[PATCH v2] KVM: SVM: improve the code readability for ASID management

[Mingwei Zhang <mizhang@google.com>]

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.

v1 -> v2:
 - change ASID bitmap size to incorporate ASID 0 [sean]
 - remove the 'fixes' line in commit message. [sean/joerg]

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>
---
 arch/x86/kvm/svm/sev.c | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 8d36f0c73071..42d46c30f313 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -63,6 +63,7 @@ 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;
 
@@ -77,11 +78,11 @@ 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;
 
 	/*
@@ -114,15 +115,15 @@ 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;
 
@@ -142,11 +143,11 @@ 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_sev_asid, min_asid);
+	if (asid > max_asid) {
 		if (retry && __sev_recycle_asids(min_asid, max_asid)) {
 			retry = false;
 			goto again;
@@ -156,11 +157,11 @@ 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);
@@ -1854,12 +1855,17 @@ 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;
@@ -1904,7 +1910,7 @@ 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);
-- 
2.32.0.554.ge1b32706d8-goog