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SubjectRe: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints
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> On Feb 4, 2019, at 3:37 PM, Alexander Duyck <alexander.h.duyck@linux.intel.com> wrote:
>
> 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.

If I understand you correctly, you “complain” that this would affect
performance.

While it might be, you may want to check whether the already available
tools can solve the problem:

1. You can use a combination of static-key and pv-ops - see for example
steal_account_process_time()

2. You can use callee-saved pv-ops.

The latter might anyhow be necessary since, IIUC, you change a very hot
path. So you may want have a look on the assembly code of free_pcp_prepare()
(or at least its code-size) before and after your changes. If they are too
big, a callee-saved function might be necessary.

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