linux-kernel - [PATCH 3/5] mmu: don't set the present bit unconditionally

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Date:	Tue, 28 Jun 2016 00:32:38 -0400
From:	Bandan Das <bsd@...hat.com>
To:	kvm@...r.kernel.org
Cc:	pbonzini@...hat.com, guangrong.xiao@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH 3/5] mmu: don't set the present bit unconditionally

To support execute only mappings on behalf of L1
hypervisors, we teach set_spte() to honor L1's valid XWR
bits. This is only if host supports EPT execute only. Reuse
ACC_USER_MASK to signify if the L1 hypervisor has the R bit
set

Signed-off-by: Bandan Das <bsd@...hat.com>
---
 arch/x86/kvm/mmu.c         | 9 +++++++--
 arch/x86/kvm/paging_tmpl.h | 2 +-
 arch/x86/kvm/vmx.c         | 2 +-
 3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 875d4f7..ee2fb16 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2516,13 +2516,17 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 		    gfn_t gfn, kvm_pfn_t pfn, bool speculative,
 		    bool can_unsync, bool host_writable)
 {
-	u64 spte;
+	u64 spte = 0;
 	int ret = 0;
+	struct kvm_mmu *context = &vcpu->arch.mmu;
+	bool execonly = !(context->guest_rsvd_check.bad_mt_xwr &
+			  (1ull << VMX_EPT_EXECUTABLE_MASK));
 
 	if (set_mmio_spte(vcpu, sptep, gfn, pfn, pte_access))
 		return 0;
 
-	spte = PT_PRESENT_MASK;
+	if (!execonly)
+		spte |= PT_PRESENT_MASK;
 	if (!speculative)
 		spte |= shadow_accessed_mask;
 
@@ -2531,6 +2535,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 	else
 		spte |= shadow_nx_mask;
 
+	/* In the EPT case, shadow_user_mask is PT_PRESENT_MASK */
 	if (pte_access & ACC_USER_MASK)
 		spte |= shadow_user_mask;
 
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index bc019f7..896118e 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -187,7 +187,7 @@ static inline unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, u64 gpte)
 #if PTTYPE == PTTYPE_EPT
 	access = ((gpte & VMX_EPT_WRITABLE_MASK) ? ACC_WRITE_MASK : 0) |
 		((gpte & VMX_EPT_EXECUTABLE_MASK) ? ACC_EXEC_MASK : 0) |
-		ACC_USER_MASK;
+		((gpte & VMX_EPT_READABLE_MASK) ? ACC_USER_MASK : 0);
 #else
 	BUILD_BUG_ON(ACC_EXEC_MASK != PT_PRESENT_MASK);
 	BUILD_BUG_ON(ACC_EXEC_MASK != 1);
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 003618e..417debc 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6366,7 +6366,7 @@ static __init int hardware_setup(void)
 	vmx_disable_intercept_msr_write_x2apic(0x83f);
 
 	if (enable_ept) {
-		kvm_mmu_set_mask_ptes(0ull,
+		kvm_mmu_set_mask_ptes(PT_PRESENT_MASK,
 			(enable_ept_ad_bits) ? VMX_EPT_ACCESS_BIT : 0ull,
 			(enable_ept_ad_bits) ? VMX_EPT_DIRTY_BIT : 0ull,
 			0ull, VMX_EPT_EXECUTABLE_MASK);
-- 
2.5.5