Re: [PATCH v5 1/8] KVM: arm64: Introduce hyp_alloc_private_va_range()

[Kalesh Singh <kaleshsingh@google.com>]

On Tue, Mar 8, 2022 at 12:21 PM Stephen Boyd <swboyd@chromium.org> wrote:
>
HI Stephen. Thanks for the review.

> Quoting Kalesh Singh (2022-03-07 10:48:59)
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index bc2aba953299..ccb2847ee2f4 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -457,22 +457,17 @@ int create_hyp_mappings(void *from, void *to, enum kvm_pgtable_prot prot)
> >         return 0;
> >  }
> >
> > -static int __create_hyp_private_mapping(phys_addr_t phys_addr, size_t size,
> > -                                       unsigned long *haddr,
> > -                                       enum kvm_pgtable_prot prot)
> > +
> > +/**
> > + * hyp_alloc_private_va_range - Allocates a private VA range.
> > + * @size:      The size of the VA range to reserve.
> > + *
> > + * The private VA range is allocated below io_map_base and
> > + * aligned based on the order of @size.
>
> Add what it returns?
>
> Return: Start address of allocated VA range or some error value... (I don't
> understand this part).
>
> It may also be a good idea to write out what VA is in the description:
>
> The private virtual address (VA) range is allocated below io_map_base
>

Ack

> > + */
> > +unsigned long hyp_alloc_private_va_range(size_t size)
> >  {
> >         unsigned long base;
> > -       int ret = 0;
> > -
> > -       if (!kvm_host_owns_hyp_mappings()) {
> > -               base = kvm_call_hyp_nvhe(__pkvm_create_private_mapping,
> > -                                        phys_addr, size, prot);
> > -               if (IS_ERR_OR_NULL((void *)base))
> > -                       return PTR_ERR((void *)base);
> > -               *haddr = base;
> > -
> > -               return 0;
> > -       }
> >
> >         mutex_lock(&kvm_hyp_pgd_mutex);
> >
> > @@ -484,29 +479,53 @@ static int __create_hyp_private_mapping(phys_addr_t phys_addr, size_t size,
> >          *
> >          * The allocated size is always a multiple of PAGE_SIZE.
> >          */
> > -       size = PAGE_ALIGN(size + offset_in_page(phys_addr));
> > -       base = io_map_base - size;
> > +       base = io_map_base - PAGE_ALIGN(size);
> > +
> > +       /* Align the allocation based on the order of its size */
> > +       base = ALIGN_DOWN(base, PAGE_SIZE << get_order(size));
> >
> >         /*
> >          * Verify that BIT(VA_BITS - 1) hasn't been flipped by
> >          * allocating the new area, as it would indicate we've
> >          * overflowed the idmap/IO address range.
> >          */
> > -       if ((base ^ io_map_base) & BIT(VA_BITS - 1))
> > -               ret = -ENOMEM;
> > +       if (!base || (base ^ io_map_base) & BIT(VA_BITS - 1))
> > +               base = (unsigned long)ERR_PTR(-ENOMEM);
>
> It looks odd to use an error pointer casted to unsigned long to return
> from an address allocation function. Why not pass a pointer for base
> like the function was written before and return an int from this
> function with 0 for success and negative error value?Otherwise some
> sort of define should made like DMA_MAPPING_ERROR and that can be used
> to indicate to the caller that the allocation failed, or a simple zero
> may work?

I wanted to keep consistent between the pkvm and traditional nvhe
code. I will refactor both *alloc_private_va_range() functions to take
a pointer and return an int error if that's preferred. There would
still be a case of this kind of cast in
__pkvm_create_private_mapping() which does return an unsigned long
address or ERR_PTR(...). It looks like it was made to return the
address to facilitate use as a hypercall (@Quentin CMIW).

>
> >         else
> >                 io_map_base = base;
> >
> >         mutex_unlock(&kvm_hyp_pgd_mutex);
> >
> > -       if (ret)
> > -               goto out;
> > +       return base;
> > +}
> > +
> > +static int __create_hyp_private_mapping(phys_addr_t phys_addr, size_t size,
> > +                                       unsigned long *haddr,
> > +                                       enum kvm_pgtable_prot prot)
> > +{
> > +       unsigned long addr;
> > +       int ret = 0;
> > +
> > +       if (!kvm_host_owns_hyp_mappings()) {
> > +               addr = kvm_call_hyp_nvhe(__pkvm_create_private_mapping,
> > +                                        phys_addr, size, prot);
> > +               if (IS_ERR((void *)addr))
>
> IS_ERR_VALUE()?

Good idea, will remove the extra cast.

>
> > +                       return PTR_ERR((void *)addr);
> > +               *haddr = addr;
> > +
> > +               return 0;
> > +       }
> > +
> > +       size += offset_in_page(phys_addr);
> > +       addr = hyp_alloc_private_va_range(size);
> > +       if (IS_ERR((void *)addr))
>
> IS_ERR_VALUE()?

Ack

>
> > +               return PTR_ERR((void *)addr);
> >
> > -       ret = __create_hyp_mappings(base, size, phys_addr, prot);
> > +       ret = __create_hyp_mappings(addr, size, phys_addr, prot);
> >         if (ret)
> >                 goto out;
> >
> > -       *haddr = base + offset_in_page(phys_addr);
> > +       *haddr = addr + offset_in_page(phys_addr);
> >  out:
> >         return ret;
>
> Would be simpler to remove the goto, or return early.
>
>         if (!ret)
>                 *haddr = addr + offset_in_page(phys_addr);
>
>         return ret;

Agreed, I'll remove the goto in the next version.

Thanks,
Kalesh
>
> >  }