linux-kernel - Re: [PATCH v2 1/3] KVM: x86: Add EMULTYPE_PF when emulation is triggered by a page fault

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Date:   Wed, 19 Feb 2020 17:01:41 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] KVM: x86: Add EMULTYPE_PF when emulation is
 triggered by a page fault

On 2/19/2020 7:03 AM, Sean Christopherson wrote:
> Add a new emulation type flag to explicitly mark emulation related to a
> page fault.  Move the propation of the GPA into the emulator from the
> page fault handler into x86_emulate_instruction, using EMULTYPE_PF as an
> indicator that cr2 is valid.  Similarly, don't propagate cr2 into the
> exception.address when it's *not* valid.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>   arch/x86/include/asm/kvm_host.h | 12 +++++++++---
>   arch/x86/kvm/mmu/mmu.c          | 10 ++--------
>   arch/x86/kvm/x86.c              | 25 +++++++++++++++++++------
>   3 files changed, 30 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 4dffbc10d3f8..10c1e8f472b6 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1370,8 +1370,9 @@ extern u64 kvm_mce_cap_supported;
>    *		   decode the instruction length.  For use *only* by
>    *		   kvm_x86_ops->skip_emulated_instruction() implementations.
>    *
> - * EMULTYPE_ALLOW_RETRY - Set when the emulator should resume the guest to
> - *			  retry native execution under certain conditions.
> + * EMULTYPE_ALLOW_RETRY_PF - Set when the emulator should resume the guest to
> + *			     retry native execution under certain conditions,
> + *			     Can only be set in conjunction with EMULTYPE_PF.
>    *
>    * EMULTYPE_TRAP_UD_FORCED - Set when emulating an intercepted #UD that was
>    *			     triggered by KVM's magic "force emulation" prefix,
> @@ -1384,13 +1385,18 @@ extern u64 kvm_mce_cap_supported;
>    *			backdoor emulation, which is opt in via module param.
>    *			VMware backoor emulation handles select instructions
>    *			and reinjects the #GP for all other cases.
> + *
> + * EMULTYPE_PF - Set when emulating MMIO by way of an intercepted #PF, in which
> + *		 case the CR2/GPA value pass on the stack is valid.
>    */
>   #define EMULTYPE_NO_DECODE	    (1 << 0)
>   #define EMULTYPE_TRAP_UD	    (1 << 1)
>   #define EMULTYPE_SKIP		    (1 << 2)
> -#define EMULTYPE_ALLOW_RETRY	    (1 << 3)
> +#define EMULTYPE_ALLOW_RETRY_PF	    (1 << 3)

How about naming it as EMULTYPE_PF_ALLOW_RETRY and exchanging the bit 
position with EMULTYPE_PF ?

>   #define EMULTYPE_TRAP_UD_FORCED	    (1 << 4)
>   #define EMULTYPE_VMWARE_GP	    (1 << 5)
> +#define EMULTYPE_PF		    (1 << 6)
> +
>   int kvm_emulate_instruction(struct kvm_vcpu *vcpu, int emulation_type);
>   int kvm_emulate_instruction_from_buffer(struct kvm_vcpu *vcpu,
>   					void *insn, int insn_len);
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7011a4e54866..258624d46588 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5416,18 +5416,12 @@ EXPORT_SYMBOL_GPL(kvm_mmu_unprotect_page_virt);
>   int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 error_code,
>   		       void *insn, int insn_len)
>   {
> -	int r, emulation_type = 0;
> +	int r, emulation_type = EMULTYPE_PF;
>   	bool direct = vcpu->arch.mmu->direct_map;
>   
>   	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root_hpa)))
>   		return RET_PF_RETRY;
>   
> -	/* With shadow page tables, fault_address contains a GVA or nGPA.  */
> -	if (vcpu->arch.mmu->direct_map) {
> -		vcpu->arch.gpa_available = true;
> -		vcpu->arch.gpa_val = cr2_or_gpa;
> -	}
> -
>   	r = RET_PF_INVALID;
>   	if (unlikely(error_code & PFERR_RSVD_MASK)) {
>   		r = handle_mmio_page_fault(vcpu, cr2_or_gpa, direct);
> @@ -5472,7 +5466,7 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 error_code,
>   	 * for L1 isn't going to magically fix whatever issue cause L2 to fail.
>   	 */
>   	if (!mmio_info_in_cache(vcpu, cr2_or_gpa, direct) && !is_guest_mode(vcpu))
> -		emulation_type = EMULTYPE_ALLOW_RETRY;
> +		emulation_type |= EMULTYPE_ALLOW_RETRY_PF;
>   emulate:
>   	/*
>   	 * On AMD platforms, under certain conditions insn_len may be zero on #NPF.
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fbabb2f06273..92af6c5a69e3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6483,10 +6483,11 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>   	gpa_t gpa = cr2_or_gpa;
>   	kvm_pfn_t pfn;
>   
> -	if (!(emulation_type & EMULTYPE_ALLOW_RETRY))
> +	if (!(emulation_type & EMULTYPE_ALLOW_RETRY_PF))
>   		return false;
>   
> -	if (WARN_ON_ONCE(is_guest_mode(vcpu)))
> +	if (WARN_ON_ONCE(is_guest_mode(vcpu)) ||
> +	    WARN_ON_ONCE(!(emulation_type & EMULTYPE_PF)))
>   		return false;
>   
>   	if (!vcpu->arch.mmu->direct_map) {
> @@ -6574,10 +6575,11 @@ static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
>   	 */
>   	vcpu->arch.last_retry_eip = vcpu->arch.last_retry_addr = 0;
>   
> -	if (!(emulation_type & EMULTYPE_ALLOW_RETRY))
> +	if (!(emulation_type & EMULTYPE_ALLOW_RETRY_PF))
>   		return false;
>   
> -	if (WARN_ON_ONCE(is_guest_mode(vcpu)))
> +	if (WARN_ON_ONCE(is_guest_mode(vcpu)) ||
> +	    WARN_ON_ONCE(!(emulation_type & EMULTYPE_PF)))
>   		return false;
>   
>   	if (x86_page_table_writing_insn(ctxt))
> @@ -6830,8 +6832,19 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>   	}
>   
>   restart:
> -	/* Save the faulting GPA (cr2) in the address field */
> -	ctxt->exception.address = cr2_or_gpa;
> +	if (emulation_type & EMULTYPE_PF) {
> +		/* Save the faulting GPA (cr2) in the address field */
> +		ctxt->exception.address = cr2_or_gpa;
> +
> +		/* With shadow page tables, cr2 contains a GVA or nGPA. */
> +		if (vcpu->arch.mmu->direct_map) {
> +			vcpu->arch.gpa_available = true;
> +			vcpu->arch.gpa_val = cr2_or_gpa;
> +		}
> +	} else {
> +		/* Sanitize the address out of an abundance of paranoia. */
> +		ctxt->exception.address = 0;
> +	}
>   
>   	r = x86_emulate_insn(ctxt);
>   
>