Hi Quentin,
On Tue, 11 Feb 2025 at 15:57, Quentin Perret <qperret@xxxxxxxxxx> wrote:
>
> Hey Fuad,
>
> On Tuesday 11 Feb 2025 at 12:11:24 (+0000), Fuad Tabba wrote:
> > Add arm64 support for handling guest page faults on guest_memfd
> > backed memslots.
> >
> > For now, the fault granule is restricted to PAGE_SIZE.
> >
> > Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> > ---
> > arch/arm64/kvm/mmu.c | 84 ++++++++++++++++++++++++++--------------
> > include/linux/kvm_host.h | 5 +++
> > virt/kvm/kvm_main.c | 5 ---
> > 3 files changed, 61 insertions(+), 33 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index b6c0acb2311c..305060518766 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -1454,6 +1454,33 @@ static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
> > return vma->vm_flags & VM_MTE_ALLOWED;
> > }
> >
> > +static kvm_pfn_t faultin_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> > + gfn_t gfn, bool write_fault, bool *writable,
> > + struct page **page, bool is_private)
> > +{
> > + kvm_pfn_t pfn;
> > + int ret;
> > +
> > + if (!is_private)
> > + return __kvm_faultin_pfn(slot, gfn, write_fault ? FOLL_WRITE : 0, writable, page);
> > +
> > + *writable = false;
> > +
> > + if (WARN_ON_ONCE(write_fault && memslot_is_readonly(slot)))
> > + return KVM_PFN_ERR_NOSLOT_MASK;
>
> I believe this check is superfluous, we should decide to report an MMIO
> exit to userspace for write faults to RO memslots and not get anywhere
> near user_mem_abort(). And nit but the error code should probably be
> KVM_PFN_ERR_RO_FAULT or something instead?
I tried to replicate the behavior of __kvm_faultin_pfn() here (but got
the wrong error!). I think you're right though that in the arm64 case,
this check isn't needed. Should I fix the return error and keep the
warning though?
> > +
> > + ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, page, NULL);
> > + if (!ret) {
> > + *writable = write_fault;
>
> In normal KVM, if we're not dirty logging we'll actively map the page as
> writable if both the memslot and the userspace mappings are writable.
> With gmem, the latter doesn't make much sense, but essentially the
> underlying page should really be writable (e.g. no CoW getting in the
> way and such?). If so, then perhaps make this
>
> *writable = !memslot_is_readonly(slot);
>
> Wdyt?
Ack.
> > + return pfn;
> > + }
> > +
> > + if (ret == -EHWPOISON)
> > + return KVM_PFN_ERR_HWPOISON;
> > +
> > + return KVM_PFN_ERR_NOSLOT_MASK;
> > +}
> > +
> > static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > struct kvm_s2_trans *nested,
> > struct kvm_memory_slot *memslot, unsigned long hva,
> > @@ -1461,25 +1488,26 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > {
> > int ret = 0;
> > bool write_fault, writable;
> > - bool exec_fault, mte_allowed;
> > + bool exec_fault, mte_allowed = false;
> > bool device = false, vfio_allow_any_uc = false;
> > unsigned long mmu_seq;
> > phys_addr_t ipa = fault_ipa;
> > struct kvm *kvm = vcpu->kvm;
> > - struct vm_area_struct *vma;
> > + struct vm_area_struct *vma = NULL;
> > short vma_shift;
> > void *memcache;
> > - gfn_t gfn;
> > + gfn_t gfn = ipa >> PAGE_SHIFT;
> > kvm_pfn_t pfn;
> > bool logging_active = memslot_is_logging(memslot);
> > - bool force_pte = logging_active || is_protected_kvm_enabled();
> > - long vma_pagesize, fault_granule;
> > + bool is_private = kvm_mem_is_private(kvm, gfn);
>
> Just trying to understand the locking rule for the xarray behind this.
> Is it kvm->srcu that protects it for reads here? Something else?
I'm not sure I follow. Which xarray are you referring to?
>
> > + bool force_pte = logging_active || is_private || is_protected_kvm_enabled();
> > + long vma_pagesize, fault_granule = PAGE_SIZE;
> > enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
> > struct kvm_pgtable *pgt;
> > struct page *page;
> > enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_HANDLE_FAULT | KVM_PGTABLE_WALK_SHARED;
> >
> > - if (fault_is_perm)
> > + if (fault_is_perm && !is_private)
>
> Nit: not strictly necessary I think.
You're right.
