Re: [PATCH 09/11] KVM: guest_memfd: Add interface for populating gmem pages with user data

[Paolo Bonzini]

On Thu, Apr 25, 2024 at 12:32 AM Sean Christopherson <seanjc@googlecom> wrote:
>
> On Thu, Apr 04, 2024, Paolo Bonzini wrote:
> > +long kvm_gmem_populate(struct kvm *kvm, gfn_t gfn, void __user *src, long npages,
> > +                    int (*post_populate)(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> > +                                         void __user *src, int order, void *opaque),
>
> Add a typedef for callback?  If only to make this prototype readable.
>
> > +long kvm_gmem_populate(struct kvm *kvm, gfn_t gfn, void __user *src, long npages,
> > +                    int (*post_populate)(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> > +                                         void __user *src, int order, void *opaque),
> > +                    void *opaque)
> > +{
> > +     struct file *file;
> > +     struct kvm_memory_slot *slot;
> > +
> > +     int ret = 0, max_order;
> > +     long i;
> > +
> > +     lockdep_assert_held(&kvm->slots_lock);
> > +     if (npages < 0)
> > +             return -EINVAL;
> > +
> > +     slot = gfn_to_memslot(kvm, gfn);
> > +     if (!kvm_slot_can_be_private(slot))
> > +             return -EINVAL;
> > +
> > +     file = kvm_gmem_get_file(slot);
> > +     if (!file)
> > +             return -EFAULT;
> > +
> > +     filemap_invalidate_lock(file->f_mapping);
> > +
> > +     npages = min_t(ulong, slot->npages - (gfn - slot->base_gfn), npages);
> > +     for (i = 0; i < npages; i += (1 << max_order)) {
> > +             gfn_t this_gfn = gfn + i;
>
> KVM usually does something like "start_gfn" or "base_gfn", and then uses "gfn"
> for the one gfn that's being processed.  And IMO that's much better because the
> propotype for kvm_gmem_populate() does not make it obvious that @gfn is the base
> of a range, not a singular gfn to process.
>
>
> > +             kvm_pfn_t pfn;
> > +
> > +             ret = __kvm_gmem_get_pfn(file, slot, this_gfn, &pfn, &max_order, false);
> > +             if (ret)
> > +                     break;
> > +
> > +             if (!IS_ALIGNED(this_gfn, (1 << max_order)) ||
> > +                 (npages - i) < (1 << max_order))
> > +                     max_order = 0;
> > +
> > +             if (post_populate) {
>
> Is there any use for this without @post_populate?  I.e. why make this optional?

Yeah, it probably does not need to be optional (before, the
copy_from_user was optionally done from kvm_gmem_populate, but not
anymore).

Paolo