Re: [PATCH v5 04/14] KVM: s390: pv: avoid stalls when making pages secure

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Am 20.09.21 um 15:24 schrieb Claudio Imbrenda:
Improve make_secure_pte to avoid stalls when the system is heavily
overcommitted. This was especially problematic in kvm_s390_pv_unpack,
because of the loop over all pages that needed unpacking.

Due to the locks being held, it was not possible to simply replace
uv_call with uv_call_sched. A more complex approach was
needed, in which uv_call is replaced with __uv_call, which does not
loop. When the UVC needs to be executed again, -EAGAIN is returned, and
the caller (or its caller) will try again.

When -EAGAIN is returned, the path is the same as when the page is in
writeback (and the writeback check is also performed, which is
harmless).

Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests")

applied this one.
---
  arch/s390/kernel/uv.c     | 29 +++++++++++++++++++++++------
  arch/s390/kvm/intercept.c |  5 +++++
  2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index aeb0a15bcbb7..68a8fbafcb9c 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -180,7 +180,7 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
  {
  	pte_t entry = READ_ONCE(*ptep);
  	struct page *page;
-	int expected, rc = 0;
+	int expected, cc = 0;
if (!pte_present(entry))
  		return -ENXIO;
@@ -196,12 +196,25 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
  	if (!page_ref_freeze(page, expected))
  		return -EBUSY;
  	set_bit(PG_arch_1, &page->flags);
-	rc = uv_call(0, (u64)uvcb);
+	/*
+	 * If the UVC does not succeed or fail immediately, we don't want to
+	 * loop for long, or we might get stall notifications.
+	 * On the other hand, this is a complex scenario and we are holding a lot of
+	 * locks, so we can't easily sleep and reschedule. We try only once,
+	 * and if the UVC returned busy or partial completion, we return
+	 * -EAGAIN and we let the callers deal with it.
+	 */
+	cc = __uv_call(0, (u64)uvcb);
  	page_ref_unfreeze(page, expected);
-	/* Return -ENXIO if the page was not mapped, -EINVAL otherwise */
-	if (rc)
-		rc = uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
-	return rc;
+	/*
+	 * Return -ENXIO if the page was not mapped, -EINVAL for other errors.
+	 * If busy or partially completed, return -EAGAIN.
+	 */
+	if (cc == UVC_CC_OK)
+		return 0;
+	else if (cc == UVC_CC_BUSY || cc == UVC_CC_PARTIAL)
+		return -EAGAIN;
+	return uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
  }
/*
@@ -254,6 +267,10 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
  	mmap_read_unlock(gmap->mm);
if (rc == -EAGAIN) {
+		/*
+		 * If we are here because the UVC returned busy or partial
+		 * completion, this is just a useless check, but it is safe.
+		 */
  		wait_on_page_writeback(page);
  	} else if (rc == -EBUSY) {
  		/*
diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
index 72b25b7cc6ae..47833ade4da5 100644
--- a/arch/s390/kvm/intercept.c
+++ b/arch/s390/kvm/intercept.c
@@ -516,6 +516,11 @@ static int handle_pv_uvc(struct kvm_vcpu *vcpu)
  	 */
  	if (rc == -EINVAL)
  		return 0;
+	/*
+	 * If we got -EAGAIN here, we simply return it. It will eventually
+	 * get propagated all the way to userspace, which should then try
+	 * again.
+	 */
  	return rc;
  }



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