Re: [PATCH v11 0/7] KVM: PPC: Driver to manage pages of secure guest

Hugh Dickins
2019-12-01T20:24:50Z

On Thu, 28 Nov 2019, Bharata B Rao wrote:
> On Mon, Nov 25, 2019 at 08:36:24AM +0530, Bharata B Rao wrote:
> > Hi,
> >
> > This is the next version of the patchset that adds required support
> > in the KVM hypervisor to run secure guests on PEF-enabled POWER platforms.
> >
>
> Here is a fix for the issue Hugh identified with the usage of ksm_madvise()
> in this patchset. It applies on top of this patchset.

It looks correct to me, and I hope will not spoil your performance in any
way that matters. But I have to say, the patch would be so much clearer,
if you just named your bool "downgraded" instead of "downgrade".

Hugh

> ----
>
> From 8a4d769bf4c61f921c79ce68923be3c403bd5862 Mon Sep 17 00:00:00 2001
> From: Bharata B Rao <bharata@linux.ibm.com>
> Date: Thu, 28 Nov 2019 09:31:54 +0530
> Subject: [PATCH 1/1] KVM: PPC: Book3S HV: Take write mmap_sem when calling
> ksm_madvise
>
> In order to prevent the device private pages (that correspond to
> pages of secure guest) from participating in KSM merging, H_SVM_PAGE_IN
> calls ksm_madvise() under read version of mmap_sem. However ksm_madvise()
> needs to be under write lock, fix this.
>
> Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
> ---
> arch/powerpc/kvm/book3s_hv_uvmem.c | 29 ++++++++++++++++++++---------
> 1 file changed, 20 insertions(+), 9 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> index f24ac3cfb34c..2de264fc3156 100644
> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> @@ -46,11 +46,10 @@
> *
> * Locking order
> *
> - * 1. srcu_read_lock(&kvm->srcu) - Protects KVM memslots
> - * 2. down_read(&kvm->mm->mmap_sem) - find_vma, migrate_vma_pages and helpers
> - * 3. mutex_lock(&kvm->arch.uvmem_lock) - protects read/writes to uvmem slots
> - * thus acting as sync-points
> - * for page-in/out
> + * 1. kvm->srcu - Protects KVM memslots
> + * 2. kvm->mm->mmap_sem - find_vma, migrate_vma_pages and helpers, ksm_madvise
> + * 3. kvm->arch.uvmem_lock - protects read/writes to uvmem slots thus acting
> + * as sync-points for page-in/out
> */
>
> /*
> @@ -344,7 +343,7 @@ static struct page *kvmppc_uvmem_get_page(unsigned long gpa, struct kvm *kvm)
> static int
> kvmppc_svm_page_in(struct vm_area_struct *vma, unsigned long start,
> unsigned long end, unsigned long gpa, struct kvm *kvm,
> - unsigned long page_shift)
> + unsigned long page_shift, bool *downgrade)
> {
> unsigned long src_pfn, dst_pfn = 0;
> struct migrate_vma mig;
> @@ -360,8 +359,15 @@ kvmppc_svm_page_in(struct vm_area_struct *vma, unsigned long start,
> mig.src = &src_pfn;
> mig.dst = &dst_pfn;
>
> + /*
> + * We come here with mmap_sem write lock held just for
> + * ksm_madvise(), otherwise we only need read mmap_sem.
> + * Hence downgrade to read lock once ksm_madvise() is done.
> + */
> ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> MADV_UNMERGEABLE, &vma->vm_flags);
> + downgrade_write(&kvm->mm->mmap_sem);
> + *downgrade = true;
> if (ret)
> return ret;
>
> @@ -456,6 +462,7 @@ unsigned long
> kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
> unsigned long flags, unsigned long page_shift)
> {
> + bool downgrade = false;
> unsigned long start, end;
> struct vm_area_struct *vma;
> int srcu_idx;
> @@ -476,7 +483,7 @@ kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
>
> ret = H_PARAMETER;
> srcu_idx = srcu_read_lock(&kvm->srcu);
> - down_read(&kvm->mm->mmap_sem);
> + down_write(&kvm->mm->mmap_sem);
>
> start = gfn_to_hva(kvm, gfn);
> if (kvm_is_error_hva(start))
> @@ -492,12 +499,16 @@ kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
> if (!vma || vma->vm_start > start || vma->vm_end < end)
> goto out_unlock;
>
> - if (!kvmppc_svm_page_in(vma, start, end, gpa, kvm, page_shift))
> + if (!kvmppc_svm_page_in(vma, start, end, gpa, kvm, page_shift,
> + &downgrade))
> ret = H_SUCCESS;
> out_unlock:
> mutex_unlock(&kvm->arch.uvmem_lock);
> out:
> - up_read(&kvm->mm->mmap_sem);
> + if (downgrade)
> + up_read(&kvm->mm->mmap_sem);
> + else
> + up_write(&kvm->mm->mmap_sem);
> srcu_read_unlock(&kvm->srcu, srcu_idx);
> return ret;
> }
> --
> 2.21.0