Date:   Wed, 06 Jul 2022 02:06:27 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v7 037/102] KVM: x86/mmu: Track shadow MMIO value/mask
 on a per-VM basis

On Mon, 2022-06-27 at 14:53 -0700, isaku.yamahata@...el.com wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> TDX will use a different shadow PTE entry value for MMIO from VMX.  Add
> members to kvm_arch and track value for MMIO per-VM instead of global
> variables.  By using the per-VM EPT entry value for MMIO, the existing VMX
> logic is kept working.
> 
> In the case of VMX VM case, the EPT entry for MMIO is non-present PTE
> (present bit cleared) without backing guest physical address (on EPT
> violation, KVM searches backing guest memory and it finds there is no
> backing guest page.) or the value to trigger EPT misconfiguration.  Once
> MMIO is triggered on the EPT entry, the EPT entry is updated to trigger EPT
> misconfiguration for the future MMIO on the same GPA.  It allows KVM to
> understand the memory access is for MMIO without searching backing guest
> pages.). And then KVM parses guest instruction to figure out
> address/value/width for MMIO.
> 
> In the case of the guest TD, the guest memory is protected so that VMM
> can't parse guest instruction to understand the value and access width for
> MMIO.  Instead, VMM sets up (Shared) EPT to trigger #VE by clearing
> the VE-suppress bit.  When the guest TD issues MMIO, #VE is injected.  Guest VE
> handler converts MMIO access into MMIO hypercall to pass
> address/value/width for MMIO to VMM. (or directly paravirtualize MMIO into
> hypercall.)  Then VMM can handle the MMIO hypercall without parsing guest
> instructions.

To me only first paragraph is needed.  It already describes _why_ we need this
patch and _how_ you are going to implement.  

The last two paragraphs only elaborate the _why_ in the first paragraph, but
they does not say this patch will do more.  And they have been explained in
previous patches so looks they are not mandatory here.

> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  4 ++++
>  arch/x86/include/asm/vmx.h      |  1 +
>  arch/x86/kvm/mmu.h              |  4 +++-
>  arch/x86/kvm/mmu/mmu.c          | 20 ++++++++++++----
>  arch/x86/kvm/mmu/paging_tmpl.h  |  2 +-
>  arch/x86/kvm/mmu/spte.c         | 41 +++++++++++++++------------------
>  arch/x86/kvm/mmu/spte.h         | 11 ++++-----
>  arch/x86/kvm/mmu/tdp_mmu.c      |  6 ++---
>  arch/x86/kvm/svm/svm.c          |  2 +-
>  arch/x86/kvm/vmx/vmx.c          |  8 +++++++
>  10 files changed, 59 insertions(+), 40 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 2c47aab72a1b..39215daa8576 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1161,6 +1161,10 @@ struct kvm_arch {
>  	 */
>  	spinlock_t mmu_unsync_pages_lock;
>  
> +	bool enable_mmio_caching;
> +	u64 shadow_mmio_value;
> +	u64 shadow_mmio_mask;
> +
>  	struct list_head assigned_dev_head;
>  	struct iommu_domain *iommu_domain;
>  	bool iommu_noncoherent;
> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> index c371ef695fcc..6231ef005a50 100644
> --- a/arch/x86/include/asm/vmx.h
> +++ b/arch/x86/include/asm/vmx.h
> @@ -511,6 +511,7 @@ enum vmcs_field {
>  #define VMX_EPT_IPAT_BIT    			(1ull << 6)
>  #define VMX_EPT_ACCESS_BIT			(1ull << 8)
>  #define VMX_EPT_DIRTY_BIT			(1ull << 9)
> +#define VMX_EPT_SUPPRESS_VE_BIT			(1ull << 63)

Both the patch title and the changelog say this patch only does per-VM MMIO
value/mask tracking.  Why do we need this bit here?

