Date:	Mon, 18 Mar 2013 21:09:43 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 5/5] KVM: MMU: fast invalid all mmio sptes

On 03/18/2013 08:46 PM, Gleb Natapov wrote:
> On Mon, Mar 18, 2013 at 08:29:29PM +0800, Xiao Guangrong wrote:
>> On 03/18/2013 05:13 PM, Gleb Natapov wrote:
>>> On Mon, Mar 18, 2013 at 04:08:50PM +0800, Xiao Guangrong wrote:
>>>> On 03/17/2013 11:02 PM, Gleb Natapov wrote:
>>>>> On Fri, Mar 15, 2013 at 11:29:53PM +0800, Xiao Guangrong wrote:
>>>>>> This patch tries to introduce a very simple and scale way to invalid all
>>>>>> mmio sptes - it need not walk any shadow pages and hold mmu-lock
>>>>>>
>>>>>> KVM maintains a global mmio invalid generation-number which is stored in
>>>>>> kvm->arch.mmio_invalid_gen and every mmio spte stores the current global
>>>>>> generation-number into his available bits when it is created
>>>>>>
>>>>>> When KVM need zap all mmio sptes, it just simply increase the global
>>>>>> generation-number. When guests do mmio access, KVM intercepts a MMIO #PF
>>>>>> then it walks the shadow page table and get the mmio spte. If the
>>>>>> generation-number on the spte does not equal the global generation-number,
>>>>>> it will go to the normal #PF handler to update the mmio spte
>>>>>>
>>>>>> Since 19 bits are used to store generation-number on mmio spte, the
>>>>>> generation-number can be round after 33554432 times. It is large enough
>>>>>> for nearly all most cases, but making the code be more strong, we zap all
>>>>>> shadow pages when the number is round
>>>>>>
>>>>> Very nice idea, but why drop Takuya patches instead of using
>>>>> kvm_mmu_zap_mmio_sptes() when generation number overflows.
>>>>
>>>> I am not sure whether it is still needed. Requesting to zap all mmio sptes for
>>>> more than 500000 times is really really rare, it nearly does not happen.
>>>> (By the way, 33554432 is wrong in the changelog, i just copy that for my origin
>>>> implantation.) And, after my patch optimizing zapping all shadow pages,
>>>> zap-all-sps should not be a problem anymore since it does not take too much lock
>>>> time.
>>>>
>>>> Your idea?
>>>>
>>> I expect 500000 to become less since I already had plans to store some
>>
>> Interesting, just curious, what are the plans? ;)
>>
> Currently we uses pio to signal that work is pending to virtio devices. The
> requirement is that signaling should be fast and PIO is fast since there
> is not need to emulate instruction. PCIE though is not really designed
> with PIO in mind, so we will have to use MMIO to do signaling. To avoid
> instruction emulation I thought about making guest access these devices using
> predefined variety of MOV instruction so that emulation can be skipped.
> The idea is to mark mmio spte to know that emulation is not needed.

How to know page-fault is caused by the predefined instruction?

