans.

:)

> set, and fails if they do not. As such, it does not make sense to have
> offline CPUs: the firmware will fail initialization because of the offlined
> ones that the kernel did not initialize.
> 
> Futher, there is a bug: SNP_INIT(_EX) require MFDM to be set in addition to
> SNPEn which the previous hotplug code did not do. Since
> k8_check_syscfg_dram_mod_en() enforces this be cleared, hotplug wouldn't
> work.
> 
> Drop the hotplug code. Collapse the __{mfd,snp}__enable() wrappers into
> their non-__ versions, since the cpu number argument is no longer needed.

Please, do not talk about *what* the patch is doing in the commit message
- that should be obvious from the diff itself. Rather, concentrate on the
*why* it needs to be done and why your patch exists.

> Signed-off-by: Tycho Andersen (AMD) <tycho@xxxxxxxxxx>
> ---
>  arch/x86/virt/svm/sev.c | 24 ++++--------------------
>  1 file changed, 4 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index a4f3a364fb65..1446011c6337 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c
> @@ -130,33 +130,26 @@ static unsigned long snp_nr_leaked_pages;
>  #undef pr_fmt
>  #define pr_fmt(fmt)	"SEV-SNP: " fmt
>  
> -static int __mfd_enable(unsigned int cpu)
> +static __init void mfd_enable(void *arg)
>  {
>  	u64 val;
>  
>  	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
> -		return 0;
> +		return;
>  
>  	rdmsrq(MSR_AMD64_SYSCFG, val);
>  
>  	val |= MSR_AMD64_SYSCFG_MFDM;
>  
>  	wrmsrq(MSR_AMD64_SYSCFG, val);
> -
> -	return 0;
>  }

While at it:

diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index 1446011c6337..f404c609582c 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -132,16 +132,10 @@ static unsigned long snp_nr_leaked_pages;
 
 static __init void mfd_enable(void *arg)
 {
-	u64 val;
-
 	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
 		return;
 
-	rdmsrq(MSR_AMD64_SYSCFG, val);
-
-	val |= MSR_AMD64_SYSCFG_MFDM;
-
-	wrmsrq(MSR_AMD64_SYSCFG, val);
+	msr_set_bit(MSR_AMD64_SYSCFG, MSR_AMD64_SYSCFG_MFDM_BIT);
 }
 
 static __init void snp_enable(void *arg)

-- 
Regards/Gruss,
    Boris.

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