On Tue, Mar 08, 2016 at 04:47:20PM +1100, Alexey Kardashevskiy wrote:
> On 03/07/2016 05:00 PM, David Gibson wrote:
> >On Mon, Mar 07, 2016 at 02:41:11PM +1100, Alexey Kardashevskiy wrote:
> >>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 <aik@xxxxxxxxx>
> >
> >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.
> >>---
> >> 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
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