Re: [PATCH 5/5] KVM: MMU: fast invalid all mmio sptes

Xiao Guangrong
2013-03-18T12:30:03Z

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? ;)

> 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@linux.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?