>  #define VMX_EPT_RWX_MASK                        (VMX_EPT_READABLE_MASK |       \
>  						 VMX_EPT_WRITABLE_MASK |       \
>  						 VMX_EPT_EXECUTABLE_MASK)
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index ccf0ba7a6387..9ba60fd79d33 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -108,7 +108,9 @@ static inline u8 kvm_get_shadow_phys_bits(void)
>  	return boot_cpu_data.x86_phys_bits;
>  }
>  
> -void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask);
> +void kvm_mmu_set_mmio_spte_mask(struct kvm *kvm, u64 mmio_value, u64 mmio_mask,
> +				u64 access_mask);
> +void kvm_mmu_set_default_mmio_spte_mask(u64 mask);
>  void kvm_mmu_set_me_spte_mask(u64 me_value, u64 me_mask);
>  void kvm_mmu_set_ept_masks(bool has_ad_bits, bool has_exec_only);
>  
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index f239b6cb5d53..496d0d30839b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2287,7 +2287,7 @@ static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
>  				return kvm_mmu_prepare_zap_page(kvm, child,
>  								invalid_list);
>  		}
> -	} else if (is_mmio_spte(pte)) {
> +	} else if (is_mmio_spte(kvm, pte)) {
>  		mmu_spte_clear_no_track(spte);
>  	}
>  	return 0;
> @@ -3067,8 +3067,13 @@ static int handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fau
>  		 * by L0 userspace (you can observe gfn > L1.MAXPHYADDR if
>  		 * and only if L1's MAXPHYADDR is inaccurate with respect to
>  		 * the hardware's).
> +		 *
> +		 * Excludes the INTEL TD guest.  Because TD memory is
> +		 * protected, the instruction can't be emulated.  Instead, use
> +		 * SPTE value without #VE suppress bit cleared
> +		 * (kvm->arch.shadow_mmio_value = 0).
>  		 */

Again, I don't think this chunk should be in this patch.  It's out-of-scope of
what the patch claims to do.

I see you will make below code change in later patch (couple of patches later):

-		if (unlikely(!vcpu->kvm->arch.enable_mmio_caching) ||
+		if (unlikely(!vcpu->kvm->arch.enable_mmio_caching &&
+			     !kvm_gfn_shared_mask(vcpu->kvm)) ||
 		    unlikely(fault->gfn > kvm_mmu_max_gfn()))
 			return RET_PF_EMULATE;

So why not putting the comment and the code change together?

> -		if (unlikely(!enable_mmio_caching) ||
> +		if (unlikely(!vcpu->kvm->arch.enable_mmio_caching) ||
>  		    unlikely(fault->gfn > kvm_mmu_max_gfn()))
>  			return RET_PF_EMULATE;
>  	}
> @@ -3200,7 +3205,8 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  		else
>  			sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
>  
> -		if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
> +		if (!is_shadow_present_pte(spte) ||
> +		    is_mmio_spte(vcpu->kvm, spte))
>  			break;
>  
>  		sp = sptep_to_sp(sptep);
> @@ -3907,7 +3913,7 @@ static int handle_mmio_page_fault(struct kvm_vcpu *vcpu, u64 addr, bool direct)
>  	if (WARN_ON(reserved))
>  		return -EINVAL;
>  
> -	if (is_mmio_spte(spte)) {
> +	if (is_mmio_spte(vcpu->kvm, spte)) {
>  		gfn_t gfn = get_mmio_spte_gfn(spte);
>  		unsigned int access = get_mmio_spte_access(spte);
>  
> @@ -4350,7 +4356,7 @@ static unsigned long get_cr3(struct kvm_vcpu *vcpu)
>  static bool sync_mmio_spte(struct kvm_vcpu *vcpu, u64 *sptep, gfn_t gfn,
>  			   unsigned int access)
>  {
> -	if (unlikely(is_mmio_spte(*sptep))) {
> +	if (unlikely(is_mmio_spte(vcpu->kvm, *sptep))) {
>  		if (gfn != get_mmio_spte_gfn(*sptep)) {
>  			mmu_spte_clear_no_track(sptep);
>  			return true;
> @@ -5864,6 +5870,10 @@ int kvm_mmu_init_vm(struct kvm *kvm)
>  	node->track_write = kvm_mmu_pte_write;
>  	node->track_flush_slot = kvm_mmu_invalidate_zap_pages_in_memslot;
>  	kvm_page_track_register_notifier(kvm, node);
> +	kvm_mmu_set_mmio_spte_mask(kvm, shadow_default_mmio_mask,
> +				   shadow_default_mmio_mask,
> +				   ACC_WRITE_MASK | ACC_USER_MASK);
> +

This (along with shadow_default_mmio_mask) looks a little bit weird.  Please
also see comments below.
 
