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

[Michael Kelley]

From: Arnd Bergmann <arnd@xxxxxxxx> Sent: Monday, March 16, 2020 1:33 AM
> 
> 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.
> 

I think there's value in keeping these functions.  There are 8 uses in
architecture independent code at the moment, which admittedly saves
only ~1/2 Mbyte of memory with a 64K page size, but we will have
additional uses with more memory savings as we get all of the
Hyper-V synthetic drivers to work with 64K page size.  Furthermore,
there's coming work that will require additional steps to share a page
between a guest and the Hyper-V host.  These functions are the right
place to put the code for the additional sharing steps.  Removing them
now in favor of a bare kmalloc() and then adding them back doesn't
seem worthwhile.

Michael