linux-kernel - [PATCH 2/2] KVM: SVM: Allow SNP guest policy to specify SINGLE

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Message-ID: <4c51018dd3e4f2c543935134d2c4f47076f109f6.1748553480.git.thomas.lendacky@amd.com>
Date: Thu, 29 May 2025 16:18:00 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>
CC: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
	<seanjc@...gle.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, "Thomas
 Gleixner" <tglx@...utronix.de>, Michael Roth <michael.roth@....com>
Subject: [PATCH 2/2] KVM: SVM: Allow SNP guest policy to specify SINGLE_SOCKET

KVM currently returns -EINVAL when it attempts to create an SNP guest if
the SINGLE_SOCKET guest policy bit is set. The reason for this action is
that KVM would need specific support (SNP_ACTIVATE_EX command support) to
achieve this when running on a system with more than one socket. However,
the SEV firmware will make the proper check and return POLICY_FAILURE
during SNP_ACTIVATE if the single socket guest policy bit is set and the
system has more than one socket:

 - System with one socket
   - Guest policy SINGLE_SOCKET == 0 ==> SNP_ACTIVATE succeeds
   - Guest policy SINGLE_SOCKET == 1 ==> SNP_ACTIVATE succeeds

 - System with more than one socket
   - Guest policy SINGLE_SOCKET == 0 ==> SNP_ACTIVATE succeeds
   - Guest policy SINGLE_SOCKET == 1 ==> SNP_ACTIVATE fails with
     POLICY_FAILURE

Remove the check for the SINGLE_SOCKET policy bit from snp_launch_start()
and allow the firmware to perform the proper checking.

This does have the effect of allowing an SNP guest with the SINGLE_SOCKET
policy bit set to run on a single socket system, but fail when run on a
system with more than one socket. However, this should not affect existing
SNP guests as setting the SINGLE_SOCKET policy bit is not allowed today.

Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
---
 arch/x86/kvm/svm/sev.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 77eb036cd6d4..4802edfc5d9e 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2197,9 +2197,6 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	if (!(params.policy & SNP_POLICY_MASK_RSVD_MBO))
 		return -EINVAL;

-	if (params.policy & SNP_POLICY_MASK_SINGLE_SOCKET)
-		return -EINVAL;
-
 	sev->policy = params.policy;

 	sev->snp_context = snp_context_create(kvm, argp);
-- 
2.46.2