KVM: SVM: improve the code readability for ASID management

KVM SEV code uses bitmaps to manage ASID states. ASID 0 was always skipped
because it is never used by VM. Thus, in existing code, ASID value and its
bitmap postion always has an 'offset-by-1' relationship.

Both SEV and SEV-ES shares the ASID space, thus KVM uses a dynamic range
[min_asid, max_asid] to handle SEV and SEV-ES ASIDs separately.

Existing code mixes the usage of ASID value and its bitmap position by
using the same variable called 'min_asid'.

Fix the min_asid usage: ensure that its usage is consistent with its name;
allocate extra size for ASID 0 to ensure that each ASID has the same value
with its bitmap position. Add comments on ASID bitmap allocation to clarify
the size change.
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Marc Orr <marcorr@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Alper Gun <alpergun@google.com>
Cc: Dionna Glaze <dionnaglaze@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Vipin Sharma <vipinsh@google.com>
Cc: Peter Gonda <pgonda@google.com>
Cc: Joerg Roedel <joro@8bytes.org>
Message-Id: <20210802180903.159381-1-mizhang@google.com>
[Fix up sev_asid_free to also index by ASID, as suggested by Sean
 Christopherson, and use nr_asids in sev_cpu_init. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -64,6 +64,7 @@ static DEFINE_MUTEX(sev_bitmap_lock);
 unsigned int max_sev_asid;
 static unsigned int min_sev_asid;
 static unsigned long sev_me_mask;
+static unsigned int nr_asids;
 static unsigned long *sev_asid_bitmap;
 static unsigned long *sev_reclaim_asid_bitmap;
 
@@ -78,11 +79,11 @@ struct enc_region {
 /* Called with the sev_bitmap_lock held, or on shutdown  */
 static int sev_flush_asids(int min_asid, int max_asid)
 {
-	int ret, pos, error = 0;
+	int ret, asid, error = 0;
 
 	/* Check if there are any ASIDs to reclaim before performing a flush */
-	pos = find_next_bit(sev_reclaim_asid_bitmap, max_asid, min_asid);
-	if (pos >= max_asid)
+	asid = find_next_bit(sev_reclaim_asid_bitmap, nr_asids, min_asid);
+	if (asid > max_asid)
 		return -EBUSY;
 
 	/*
@@ -115,15 +116,15 @@ static bool __sev_recycle_asids(int min_asid, int max_asid)
 
 	/* The flush process will flush all reclaimable SEV and SEV-ES ASIDs */
 	bitmap_xor(sev_asid_bitmap, sev_asid_bitmap, sev_reclaim_asid_bitmap,
-		   max_sev_asid);
-	bitmap_zero(sev_reclaim_asid_bitmap, max_sev_asid);
+		   nr_asids);
+	bitmap_zero(sev_reclaim_asid_bitmap, nr_asids);
 
 	return true;
 }
 
 static int sev_asid_new(struct kvm_sev_info *sev)
 {
-	int pos, min_asid, max_asid, ret;
+	int asid, min_asid, max_asid, ret;
 	bool retry = true;
 	enum misc_res_type type;
 
@@ -143,11 +144,11 @@ static int sev_asid_new(struct kvm_sev_info *sev)
 	 * 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 ? 0 : 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:
-	pos = find_next_zero_bit(sev_asid_bitmap, max_sev_asid, min_asid);
-	if (pos >= max_asid) {
+	asid = find_next_zero_bit(sev_asid_bitmap, max_asid + 1, min_asid);
+	if (asid > max_asid) {
 		if (retry && __sev_recycle_asids(min_asid, max_asid)) {
 			retry = false;
 			goto again;
@@ -157,11 +158,11 @@ static int sev_asid_new(struct kvm_sev_info *sev)
 		goto e_uncharge;
 	}
 
-	__set_bit(pos, sev_asid_bitmap);
+	__set_bit(asid, sev_asid_bitmap);
 
 	mutex_unlock(&sev_bitmap_lock);
 
-	return pos + 1;
+	return asid;
 e_uncharge:
 	misc_cg_uncharge(type, sev->misc_cg, 1);
 	put_misc_cg(sev->misc_cg);
@@ -179,13 +180,12 @@ static int sev_get_asid(struct kvm *kvm)
 static void sev_asid_free(struct kvm_sev_info *sev)
 {
 	struct svm_cpu_data *sd;
-	int cpu, pos;
+	int cpu;
 	enum misc_res_type type;
 
 	mutex_lock(&sev_bitmap_lock);
 
-	pos = sev->asid - 1;
-	__set_bit(pos, sev_reclaim_asid_bitmap);
+	__set_bit(sev->asid, sev_reclaim_asid_bitmap);
 
 	for_each_possible_cpu(cpu) {
 		sd = per_cpu(svm_data, cpu);
@@ -1857,12 +1857,17 @@ void __init sev_hardware_setup(void)
 	min_sev_asid = edx;
 	sev_me_mask = 1UL << (ebx & 0x3f);
 
-	/* Initialize SEV ASID bitmaps */
-	sev_asid_bitmap = bitmap_zalloc(max_sev_asid, GFP_KERNEL);
+	/*
+	 * Initialize SEV ASID bitmaps. Allocate space for ASID 0 in the bitmap,
+	 * even though it's never used, so that the bitmap is indexed by the
+	 * actual ASID.
+	 */
+	nr_asids = max_sev_asid + 1;
+	sev_asid_bitmap = bitmap_zalloc(nr_asids, GFP_KERNEL);
 	if (!sev_asid_bitmap)
 		goto out;
 
-	sev_reclaim_asid_bitmap = bitmap_zalloc(max_sev_asid, GFP_KERNEL);
+	sev_reclaim_asid_bitmap = bitmap_zalloc(nr_asids, GFP_KERNEL);
 	if (!sev_reclaim_asid_bitmap) {
 		bitmap_free(sev_asid_bitmap);
 		sev_asid_bitmap = NULL;
@@ -1907,7 +1912,7 @@ void sev_hardware_teardown(void)
 		return;
 
 	/* No need to take sev_bitmap_lock, all VMs have been destroyed. */
-	sev_flush_asids(0, max_sev_asid);
+	sev_flush_asids(1, max_sev_asid);
 
 	bitmap_free(sev_asid_bitmap);
 	bitmap_free(sev_reclaim_asid_bitmap);
@@ -1921,7 +1926,7 @@ int sev_cpu_init(struct svm_cpu_data *sd)
 	if (!sev_enabled)
 		return 0;
 
-	sd->sev_vmcbs = kcalloc(max_sev_asid + 1, sizeof(void *), GFP_KERNEL);
+	sd->sev_vmcbs = kcalloc(nr_asids, sizeof(void *), GFP_KERNEL);
 	if (!sd->sev_vmcbs)
 		return -ENOMEM;