Re: [PATCH v6 04/10] arm64: hyperv: Add memory alloc/free functions for Hyper-V size pages

[Arnd Bergmann]

On Mon, Mar 16, 2020 at 9:30 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> On 2020-03-16 08:22, Arnd Bergmann wrote:
> > On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@xxxxxxxxxxxxx>
> > wrote:
> >>  /*
> >> + * Functions for allocating and freeing memory with size and
> >> + * alignment HV_HYP_PAGE_SIZE. These functions are needed because
> >> + * the guest page size may not be the same as the Hyper-V page
> >> + * size. We depend upon kmalloc() aligning power-of-two size
> >> + * allocations to the allocation size boundary, so that the
> >> + * allocated memory appears to Hyper-V as a page of the size
> >> + * it expects.
> >> + *
> >> + * These functions are used by arm64 specific code as well as
> >> + * arch independent Hyper-V drivers.
> >> + */
> >> +
> >> +void *hv_alloc_hyperv_page(void)
> >> +{
> >> +       BUILD_BUG_ON(PAGE_SIZE <  HV_HYP_PAGE_SIZE);
> >> +       return kmalloc(HV_HYP_PAGE_SIZE, GFP_KERNEL);
> >> +}
> >> +EXPORT_SYMBOL_GPL(hv_alloc_hyperv_page);
> >
> > I don't think there is any guarantee that kmalloc() returns
> > page-aligned
> > allocations in general.
>
> I believe that guarantee came with 59bb47985c1db ("mm, sl[aou]b:
> guarantee
> natural alignment for kmalloc(power-of-two)").
>
> > How about using get_free_pages() to implement this?
>
> This would certainly work, at the expense of a lot of wasted memory when
> PAGE_SIZE isn't 4k.

I'm sure this is the least of your problems when the guest runs with
a large base page size, you've already wasted most of your memory
otherwise then.

    Arnd