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

From: David Gibson <[email-masked]>
Date: Thu, 10 Mar 2016 10:46:27 +1100

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 <[email-masked]>
> >>>
> >>>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