>  	return 0;
>  }
>  
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index ee2fb0c073f3..62ae590d4e5b 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -1032,7 +1032,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>  		gfn_t gfn;
>  
>  		if (!is_shadow_present_pte(sp->spt[i]) &&
> -		    !is_mmio_spte(sp->spt[i]))
> +		    !is_mmio_spte(vcpu->kvm, sp->spt[i]))
>  			continue;
>  
>  		pte_gpa = first_pte_gpa + i * sizeof(pt_element_t);
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index bd441458153f..5194aef60c1f 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -29,8 +29,7 @@ u64 __read_mostly shadow_x_mask; /* mutual exclusive with nx_mask */
>  u64 __read_mostly shadow_user_mask;
>  u64 __read_mostly shadow_accessed_mask;
>  u64 __read_mostly shadow_dirty_mask;
> -u64 __read_mostly shadow_mmio_value;
> -u64 __read_mostly shadow_mmio_mask;
> +u64 __read_mostly shadow_default_mmio_mask;

This shadow_default_mmio_mask looks a little bit weird.  Please also see below.

>  u64 __read_mostly shadow_mmio_access_mask;
>  u64 __read_mostly shadow_present_mask;
>  u64 __read_mostly shadow_me_value;
> @@ -62,10 +61,11 @@ u64 make_mmio_spte(struct kvm_vcpu *vcpu, u64 gfn, unsigned int access)
>  	u64 spte = generation_mmio_spte_mask(gen);
>  	u64 gpa = gfn << PAGE_SHIFT;
>  
> -	WARN_ON_ONCE(!shadow_mmio_value);
> +	WARN_ON_ONCE(!vcpu->kvm->arch.shadow_mmio_value &&
> +		     !kvm_gfn_shared_mask(vcpu->kvm));

Chunk shouldn't belong to  this patch.

>  
>  	access &= shadow_mmio_access_mask;
> -	spte |= shadow_mmio_value | access;
> +	spte |= vcpu->kvm->arch.shadow_mmio_value | access;
>  	spte |= gpa | shadow_nonpresent_or_rsvd_mask;
>  	spte |= (gpa & shadow_nonpresent_or_rsvd_mask)
>  		<< SHADOW_NONPRESENT_OR_RSVD_MASK_LEN;
> @@ -337,7 +337,8 @@ u64 mark_spte_for_access_track(u64 spte)
>  	return spte;
>  }
>  
> -void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
> +void kvm_mmu_set_mmio_spte_mask(struct kvm *kvm, u64 mmio_value, u64 mmio_mask,
> +				u64 access_mask)
>  {
>  	BUG_ON((u64)(unsigned)access_mask != access_mask);
>  	WARN_ON(mmio_value & shadow_nonpresent_or_rsvd_lower_gfn_mask);
> @@ -366,11 +367,9 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
>  	    WARN_ON(mmio_value && (__REMOVED_SPTE & mmio_mask) == mmio_value))
>  		mmio_value = 0;
>  
> -	if (!mmio_value)
> -		enable_mmio_caching = false;
> -
> -	shadow_mmio_value = mmio_value;
> -	shadow_mmio_mask  = mmio_mask;
> +	kvm->arch.enable_mmio_caching = !!mmio_value;
> +	kvm->arch.shadow_mmio_value = mmio_value;
> +	kvm->arch.shadow_mmio_mask = mmio_mask;
>  	shadow_mmio_access_mask = access_mask;
>  }
>  EXPORT_SYMBOL_GPL(kvm_mmu_set_mmio_spte_mask);
> @@ -393,24 +392,18 @@ void kvm_mmu_set_ept_masks(bool has_ad_bits, bool has_exec_only)
>  	shadow_dirty_mask	= has_ad_bits ? VMX_EPT_DIRTY_BIT : 0ull;
>  	shadow_nx_mask		= 0ull;
>  	shadow_x_mask		= VMX_EPT_EXECUTABLE_MASK;
> -	shadow_present_mask	= has_exec_only ? 0ull : VMX_EPT_READABLE_MASK;
> +	/* VMX_EPT_SUPPRESS_VE_BIT is needed for W or X violation. */
> +	shadow_present_mask	=
> +		(has_exec_only ? 0ull : VMX_EPT_READABLE_MASK) | VMX_EPT_SUPPRESS_VE_BIT;

