Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752398Ab3CRNJw (ORCPT ); Mon, 18 Mar 2013 09:09:52 -0400 Received: from e23smtp08.au.ibm.com ([202.81.31.141]:60441 "EHLO e23smtp08.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751190Ab3CRNJu (ORCPT ); Mon, 18 Mar 2013 09:09:50 -0400 Message-ID: <51471217.7070801@linux.vnet.ibm.com> Date: Mon, 18 Mar 2013 21:09:43 +0800 From: Xiao Guangrong User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2 MIME-Version: 1.0 To: Gleb Natapov CC: Marcelo Tosatti , LKML , KVM Subject: Re: [PATCH 5/5] KVM: MMU: fast invalid all mmio sptes References: <51433D98.4050605@linux.vnet.ibm.com> <51433E71.2070107@linux.vnet.ibm.com> <20130317150239.GM11223@redhat.com> <5146CB92.7060203@linux.vnet.ibm.com> <20130318091303.GA1276@redhat.com> <514708A9.8040009@linux.vnet.ibm.com> <20130318124616.GH4020@redhat.com> In-Reply-To: <20130318124616.GH4020@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13031813-5140-0000-0000-000002E83901 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 8404 Lines: 194 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 >>>>>> --- >>>>>> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/