Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

On Mon, 2019-02-04 at 15:00 -0800, Nadav Amit wrote:
> > On Feb 4, 2019, at 10:15 AM, Alexander Duyck <alexander.duyck@gmail.com> wrote:
> > 
> > From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > 
> > Add guest support for providing free memory hints to the KVM hypervisor for
> > freed pages huge TLB size or larger. I am restricting the size to
> > huge TLB order and larger because the hypercalls are too expensive to be
> > performing one per 4K page. Using the huge TLB order became the obvious
> > choice for the order to use as it allows us to avoid fragmentation of higher
> > order memory on the host.
> > 
> > I have limited the functionality so that it doesn't work when page
> > poisoning is enabled. I did this because a write to the page after doing an
> > MADV_DONTNEED would effectively negate the hint, so it would be wasting
> > cycles to do so.
> > 
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > ---
> > arch/x86/include/asm/page.h |   13 +++++++++++++
> > arch/x86/kernel/kvm.c       |   23 +++++++++++++++++++++++
> > 2 files changed, 36 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
> > index 7555b48803a8..4487ad7a3385 100644
> > --- a/arch/x86/include/asm/page.h
> > +++ b/arch/x86/include/asm/page.h
> > @@ -18,6 +18,19 @@
> > 
> > struct page;
> > 
> > +#ifdef CONFIG_KVM_GUEST
> > +#include <linux/jump_label.h>
> > +extern struct static_key_false pv_free_page_hint_enabled;
> > +
> > +#define HAVE_ARCH_FREE_PAGE
> > +void __arch_free_page(struct page *page, unsigned int order);
> > +static inline void arch_free_page(struct page *page, unsigned int order)
> > +{
> > +	if (static_branch_unlikely(&pv_free_page_hint_enabled))
> > +		__arch_free_page(page, order);
> > +}
> > +#endif
> 
> This patch and the following one assume that only KVM should be able to hook
> to these events. I do not think it is appropriate for __arch_free_page() to
> effectively mean “kvm_guest_free_page()”.
> 
> Is it possible to use the paravirt infrastructure for this feature,
> similarly to other PV features? It is not the best infrastructure, but at least
> it is hypervisor-neutral.

I could probably tie this into the paravirt infrastructure, but if I
did so I would probably want to pull the checks for the page order out
of the KVM specific bits and make it something we handle in the inline.
Doing that I would probably make it a paravirtual hint that only
operates at the PMD level. That way we wouldn't incur the cost of the
paravirt infrastructure at the per 4K page level.