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

[Nadav Amit <nadav.amit@gmail.com>]

> 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.