Re: [PATCH 06/11] KVM: MMU: let page fault handler be aware tracked page

From: Kai Huang <kai.huang@linux.intel.com>
To: Xiao Guangrong <guangrong.xiao@linux.intel.com>, pbonzini@redhat.com
Cc: gleb@kernel.org, mtosatti@redhat.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 06/11] KVM: MMU: let page fault handler be aware tracked page
Date: Tue, 15 Dec 2015 16:11:01 +0800	[thread overview]
Message-ID: <566FCB15.1000004@linux.intel.com> (raw)
In-Reply-To: <1448907973-36066-7-git-send-email-guangrong.xiao@linux.intel.com>


On 12/01/2015 02:26 AM, Xiao Guangrong wrote:
> The page fault caused by write access on the write tracked page can not
> be fixed, it always need to be emulated. page_fault_handle_page_track()
> is the fast path we introduce here to skip holding mmu-lock and shadow
> page table walking
Why can it be out side of mmu-lock? Is it OK that some other thread 
removes tracking of this page simultaneously? Shall we assuming the 
emulation code should handle this case?

Even it works for write protection, is it OK for other purpose tracking 
(as your tracking framework can be extended for other purpose)?
>
> However, if the page table is not present, it is worth making the page
> table entry present and readonly to make the read access happy
It's  fine for tracking write from guest. But what if I want to track 
every read from guest? Probably I am exaggerating :)

