linux-kernel - Re: [PATCH] KVM: x86: fix bogus warning about reserved bits

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Date:	Wed, 23 Sep 2015 11:36:47 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: fix bogus warning about reserved bits



On 23/09/2015 09:56, Borislav Petkov wrote:
> On Tue, Sep 22, 2015 at 11:04:38PM +0200, Paolo Bonzini wrote:
>> Let's add more debugging output:
> 
> Here you go:
> 
> [   50.474002] walk_shadow_page_get_mmio_spte: detect reserved bits on spte, addr 0xb8000 (level 4, 0xf0000000000f8)
> [   50.484249] walk_shadow_page_get_mmio_spte: detect reserved bits on spte, addr 0xb8000 (level 3, 0xf000000000078)
> [   50.494492] walk_shadow_page_get_mmio_spte: detect reserved bits on spte, addr 0xb8000 (level 2, 0xf000000000078)

And another patch, which both cranks up the debugging a bit and
tries another fix:

diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index dd05b9cef6ae..b2f49bb15ba1 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -105,8 +105,15 @@ static inline bool guest_cpuid_has_x2apic(struct kvm_vcpu *vcpu)
 static inline bool guest_cpuid_is_amd(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best;
+	static bool first;
 
 	best = kvm_find_cpuid_entry(vcpu, 0, 0);
+	if (first && best) {
+		printk("cpuid(0).ebx = %x\n", best->ebx);
+		first = false;
+	} else if (first)
+		printk_ratelimited("cpuid(0) not initialized yet\n");
+
 	return best && best->ebx == X86EMUL_CPUID_VENDOR_AuthenticAMD_ebx;
 }
 
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index bf1122e9c7bf..f50b280ffee1 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3625,7 +3625,7 @@ static void
 __reset_rsvds_bits_mask(struct kvm_vcpu *vcpu,
 			struct rsvd_bits_validate *rsvd_check,
 			int maxphyaddr, int level, bool nx, bool gbpages,
-			bool pse)
+			bool pse, bool amd)
 {
 	u64 exb_bit_rsvd = 0;
 	u64 gbpages_bit_rsvd = 0;
@@ -3642,7 +3642,7 @@ __reset_rsvds_bits_mask(struct kvm_vcpu *vcpu,
 	 * Non-leaf PML4Es and PDPEs reserve bit 8 (which would be the G bit for
 	 * leaf entries) on AMD CPUs only.
 	 */
-	if (guest_cpuid_is_amd(vcpu))
+	if (amd)
 		nonleaf_bit8_rsvd = rsvd_bits(8, 8);
 
 	switch (level) {
@@ -3710,7 +3710,7 @@ static void reset_rsvds_bits_mask(struct kvm_vcpu *vcpu,
 	__reset_rsvds_bits_mask(vcpu, &context->guest_rsvd_check,
 				cpuid_maxphyaddr(vcpu), context->root_level,
 				context->nx, guest_cpuid_has_gbpages(vcpu),
-				is_pse(vcpu));
+				is_pse(vcpu), guest_cpuid_is_amd(vcpu));
 }
 
 static void
@@ -3760,13 +3760,25 @@ static void reset_rsvds_bits_mask_ept(struct kvm_vcpu *vcpu,
 void
 reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
 {
+	/*
+	 * Passing "true" to the last argument is okay; it adds a check
+	 * on bit 8 of the SPTEs which KVM doesn't use anyway.
+	 */
 	__reset_rsvds_bits_mask(vcpu, &context->shadow_zero_check,
 				boot_cpu_data.x86_phys_bits,
 				context->shadow_root_level, context->nx,
-				guest_cpuid_has_gbpages(vcpu), is_pse(vcpu));
+				guest_cpuid_has_gbpages(vcpu), is_pse(vcpu),
+				true);
 }
 EXPORT_SYMBOL_GPL(reset_shadow_zero_bits_mask);
 
+static inline bool
+boot_cpu_is_amd(void)
+{
+	WARN_ON_ONCE(!tdp_enabled);
+	return shadow_x_mask != 0;
+}
+
 /*
  * the direct page table on host, use as much mmu features as
  * possible, however, kvm currently does not do execution-protection.
@@ -3775,11 +3787,11 @@ static void
 reset_tdp_shadow_zero_bits_mask(struct kvm_vcpu *vcpu,
 				struct kvm_mmu *context)
 {
-	if (guest_cpuid_is_amd(vcpu))
+	if (boot_cpu_is_amd())
 		__reset_rsvds_bits_mask(vcpu, &context->shadow_zero_check,
 					boot_cpu_data.x86_phys_bits,
 					context->shadow_root_level, false,
-					cpu_has_gbpages, true);
+					cpu_has_gbpages, true, true);
 	else
 		__reset_rsvds_bits_mask_ept(&context->shadow_zero_check,
 					    boot_cpu_data.x86_phys_bits,



Applies on top of everything else you've got already.

Paolo
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