Re: [PATCH kernel v2] KVM: PPC: Allocate guest TCEs on demand too

We already allocate hardware TCE tables in multiple levels and skip
intermediate levels when we can, now it is a turn of the KVM TCE tables.
Thankfully these are allocated already in 2 levels.

This moves the table's last level allocation from the creating helper to
kvmppc_tce_put() and kvm_spapr_tce_fault().

This adds kvmppc_rm_ioba_validate() to do an additional test if
the consequent kvmppc_tce_put() needs a page which has not been allocated;
if this is the case, we bail out to virtual mode handlers.

Signed-off-by: Alexey Kardashevskiy <redacted>
---
Changes:
v2:
* added kvm mutex around alloc_page to prevent races; in both place we
test the pointer, if NULL, then take a lock and check again so on a fast
path we do not take a lock at all


---
For NVLink2 passthrough guests with 128TiB DMA windows and very fragmented
system RAM the difference is gigabytes of RAM.
---
 arch/powerpc/kvm/book3s_64_vio.c    | 29 ++++++------
 arch/powerpc/kvm/book3s_64_vio_hv.c | 69 ++++++++++++++++++++++++++---
 2 files changed, 79 insertions(+), 19 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
index f02b049..7eed8c9 100644
--- a/arch/powerpc/kvm/book3s_64_vio.c
+++ b/arch/powerpc/kvm/book3s_64_vio.c

@@ -228,7 +228,8 @@ static void release_spapr_tce_table(struct rcu_head *head)
 	unsigned long i, npages = kvmppc_tce_pages(stt->size);
 
 	for (i = 0; i < npages; i++)
-		__free_page(stt->pages[i]);
+		if (stt->pages[i])
+			__free_page(stt->pages[i]);
 
 	kfree(stt);
 }

@@ -242,6 +243,20 @@ static vm_fault_t kvm_spapr_tce_fault(struct vm_fault *vmf)
 		return VM_FAULT_SIGBUS;
 
 	page = stt->pages[vmf->pgoff];
+	if (!page) {
+		mutex_lock(&stt->kvm->lock);
+		page = stt->pages[vmf->pgoff];
+		if (!page) {
+			page  = alloc_page(GFP_KERNEL | __GFP_ZERO);
+			if (!page) {
+				mutex_unlock(&stt->kvm->lock);
+				return VM_FAULT_OOM;
+			}
+			stt->pages[vmf->pgoff] = page;
+		}
+		mutex_unlock(&stt->kvm->lock);
+	}
+
 	get_page(page);
 	vmf->page = page;
 	return 0;

@@ -296,7 +311,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
 	struct kvmppc_spapr_tce_table *siter;
 	unsigned long npages, size = args->size;
 	int ret = -ENOMEM;
-	int i;
 
 	if (!args->size || args->page_shift < 12 || args->page_shift > 34 ||
 		(args->offset + args->size > (ULLONG_MAX >> args->page_shift)))

@@ -320,12 +334,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
 	stt->kvm = kvm;
 	INIT_LIST_HEAD_RCU(&stt->iommu_tables);
 
-	for (i = 0; i < npages; i++) {
-		stt->pages[i] = alloc_page(GFP_KERNEL | __GFP_ZERO);
-		if (!stt->pages[i])
-			goto fail;
-	}
-
 	mutex_lock(&kvm->lock);
 
 	/* Check this LIOBN hasn't been previously allocated */

@@ -352,11 +360,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
 	if (ret >= 0)
 		return ret;
 
- fail:
-	for (i = 0; i < npages; i++)
-		if (stt->pages[i])
-			__free_page(stt->pages[i]);
-
 	kfree(stt);
  fail_acct:
 	kvmppc_account_memlimit(kvmppc_stt_pages(npages), false);

diff --git a/arch/powerpc/kvm/book3s_64_vio_hv.c b/arch/powerpc/kvm/book3s_64_vio_hv.c
index 2206bc7..a0912d5 100644
--- a/arch/powerpc/kvm/book3s_64_vio_hv.c
+++ b/arch/powerpc/kvm/book3s_64_vio_hv.c

@@ -158,23 +158,76 @@ static u64 *kvmppc_page_address(struct page *page)
 	return (u64 *) page_address(page);
 }
 
+/*
+ * TCEs pages are allocated in kvmppc_tce_put() which won't be able to do so
+ * in real mode.
+ * Check if kvmppc_tce_put() can succeed in real mode, i.e. a TCEs page is
+ * allocated or not required (when clearing a tce entry).
+ */
+static long kvmppc_rm_ioba_validate(struct kvmppc_spapr_tce_table *stt,
+		unsigned long ioba, unsigned long npages, bool clearing)
+{
+	unsigned long i, sttpage, sttpages;
+	unsigned long ret = kvmppc_ioba_validate(stt, ioba, npages);
+
+	if (ret)
+		return ret;
+	/*
+	 * clearing==true says kvmppc_tce_put won't be allocating pages
+	 * for empty tces.
+	 */
+	if (clearing)
+		return H_SUCCESS;
+
+	sttpage = ((ioba >> stt->page_shift) - stt->offset) / TCES_PER_PAGE;
+	sttpages = (npages + TCES_PER_PAGE - 1) / TCES_PER_PAGE;