On Wed, Mar 09, 2016 at 07:55:53PM +1100, Alexey Kardashevskiy wrote:
> On 03/08/2016 05:30 PM, David Gibson wrote:
> >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.
>
>
> So? The code is still correct - the "preregistered branch" does not touch
> NULL pointer, it remains NULL and unlock_rmap() is not called. I agree the
> patch is not the easiest to read but how can I improve it to get your "rb"?
> Replace "if(rmap)" with "if (kvmppc_preregistered(vcpu))"? Move that loop
> between lock_rmap/unlock_rmap to a helper? kvmppc_rm_h_put_tce_indirect() is
> not big enough to justify splitting, the comments inside it are though...
Sorry, I wasn't clear. I no longer have a specific objection. I left
out the R-b, since there have been enough other comments on the series
that I was expecting a respin, so I was planning to re-review in the
context of an updated series.
--
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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