[PATCH] KVM: SEV: 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, 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;
adjust its value before using it as a bitmap position. Add comments on ASID
bitmap allocation to clarify the skipping-ASID-0 property.

Fixes: 80675b3ad45f (KVM: SVM: Update ASID allocation to support SEV-ES guests)
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>
Ce: Peter Gonda <pgonda@google.com>
---
 arch/x86/kvm/svm/sev.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 8d36f0c73071..e3902283cbf7 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -80,7 +80,7 @@ static int sev_flush_asids(int min_asid, int max_asid)
 	int ret, pos, 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);
+	pos = find_next_bit(sev_reclaim_asid_bitmap, max_asid, min_asid - 1);
 	if (pos >= max_asid)
 		return -EBUSY;
 
@@ -142,10 +142,10 @@ 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);
+	pos = find_next_zero_bit(sev_asid_bitmap, max_sev_asid, min_asid - 1);
 	if (pos >= max_asid) {
 		if (retry && __sev_recycle_asids(min_asid, max_asid)) {
 			retry = false;
@@ -1854,7 +1854,10 @@ void __init sev_hardware_setup(void)
 	min_sev_asid = edx;
 	sev_me_mask = 1UL << (ebx & 0x3f);
 
-	/* Initialize SEV ASID bitmaps */
+	/*
+	 * Initialize SEV ASID bitmaps. Note: ASID 0 is skipped since it is
+	 * never used by any VM, thus: ASID value == ASID position + 1;
+	 */
 	sev_asid_bitmap = bitmap_zalloc(max_sev_asid, GFP_KERNEL);
 	if (!sev_asid_bitmap)
 		goto out;
@@ -1904,7 +1907,7 @@ 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