Re: [PATCH v4 2/7] KVM: x86/mmu: Factor out allocating memslot rmap

[David Hildenbrand]

On 11.05.21 20:17, Ben Gardon wrote:
> On Tue, May 11, 2021 at 10:56 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > On Tue, May 11, 2021, Ben Gardon wrote:
> > > Small refactor to facilitate allocating rmaps for all memslots at once.
> > >
> > > No functional change expected.
> > >
> > > Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
> > > ---
> > >   arch/x86/kvm/x86.c | 39 ++++++++++++++++++++++++++++++---------
> > >   1 file changed, 30 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 1e1f4f31e586..cc0440b5b35d 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -10911,10 +10911,35 @@ void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot)
> > >        kvm_page_track_free_memslot(slot);
> > >   }
> > >
> > > +static int memslot_rmap_alloc(struct kvm_memory_slot *slot,
> > > +                           unsigned long npages)
> > > +{
> > > +     int i;
> > > +
> > > +     for (i = 0; i < KVM_NR_PAGE_SIZES; ++i) {
> > > +             int lpages;
> > > +             int level = i + 1;
> > > +
> > > +             lpages = gfn_to_index(slot->base_gfn + npages - 1,
> > > +                                   slot->base_gfn, level) + 1;
> >
> > Might as well assign lpages at its declaration, i.e.
> >
> >                  int lpages = gfn_to_index(slot->base_gfn + npages - 1,
> >                                            slot->base_gfn, level) + 1;
>
> I'll do this if I end up sending out a v5.
>
> > > +
> > > +             slot->arch.rmap[i] =
> > > +                     kvcalloc(lpages, sizeof(*slot->arch.rmap[i]),
> > > +                              GFP_KERNEL_ACCOUNT);
> >
> > Eh, I don't think avoiding a 3 char overrun is worth splitting across three lines.
> > E.g. this is perfectly readable
> >
> >                  slot->arch.rmap[i] = kvcalloc(lpages, sizeof(*slot->arch.rmap[i]),
> >                                                GFP_KERNEL_ACCOUNT);
> >
> > Alternatively, the rmap size could be captured in a local var, e.g.
> >
> >          const int sz = sizeof(*slot->arch.rmap[0]);
> >
> >          ...
> >
> >                  slot->arch.rmap[i] = kvcalloc(lpages, sz, GFP_KERNEL_ACCOUNT);
>
> I like this suggestion. Much nicer. Will incorporate if I send a v5.
>
> >                  if (!slot->arch.rmap[i]) {
> >                          memslot_rmap_free(slot);
> >                          return -ENOMEM;
> >                  }
> >
> > > +             if (!slot->arch.rmap[i]) {
> > > +                     memslot_rmap_free(slot);
> > > +                     return -ENOMEM;
> >
> > Reaaaally getting into nitpicks, what do you think about changing this to a goto
> > with the error handling at the bottom?  Obviously not necessary by any means,
> > but for me it makes it easier to see that all rmaps are freed on failure.  My
> > eyes skipped over that on the first read through.  E.g.
> >
> >                  if (!slot_arch.rmap[i])
> >                          goto err;
> >          }
> >
> >          return 0;
> >
> > err:
> >          memslot_rmap_free(slot);
> >          return -ENOMEM;
>
> Lol, I had a goto in v3, but David Hildenbrand suggested removing it
> and putting the free in the loop. I think I like it more this way too.

No strong opinion, I tend to stick to 
Documentation/process/coding-style.rst which states

"The goto statement comes in handy when a function exits from multiple 
locations and some common work such as cleanup has to be done."

As we only have a single error exit and no complicated locking, at least 
for me the "goto" makes it unnecessary hard to read.


--
Thanks,

David / dhildenb