Thanks,
-Kai
>
> mmu_need_write_protect() need to be cooked to avoid page becoming writable
> when making page table present or sync/prefetch shadow page table entries
>
> Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
> ---
>   arch/x86/include/asm/kvm_page_track.h |  2 ++
>   arch/x86/kvm/mmu.c                    | 44 +++++++++++++++++++++++++++++------
>   arch/x86/kvm/page_track.c             | 14 +++++++++++
>   arch/x86/kvm/paging_tmpl.h            |  3 +++
>   4 files changed, 56 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
> index 9cc17c6..f223201 100644
> --- a/arch/x86/include/asm/kvm_page_track.h
> +++ b/arch/x86/include/asm/kvm_page_track.h
> @@ -15,4 +15,6 @@ void kvm_page_track_add_page(struct kvm *kvm, gfn_t gfn,
>   			     enum kvm_page_track_mode mode);
>   void kvm_page_track_remove_page(struct kvm *kvm, gfn_t gfn,
>   				enum kvm_page_track_mode mode);
> +bool kvm_page_track_check_mode(struct kvm_vcpu *vcpu, gfn_t gfn,
> +			       enum kvm_page_track_mode mode);
>   #endif
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 39809b8..b23f9fc 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -41,6 +41,7 @@
>   #include <asm/cmpxchg.h>
>   #include <asm/io.h>
>   #include <asm/vmx.h>
> +#include <asm/kvm_page_track.h>
>   
>   /*
>    * When setting this variable to true it enables Two-Dimensional-Paging
> @@ -2456,25 +2457,29 @@ static void kvm_unsync_pages(struct kvm_vcpu *vcpu,  gfn_t gfn)
>   	}
>   }
>   
> -static int mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
> -				  bool can_unsync)
> +static bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
> +				   bool can_unsync)
>   {
>   	struct kvm_mmu_page *s;
>   	bool need_unsync = false;
>   
> +	if (kvm_page_track_check_mode(vcpu, gfn, KVM_PAGE_TRACK_WRITE))
> +		return true;
> +
>   	for_each_gfn_indirect_valid_sp(vcpu->kvm, s, gfn) {
>   		if (!can_unsync)
> -			return 1;
> +			return true;
>   
>   		if (s->role.level != PT_PAGE_TABLE_LEVEL)
> -			return 1;
> +			return true;
>   
>   		if (!s->unsync)
>   			need_unsync = true;
>   	}
>   	if (need_unsync)
>   		kvm_unsync_pages(vcpu, gfn);
> -	return 0;
> +
> +	return false;
>   }
>   
>   static bool kvm_is_mmio_pfn(pfn_t pfn)
> @@ -3388,10 +3393,30 @@ int handle_mmio_page_fault(struct kvm_vcpu *vcpu, u64 addr, bool direct)
>   }
>   EXPORT_SYMBOL_GPL(handle_mmio_page_fault);
>   
> +static bool page_fault_handle_page_track(struct kvm_vcpu *vcpu,
> +					 u32 error_code, gfn_t gfn)
> +{
> +	if (unlikely(error_code & PFERR_RSVD_MASK))
> +		return false;
> +
> +	if (!(error_code & PFERR_PRESENT_MASK) ||
> +	      !(error_code & PFERR_WRITE_MASK))
> +		return false;
> +
> +	/*
> +	 * guest is writing the page which is write tracked which can
> +	 * not be fixed by page fault handler.
> +	 */
> +	if (kvm_page_track_check_mode(vcpu, gfn, KVM_PAGE_TRACK_WRITE))
> +		return true;
> +
> +	return false;
> +}
> +
>   static int nonpaging_page_fault(struct kvm_vcpu *vcpu, gva_t gva,
>   				u32 error_code, bool prefault)
>   {
> -	gfn_t gfn;
> +	gfn_t gfn = gva >> PAGE_SHIFT;
>   	int r;
>   
>   	pgprintk("%s: gva %lx error %x\n", __func__, gva, error_code);
> @@ -3403,13 +3428,15 @@ static int nonpaging_page_fault(struct kvm_vcpu *vcpu, gva_t gva,
>   			return r;
>   	}
>   
> +	if (page_fault_handle_page_track(vcpu, error_code, gfn))
> +		return 1;
> +
>   	r = mmu_topup_memory_caches(vcpu);
>   	if (r)
>   		return r;
>   
>   	MMU_WARN_ON(!VALID_PAGE(vcpu->arch.mmu.root_hpa));
>   
> -	gfn = gva >> PAGE_SHIFT;
>   
>   	return nonpaging_map(vcpu, gva & PAGE_MASK,
>   			     error_code, gfn, prefault);
> @@ -3493,6 +3520,9 @@ static int tdp_page_fault(struct kvm_vcpu *vcpu, gva_t gpa, u32 error_code,
>   			return r;
>   	}
>   
> +	if (page_fault_handle_page_track(vcpu, error_code, gfn))
> +		return 1;
> +
>   	r = mmu_topup_memory_caches(vcpu);
>   	if (r)
>   		return r;
> diff --git a/arch/x86/kvm/page_track.c b/arch/x86/kvm/page_track.c
> index ad510db..dc2da12 100644
> --- a/arch/x86/kvm/page_track.c
> +++ b/arch/x86/kvm/page_track.c
> @@ -151,3 +151,17 @@ void kvm_page_track_remove_page(struct kvm *kvm, gfn_t gfn,
>   		spin_unlock(&kvm->mmu_lock);
>   	}
>   }
> +
> +/*
> + * check if the corresponding access on the specified guest page is tracked.
> + */
> +bool kvm_page_track_check_mode(struct kvm_vcpu *vcpu, gfn_t gfn,
> +			       enum kvm_page_track_mode mode)
> +{
> +	struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
> +	int index = gfn_to_index(gfn, slot->base_gfn, PT_PAGE_TABLE_LEVEL);
> +
> +	WARN_ON(!check_mode(mode));
> +
> +	return !!ACCESS_ONCE(slot->arch.gfn_track[mode][index]);
> +}
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 91e939b..ac85682 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -735,6 +735,9 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gva_t addr, u32 error_code,
>   		return 0;
>   	}
>   
> +	if (page_fault_handle_page_track(vcpu, error_code, walker.gfn))
> +		return 1;
> +
>   	vcpu->arch.write_fault_to_shadow_pgtable = false;
>   
>   	is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu,