linux-kernel - [PATCH v16 4/6] KVM: s390: pv: avoid export before import if possible

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Date:   Fri, 11 Nov 2022 18:06:30 +0100
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     kvm@...r.kernel.org
Cc:     borntraeger@...ibm.com, frankja@...ux.ibm.com, thuth@...hat.com,
        david@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, scgl@...ux.ibm.com,
        seiden@...ux.ibm.com, nrb@...ux.ibm.com
Subject: [PATCH v16 4/6] KVM: s390: pv: avoid export before import if possible

If the appropriate UV feature bit is set, there is no need to perform
an export before import.

The misc feature indicates, among other things, that importing a shared
page from a different protected VM will automatically also transfer its
ownership.

Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
Reviewed-by: Nico Boehr <nrb@...ux.ibm.com>
Reviewed-by: Janosch Frank <frankja@...ux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@...ux.ibm.com>
---
 arch/s390/kernel/uv.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index f9810d2a267c..9f18a4af9c13 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -255,6 +255,13 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
  */
 static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_struct *mm)
 {
+	/*
+	 * The misc feature indicates, among other things, that importing a
+	 * shared page from a different protected VM will automatically also
+	 * transfer its ownership.
+	 */
+	if (test_bit_inv(BIT_UV_FEAT_MISC, &uv_info.uv_feature_indications))
+		return false;
 	if (uvcb->cmd == UVC_CMD_UNPIN_PAGE_SHARED)
 		return false;
 	return atomic_read(&mm->context.protected_count) > 1;
-- 
2.38.1