KVM: SVM: Add support for allowing zero SEV ASIDs

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
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240104190520.62510-1-Ashish.Kalra@amd.com
Link: https://lore.kernel.org/r/20240131235609.4161407-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index eeef43c795d8dd11eed3e8668fee9774d87ac305..5f8312edee36b1a2385de032114c63f4209494af 100644 (file)
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -144,10 +144,21 @@ static void sev_misc_cg_uncharge(struct kvm_sev_info *sev)
 
 static int sev_asid_new(struct kvm_sev_info *sev)
 {
-       unsigned int asid, min_asid, max_asid;
+       /*
+        * 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();
        ret = sev_misc_cg_try_charge(sev);
@@ -159,12 +170,6 @@ 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) {
@@ -2234,8 +2239,10 @@ 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? */
@@ -2266,7 +2273,9 @@ 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",