Again, this chunk shouldn't be in this patch.

>  	shadow_acc_track_mask	= VMX_EPT_RWX_MASK;
>  	shadow_host_writable_mask = EPT_SPTE_HOST_WRITABLE;
>  	shadow_mmu_writable_mask  = EPT_SPTE_MMU_WRITABLE;
> -
> -	/*
> -	 * EPT Misconfigurations are generated if the value of bits 2:0
> -	 * of an EPT paging-structure entry is 110b (write/execute).
> -	 */
> -	kvm_mmu_set_mmio_spte_mask(VMX_EPT_MISCONFIG_WX_VALUE,
> -				   VMX_EPT_RWX_MASK, 0);
>  }
>  EXPORT_SYMBOL_GPL(kvm_mmu_set_ept_masks);
>  
>  void kvm_mmu_reset_all_pte_masks(void)
>  {
>  	u8 low_phys_bits;
> -	u64 mask;
>  
>  	shadow_phys_bits = kvm_get_shadow_phys_bits();
>  
> @@ -459,9 +452,13 @@ void kvm_mmu_reset_all_pte_masks(void)
>  	 * PTEs and so the reserved PA approach must be disabled.
>  	 */
>  	if (shadow_phys_bits < 52)
> -		mask = BIT_ULL(51) | PT_PRESENT_MASK;
> +		shadow_default_mmio_mask = BIT_ULL(51) | PT_PRESENT_MASK;
>  	else
> -		mask = 0;
> +		shadow_default_mmio_mask = 0;
> +}

Shadow_default_mmio_mask alone looks a little bit weird with per-VM MMIO
tracking.  I think it can be removed by moving this code to vmx_vm_init(), and
call it as VM's MMIO mask/value for non-EPT case.  If EPT is enabled, it can
override using new mask/value.

>  
> -	kvm_mmu_set_mmio_spte_mask(mask, mask, ACC_WRITE_MASK | ACC_USER_MASK);
> +void kvm_mmu_set_default_mmio_spte_mask(u64 mask)
> +{
> +	shadow_default_mmio_mask = mask;
>  }
> +EXPORT_SYMBOL_GPL(kvm_mmu_set_default_mmio_spte_mask);
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 1bfedbe0585f..96312ab4fffb 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -5,8 +5,6 @@
>  
>  #include "mmu_internal.h"
>  
> -extern bool __read_mostly enable_mmio_caching;
> -
>  /*
>   * A MMU present SPTE is backed by actual memory and may or may not be present
>   * in hardware.  E.g. MMIO SPTEs are not considered present.  Use bit 11, as it
> @@ -160,8 +158,7 @@ extern u64 __read_mostly shadow_x_mask; /* mutual exclusive with nx_mask */
>  extern u64 __read_mostly shadow_user_mask;
>  extern u64 __read_mostly shadow_accessed_mask;
>  extern u64 __read_mostly shadow_dirty_mask;
> -extern u64 __read_mostly shadow_mmio_value;
> -extern u64 __read_mostly shadow_mmio_mask;
> +extern u64 __read_mostly shadow_default_mmio_mask;
>  extern u64 __read_mostly shadow_mmio_access_mask;
>  extern u64 __read_mostly shadow_present_mask;
>  extern u64 __read_mostly shadow_me_value;
> @@ -233,10 +230,10 @@ static inline bool is_removed_spte(u64 spte)
>   */
>  extern u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;
>  
> -static inline bool is_mmio_spte(u64 spte)
> +static inline bool is_mmio_spte(struct kvm *kvm, u64 spte)
>  {
> -	return (spte & shadow_mmio_mask) == shadow_mmio_value &&
> -	       likely(enable_mmio_caching);
> +	return (spte & kvm->arch.shadow_mmio_mask) == kvm->arch.shadow_mmio_value &&
> +		likely(kvm->arch.enable_mmio_caching || kvm_gfn_shared_mask(kvm));
>  }