Thanks,
/fuad
> > fault_granule = kvm_vcpu_trap_get_perm_fault_granule(vcpu);
> > write_fault = kvm_is_write_fault(vcpu);
> > exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
> > @@ -1510,24 +1538,30 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > return ret;
> > }
> >
> > + mmap_read_lock(current->mm);
> > +
> > /*
> > * Let's check if we will get back a huge page backed by hugetlbfs, or
> > * get block mapping for device MMIO region.
> > */
> > - mmap_read_lock(current->mm);
> > - vma = vma_lookup(current->mm, hva);
> > - if (unlikely(!vma)) {
> > - kvm_err("Failed to find VMA for hva 0x%lx\n", hva);
> > - mmap_read_unlock(current->mm);
> > - return -EFAULT;
> > - }
> > + if (!is_private) {
> > + vma = vma_lookup(current->mm, hva);
> > + if (unlikely(!vma)) {
> > + kvm_err("Failed to find VMA for hva 0x%lx\n", hva);
> > + mmap_read_unlock(current->mm);
> > + return -EFAULT;
> > + }
> >
> > - /*
> > - * logging_active is guaranteed to never be true for VM_PFNMAP
> > - * memslots.
> > - */
> > - if (WARN_ON_ONCE(logging_active && (vma->vm_flags & VM_PFNMAP)))
> > - return -EFAULT;
> > + /*
> > + * logging_active is guaranteed to never be true for VM_PFNMAP
> > + * memslots.
> > + */
> > + if (WARN_ON_ONCE(logging_active && (vma->vm_flags & VM_PFNMAP)))
> > + return -EFAULT;
> > +
> > + vfio_allow_any_uc = vma->vm_flags & VM_ALLOW_ANY_UNCACHED;
> > + mte_allowed = kvm_vma_mte_allowed(vma);
> > + }
> >
> > if (force_pte)
> > vma_shift = PAGE_SHIFT;
> > @@ -1597,18 +1631,13 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > ipa &= ~(vma_pagesize - 1);
> > }
> >
> > - gfn = ipa >> PAGE_SHIFT;
> > - mte_allowed = kvm_vma_mte_allowed(vma);
> > -
> > - vfio_allow_any_uc = vma->vm_flags & VM_ALLOW_ANY_UNCACHED;
> > -
> > /* Don't use the VMA after the unlock -- it may have vanished */
> > vma = NULL;
> >
> > /*
> > * Read mmu_invalidate_seq so that KVM can detect if the results of
> > - * vma_lookup() or __kvm_faultin_pfn() become stale prior to
> > - * acquiring kvm->mmu_lock.
> > + * vma_lookup() or faultin_pfn() become stale prior to acquiring
> > + * kvm->mmu_lock.
> > *
> > * Rely on mmap_read_unlock() for an implicit smp_rmb(), which pairs
> > * with the smp_wmb() in kvm_mmu_invalidate_end().
> > @@ -1616,8 +1645,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > mmu_seq = vcpu->kvm->mmu_invalidate_seq;
> > mmap_read_unlock(current->mm);
> >
> > - pfn = __kvm_faultin_pfn(memslot, gfn, write_fault ? FOLL_WRITE : 0,
> > - &writable, &page);
> > + pfn = faultin_pfn(kvm, memslot, gfn, write_fault, &writable, &page, is_private);
> > if (pfn == KVM_PFN_ERR_HWPOISON) {
> > kvm_send_hwpoison_signal(hva, vma_shift);
> > return 0;
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 39fd6e35c723..415c6274aede 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1882,6 +1882,11 @@ static inline int memslot_id(struct kvm *kvm, gfn_t gfn)
> > return gfn_to_memslot(kvm, gfn)->id;
> > }
> >
> > +static inline bool memslot_is_readonly(const struct kvm_memory_slot *slot)
> > +{
> > + return slot->flags & KVM_MEM_READONLY;
> > +}
> > +
> > static inline gfn_t
> > hva_to_gfn_memslot(unsigned long hva, struct kvm_memory_slot *slot)
> > {
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 38f0f402ea46..3e40acb9f5c0 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -2624,11 +2624,6 @@ unsigned long kvm_host_page_size(struct kvm_vcpu *vcpu, gfn_t gfn)
> > return size;
> > }
> >
> > -static bool memslot_is_readonly(const struct kvm_memory_slot *slot)
> > -{
> > - return slot->flags & KVM_MEM_READONLY;
> > -}
> > -
> > static unsigned long __gfn_to_hva_many(const struct kvm_memory_slot *slot, gfn_t gfn,
> > gfn_t *nr_pages, bool write)
> > {
> > --
> > 2.48.1.502.g6dc24dfdaf-goog
> >
