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

[Sean Christopherson]

On Wed, Feb 19, 2020 at 05:01:41PM +0800, Xiaoyao Li wrote:
> 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@xxxxxxxxx>
> >---
> >  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 ?

Hmm, EMULTYPE_PF_ALLOW_RETRY does sound better.  I'm on the fence regarding
shuffling the bits.  If I were to shuffle the bits, I'd do a more thorough
reorder so that the #UD and #PF types are consecutive, e.g.

	#define EMULTYPE_NO_DECODE	    (1 << 0)
	#define EMULTYPE_TRAP_UD	    (1 << 1)
	#define EMULTYPE_TRAP_UD_FORCED	    (1 << 2)
	#define EMULTYPE_SKIP		    (1 << 3)
	#define EMULTYPE_VMWARE_GP	    (1 << 4)
	#define EMULTYPE_PF		    (1 << 5)
	#define EMULTYPE_PF_ALLOW_RETRY	    (1 << 6)

Part of me really wants to do that, the other part of me thinks it's
unnecessary thrash.

> >  #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);
> >
>