This chunk (checking kvm_gfn_shared_mask(kvm)) should not be in this patch. 

>  
>  static inline bool is_shadow_present_pte(u64 pte)
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 2ca03ec3bf52..82f1bfac7ee6 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -569,8 +569,8 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
>  		 * impact the guest since both the former and current SPTEs
>  		 * are nonpresent.
>  		 */
> -		if (WARN_ON(!is_mmio_spte(old_spte) &&
> -			    !is_mmio_spte(new_spte) &&
> +		if (WARN_ON(!is_mmio_spte(kvm, old_spte) &&
> +			    !is_mmio_spte(kvm, new_spte) &&
>  			    !is_removed_spte(new_spte)))
>  			pr_err("Unexpected SPTE change! Nonpresent SPTEs\n"
>  			       "should not be replaced with another,\n"
> @@ -1108,7 +1108,7 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
>  	}
>  
>  	/* If a MMIO SPTE is installed, the MMIO will need to be emulated. */
> -	if (unlikely(is_mmio_spte(new_spte))) {
> +	if (unlikely(is_mmio_spte(vcpu->kvm, new_spte))) {
>  		vcpu->stat.pf_mmio_spte_created++;
>  		trace_mark_mmio_spte(rcu_dereference(iter->sptep), iter->gfn,
>  				     new_spte);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 815a07c594f1..0abc43d6a115 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4870,7 +4870,7 @@ static __init void svm_adjust_mmio_mask(void)
>  	 */
>  	mask = (mask_bit < 52) ? rsvd_bits(mask_bit, 51) | PT_PRESENT_MASK : 0;
>  
> -	kvm_mmu_set_mmio_spte_mask(mask, mask, PT_WRITABLE_MASK | PT_USER_MASK);
> +	kvm_mmu_set_default_mmio_spte_mask(mask);

SVM doesn't need shadow_default_mmio_mask.  Instead, it can define a local
variable in svm.c, and call kvm_mmu_set_mmio_spte_mask(mask, mask,
PT_WRITABLE_MASK | PT_USER_MASK) in svm_vm_init().

>  }
>  
>  static __init void svm_set_cpu_caps(void)
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 1d87885245cc..e2415ac55317 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7289,6 +7289,14 @@ int vmx_vm_init(struct kvm *kvm)
>  	if (!ple_gap)
>  		kvm->arch.pause_in_guest = true;
>  
> +	/*
> +	 * EPT Misconfigurations can be generated if the value of bits 2:0
> +	 * of an EPT paging-structure entry is 110b (write/execute).
> +	 */
> +	if (enable_ept)
> +		kvm_mmu_set_mmio_spte_mask(kvm, VMX_EPT_MISCONFIG_WX_VALUE,
> +					   VMX_EPT_RWX_MASK, 0);
> +

As commented above, I think we can remove shadow_default_mmio_mask by moving the
logic in kvm_mmu_reset_all_pte_mask() here.

Or use SVM similar way, use a local variable 'mask' in vmx.c, calculate the
'mask' during hardware_setup(), and use it here for non-EPT case.


>  	if (boot_cpu_has(X86_BUG_L1TF) && enable_ept) {
>  		switch (l1tf_mitigation) {
>  		case L1TF_MITIGATION_OFF:

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