> 
>>> information in mmio spte. Even if all zap-all-sptes becomes faster we
>>> still needlessly zap all sptes while we can zap only mmio.
>>
>> Okay.
>>
>>>
>>>>>
>>>>>
>>>>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>>>>>> ---
>>>>>>  arch/x86/include/asm/kvm_host.h |    2 +
>>>>>>  arch/x86/kvm/mmu.c              |   61 +++++++++++++++++++++++++++++++++------
>>>>>>  arch/x86/kvm/mmutrace.h         |   17 +++++++++++
>>>>>>  arch/x86/kvm/paging_tmpl.h      |    7 +++-
>>>>>>  arch/x86/kvm/vmx.c              |    4 ++
>>>>>>  arch/x86/kvm/x86.c              |    6 +--
>>>>>>  6 files changed, 82 insertions(+), 15 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>>>>> index ef7f4a5..572398e 100644
>>>>>> --- a/arch/x86/include/asm/kvm_host.h
>>>>>> +++ b/arch/x86/include/asm/kvm_host.h
>>>>>> @@ -529,6 +529,7 @@ struct kvm_arch {
>>>>>>  	unsigned int n_requested_mmu_pages;
>>>>>>  	unsigned int n_max_mmu_pages;
>>>>>>  	unsigned int indirect_shadow_pages;
>>>>>> +	unsigned int mmio_invalid_gen;
>>>>> Why invalid? Should be mmio_valid_gen or mmio_current_get.
>>>>
>>>> mmio_invalid_gen is only updated in kvm_mmu_invalidate_mmio_sptes,
>>>> so i named it as _invalid_. But mmio_valid_gen is good for me.
>>>>
>>> It holds currently valid value though, so calling it "invalid" is
>>> confusing.
>>
>> I agree.
>>
>>>
>>>>>
>>>>>>  	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
>>>>>>  	/*
>>>>>>  	 * Hash table of struct kvm_mmu_page.
>>>>>> @@ -765,6 +766,7 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot);
>>>>>>  void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
>>>>>>  				     struct kvm_memory_slot *slot,
>>>>>>  				     gfn_t gfn_offset, unsigned long mask);
>>>>>> +void kvm_mmu_invalid_mmio_spte(struct kvm *kvm);
>>>>> Agree with Takuya that kvm_mmu_invalidate_mmio_sptes() is a better name.
>>>>
>>>> Me too.
>>>>
>>>>>
>>>>>>  void kvm_mmu_zap_all(struct kvm *kvm);
>>>>>>  unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
>>>>>>  void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages);
>>>>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>>>>> index 13626f4..7093a92 100644
>>>>>> --- a/arch/x86/kvm/mmu.c
>>>>>> +++ b/arch/x86/kvm/mmu.c
>>>>>> @@ -234,12 +234,13 @@ static unsigned int get_mmio_spte_generation(u64 spte)
>>>>>>  static void mark_mmio_spte(struct kvm *kvm, u64 *sptep, u64 gfn,
>>>>>>  			   unsigned access)
>>>>>>  {
>>>>>> -	u64 mask = generation_mmio_spte_mask(0);
>>>>>> +	unsigned int gen = ACCESS_ONCE(kvm->arch.mmio_invalid_gen);
>>>>>> +	u64 mask = generation_mmio_spte_mask(gen);
>>>>>>
>>>>>>  	access &= ACC_WRITE_MASK | ACC_USER_MASK;
>>>>>>  	mask |= shadow_mmio_mask | access | gfn << PAGE_SHIFT;
>>>>>>
>>>>>> -	trace_mark_mmio_spte(sptep, gfn, access, 0);
>>>>>> +	trace_mark_mmio_spte(sptep, gfn, access, gen);
>>>>>>  	mmu_spte_set(sptep, mask);
>>>>>>  }
>>>>>>
>>>>>> @@ -269,6 +270,34 @@ static bool set_mmio_spte(struct kvm *kvm, u64 *sptep, gfn_t gfn,
>>>>>>  	return false;
>>>>>>  }
>>>>>>
>>>>>> +static bool check_mmio_spte(struct kvm *kvm, u64 spte)
>>>>>> +{
>>>>>> +	return get_mmio_spte_generation(spte) ==
>>>>>> +		ACCESS_ONCE(kvm->arch.mmio_invalid_gen);
>>>>>> +}
>>>>>> +
>>>>>> +/*
>>>>>> + * The caller should protect concurrent access on
>>>>>> + * kvm->arch.mmio_invalid_gen. Currently, it is used by
>>>>>> + * kvm_arch_commit_memory_region and protected by kvm->slots_lock.
>>>>>> + */
>>>>>> +void kvm_mmu_invalid_mmio_spte(struct kvm *kvm)
>>>>>> +{
>>>>>> +	/* Ensure update memslot has been completed. */
>>>>>> +	smp_mb();
>>>>> What barrier this one is paired with?
>>>>
>>>> It is paired with nothing. :)
>>>>
>>>> I used mb here just for avoid increasing the generation-number before updating
>>>> the memslot. But on other sides (storing gen and checking gen), we do not need
>>>> to care it - the worse case is that we emulate a memory-accessed instruction.
>>>>
>>> Are you warring that compiler can reorder instructions and put
>>> instruction that increase generation number before updating memslot?
>>> If yes then you need to use barrier() here. Or are you warring that
>>> update may be seen in different order by another cpu? Then you need to
>>> put another barring in the code that access memslot/generation number
>>> and cares about the order.
>>
>> After more thinking, maybe i missed something. The correct order should be:
>>
>> The write side:
>> update kvm->memslots
>> smp_wmb()
>> kvm->mmio_invalid_gen++
>>
>> The read side:
>> read kvm->mmio_invalid_gen++
>> smp_rmb();
>> search gfn in memslots (read all memslots)
>>
>> Otherwise, mmio spte would cache a newest generation-number and obsolete
>> memslot info.
>>
>> But we read memslots out of mmu-lock on page fault path, we should pass
>> mmio_invalid_gen to the page fault hander. In order to simplify the code,
>> let's save the generation-number into kvm_memslots, then they can protected
>> by SRCU. How about this?
>>
> Make sense and in fact we already have generation number there which is
> used for gfn_to_hva_cache. The problem is that gfn_to_hva cache does not
> expect generation number to wrap, but with modulo arithmetic we can make
> it wrap only for mmio sptes.

Reusing the existing generation number can cause mmio spte invalid even if
memslot is deleted but i guess it is not too bad.


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