Re: [PATCH v2] x86/sev: Add support for allowing zero SEV ASIDs.

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On 1/3/2024 11:28 PM, Tom Lendacky wrote:

On 1/3/24 20:46, Ashish Kalra wrote:
From: Ashish Kalra <ashish.kalra@xxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
  arch/x86/kvm/svm/sev.c | 41 +++++++++++++++++++++++++----------------
  1 file changed, 25 insertions(+), 16 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 4900c078045a..651d671ff8ae 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -143,8 +143,21 @@ 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;
+

Remove this blank line.

      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 +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) {
@@ -246,21 +253,20 @@ 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 +2235,10 @@ 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) {

Shouldn't this be: if (min_sev_asid <= max_sev_asid) ?

You only want to do the misc_cg_set_capactity() call if you can have SEV guests.


Yes, it should be, will fix it and post v3.

Thanks, Ashish

+        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 +2269,8 @@ 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",




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