Re: [PATCH kernel 3/9] KVM: PPC: Use preregistered memory API to access TCE list

On Tue, Mar 08, 2016 at 04:47:20PM +1100, Alexey Kardashevskiy wrote:

On 03/07/2016 05:00 PM, David Gibson wrote:

quoted

On Mon, Mar 07, 2016 at 02:41:11PM +1100, Alexey Kardashevskiy wrote:

quoted

VFIO on sPAPR already implements guest memory pre-registration
when the entire guest RAM gets pinned. This can be used to translate
the physical address of a guest page containing the TCE list
from H_PUT_TCE_INDIRECT.

This makes use of the pre-registrered memory API to access TCE list
pages in order to avoid unnecessary locking on the KVM memory
reverse map.

Signed-off-by: Alexey Kardashevskiy <redacted>

Ok.. so, what's the benefit of not having to lock the rmap?

Less locking -> less racing == good, no?

Well.. maybe.  The increased difficulty in verifying that the code is
correct isn't always a good price to pay.

quoted

quoted

---
 arch/powerpc/kvm/book3s_64_vio_hv.c | 86 ++++++++++++++++++++++++++++++-------
 1 file changed, 70 insertions(+), 16 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_64_vio_hv.c b/arch/powerpc/kvm/book3s_64_vio_hv.c
index 44be73e..af155f6 100644
--- a/arch/powerpc/kvm/book3s_64_vio_hv.c
+++ b/arch/powerpc/kvm/book3s_64_vio_hv.c

@@ -180,6 +180,38 @@ long kvmppc_gpa_to_ua(struct kvm *kvm, unsigned long gpa,
 EXPORT_SYMBOL_GPL(kvmppc_gpa_to_ua);

 #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
+static mm_context_t *kvmppc_mm_context(struct kvm_vcpu *vcpu)
+{
+	struct task_struct *task;
+
+	task = vcpu->arch.run_task;
+	if (unlikely(!task || !task->mm))
+		return NULL;
+
+	return &task->mm->context;
+}
+
+static inline bool kvmppc_preregistered(struct kvm_vcpu *vcpu)
+{
+	mm_context_t *mm = kvmppc_mm_context(vcpu);
+
+	if (unlikely(!mm))
+		return false;
+
+	return mm_iommu_preregistered(mm);
+}
+
+static struct mm_iommu_table_group_mem_t *kvmppc_rm_iommu_lookup(
+		struct kvm_vcpu *vcpu, unsigned long ua, unsigned long size)
+{
+	mm_context_t *mm = kvmppc_mm_context(vcpu);
+
+	if (unlikely(!mm))
+		return NULL;
+
+	return mm_iommu_lookup_rm(mm, ua, size);
+}
+
 long kvmppc_h_put_tce(struct kvm_vcpu *vcpu, unsigned long liobn,
 		      unsigned long ioba, unsigned long tce)
 {

@@ -261,23 +293,44 @@ long kvmppc_rm_h_put_tce_indirect(struct kvm_vcpu *vcpu,
 	if (ret != H_SUCCESS)
 		return ret;

-	if (kvmppc_gpa_to_ua(vcpu->kvm, tce_list, &ua, &rmap))
-		return H_TOO_HARD;
+	if (kvmppc_preregistered(vcpu)) {
+		/*
+		 * We get here if guest memory was pre-registered which
+		 * is normally VFIO case and gpa->hpa translation does not
+		 * depend on hpt.
+		 */
+		struct mm_iommu_table_group_mem_t *mem;

-	rmap = (void *) vmalloc_to_phys(rmap);
+		if (kvmppc_gpa_to_ua(vcpu->kvm, tce_list, &ua, NULL))
+			return H_TOO_HARD;

-	/*
-	 * Synchronize with the MMU notifier callbacks in
-	 * book3s_64_mmu_hv.c (kvm_unmap_hva_hv etc.).
-	 * While we have the rmap lock, code running on other CPUs
-	 * cannot finish unmapping the host real page that backs
-	 * this guest real page, so we are OK to access the host
-	 * real page.
-	 */
-	lock_rmap(rmap);
-	if (kvmppc_rm_ua_to_hpa(vcpu, ua, &tces)) {
-		ret = H_TOO_HARD;
-		goto unlock_exit;
+		mem = kvmppc_rm_iommu_lookup(vcpu, ua, IOMMU_PAGE_SIZE_4K);
+		if (!mem || mm_iommu_rm_ua_to_hpa(mem, ua, &tces))
+			return H_TOO_HARD;
+	} else {
+		/*
+		 * This is emulated devices case.
+		 * We do not require memory to be preregistered in this case
+		 * so lock rmap and do __find_linux_pte_or_hugepte().
+		 */
+		if (kvmppc_gpa_to_ua(vcpu->kvm, tce_list, &ua, &rmap))
+			return H_TOO_HARD;
+
+		rmap = (void *) vmalloc_to_phys(rmap);
+
+		/*
+		 * Synchronize with the MMU notifier callbacks in
+		 * book3s_64_mmu_hv.c (kvm_unmap_hva_hv etc.).
+		 * While we have the rmap lock, code running on other CPUs
+		 * cannot finish unmapping the host real page that backs
+		 * this guest real page, so we are OK to access the host
+		 * real page.
+		 */
+		lock_rmap(rmap);
+		if (kvmppc_rm_ua_to_hpa(vcpu, ua, &tces)) {
+			ret = H_TOO_HARD;
+			goto unlock_exit;
+		}
 	}

 	for (i = 0; i < npages; ++i) {

@@ -291,7 +344,8 @@ long kvmppc_rm_h_put_tce_indirect(struct kvm_vcpu *vcpu,
 	}

 unlock_exit:
-	unlock_rmap(rmap);
+	if (rmap)

I don't see where rmap is initialized to NULL in the case where it's
not being used.

@rmap is not new to this function, and it has always been initialized to
NULL as it was returned via a pointer from kvmppc_gpa_to_ua().

This comment confuses me.  Looking closer at the code I see you're
right, and it's initialized to NULL where defined, which I missed.

But that has nothing to do with being returned by pointer from
kvmppc_gpa_to_ua(), since one of your branches in the new code no
longer passes &rmap to that function.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson