[PATCH v3] x86/sev: Add support for allowing zero SEV ASIDs.

[Ashish Kalra <Ashish.Kalra@amd.com>]

From: Ashish Kalra <ashish.kalra@amd.com>

Some BIOSes allow the end user to set the minimum SEV ASID value
(CPUID 0x8000001F_EDX) to be greater than the maximum number of
encrypted guests, or maximum SEV ASID value (CPUID 0x8000001F_ECX)
in order to dedicate all the SEV ASIDs to SEV-ES or SEV-SNP.

The SEV support, as coded, does not handle the case where the minimum
SEV ASID value can be greater than the maximum SEV ASID value.
As a result, the following confusing message is issued:

[   30.715724] kvm_amd: SEV enabled (ASIDs 1007 - 1006)

Fix the support to properly handle this case.

Fixes: 916391a2d1dc ("KVM: SVM: Add support for SEV-ES capability in KVM")
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Cc: stable@vger.kernel.org
---
 arch/x86/kvm/svm/sev.c | 40 ++++++++++++++++++++++++----------------
 1 file changed, 24 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 4900c078045a..2112c94bac76 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -143,8 +143,20 @@ static void sev_misc_cg_uncharge(struct kvm_sev_info *sev)
 
 static int sev_asid_new(struct kvm_sev_info *sev)
 {
-	int asid, min_asid, max_asid, ret;
+	/*
+	 * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
+	 * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.
+	 * Note: min ASID can end up larger than the max if basic SEV support is
+	 * effectively disabled by disallowing use of ASIDs for SEV guests.
+	 */
+	unsigned int min_asid = sev->es_active ? 1 : min_sev_asid;
+	unsigned int max_asid = sev->es_active ? min_sev_asid - 1 : max_sev_asid;
+	unsigned int asid;
 	bool retry = true;
+	int ret;
+
+	if (min_asid > max_asid)
+		return -ENOTTY;
 
 	WARN_ON(sev->misc_cg);
 	sev->misc_cg = get_current_misc_cg();
@@ -157,12 +169,6 @@ static int sev_asid_new(struct kvm_sev_info *sev)
 
 	mutex_lock(&sev_bitmap_lock);
 
-	/*
-	 * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
-	 * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.
-	 */
-	min_asid = sev->es_active ? 1 : min_sev_asid;
-	max_asid = sev->es_active ? min_sev_asid - 1 : max_sev_asid;
 again:
 	asid = find_next_zero_bit(sev_asid_bitmap, max_asid + 1, min_asid);
 	if (asid > max_asid) {
@@ -246,21 +252,20 @@ static void sev_unbind_asid(struct kvm *kvm, unsigned int handle)
 static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
 {
 	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
-	int asid, ret;
+	int ret;
 
 	if (kvm->created_vcpus)
 		return -EINVAL;
 
-	ret = -EBUSY;
 	if (unlikely(sev->active))
-		return ret;
+		return -EINVAL;
 
 	sev->active = true;
 	sev->es_active = argp->id == KVM_SEV_ES_INIT;
-	asid = sev_asid_new(sev);
-	if (asid < 0)
+	ret = sev_asid_new(sev);
+	if (ret < 0)
 		goto e_no_asid;
-	sev->asid = asid;
+	sev->asid = ret;
 
 	ret = sev_platform_init(&argp->error);
 	if (ret)
@@ -2229,8 +2234,10 @@ void __init sev_hardware_setup(void)
 		goto out;
 	}
 
-	sev_asid_count = max_sev_asid - min_sev_asid + 1;
-	WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count));
+	if (min_sev_asid <= max_sev_asid) {
+		sev_asid_count = max_sev_asid - min_sev_asid + 1;
+		WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count));
+	}
 	sev_supported = true;
 
 	/* SEV-ES support requested? */
@@ -2261,7 +2268,8 @@ void __init sev_hardware_setup(void)
 out:
 	if (boot_cpu_has(X86_FEATURE_SEV))
 		pr_info("SEV %s (ASIDs %u - %u)\n",
-			sev_supported ? "enabled" : "disabled",
+			sev_supported ? (min_sev_asid <= max_sev_asid ?  "enabled" : "unusable")
+			: "disabled",
 			min_sev_asid, max_sev_asid);
 	if (boot_cpu_has(X86_FEATURE_SEV_ES))
 		pr_info("SEV-ES %s (ASIDs %u - %u)\n",
-- 
2.34.1