Re: [PATCH kernel v4 09/10] KVM: PPC: Use preregistered memory API to access TCE list

David Gibson
2017-02-09T04:00:51Z

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On Tue, Feb 07, 2017 at 06:17:10PM +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 as we know that all of guest memory is pinned and
> we have a flat array mapping GPA to HPA which makes it simpler and
> quicker to index into that array (even with looking up the
> kernel page tables in vmalloc_to_phys) than it is to find the memslot,
> lock the rmap entry, look up the user page tables, and unlock the rmap
> entry. Note that the rmap pointer is initialized to NULL
> where declared (not in this patch).
>
> If a requested chunk of memory has not been preregistered, this will
> fall back to non-preregistered case and lock rmap.
>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
> Changes:
> v4:
> * removed oneline inlines
> * now falls back to locking rmap if TCE list is not in preregistered memory
>
> v2:
> * updated the commit log with David's comment
> ---
> arch/powerpc/kvm/book3s_64_vio_hv.c | 58 +++++++++++++++++++++++++++----------
> 1 file changed, 42 insertions(+), 16 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_64_vio_hv.c b/arch/powerpc/kvm/book3s_64_vio_hv.c
> index f8a54b7c788e..dc1c66fda941 100644
> --- a/arch/powerpc/kvm/book3s_64_vio_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_vio_hv.c
> @@ -239,6 +239,7 @@ long kvmppc_rm_h_put_tce_indirect(struct kvm_vcpu *vcpu,
> long i, ret = H_SUCCESS;
> unsigned long tces, entry, tce, ua = 0;
> unsigned long *rmap = NULL;
> + bool prereg = false;
>
> stt = kvmppc_find_table(vcpu->kvm, liobn);
> if (!stt)
> @@ -259,23 +260,47 @@ 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 (mm_iommu_preregistered(vcpu->kvm->mm)) {
> + /*
> + * 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 = mm_iommu_lookup_rm(vcpu->kvm->mm, ua, IOMMU_PAGE_SIZE_4K);
> + if (mem)
> + prereg = mm_iommu_ua_to_hpa_rm(mem, ua, &tces) == 0;
> + }
> +
> + if (!prereg) {
> + /*
> + * This is usually a case of a guest with emulated devices only
> + * when TCE list is not in preregistered memory.
> + * 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) {
> @@ -293,7 +318,8 @@ long kvmppc_rm_h_put_tce_indirect(struct kvm_vcpu *vcpu,
> }
>
> unlock_exit:
> - unlock_rmap(rmap);
> + if (rmap)
> + unlock_rmap(rmap);
>
> return ret;
> }

--
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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