Subject: Re: linux-next: manual merge of the kvm tree with the tip tree

From: Paolo Bonzini <[email-masked]>
Date: Fri, 25 Aug 2017 08:39:22 +0200

On 25/08/2017 06:39, Stephen Rothwell wrote:
> Hi all,
> 
> Today's linux-next merge of the kvm tree got a conflict in:
> 
>   arch/x86/kvm/mmu.h
> 
> between commit:
> 
>   d0ec49d4de90 ("kvm/x86/svm: Support Secure Memory Encryption within KVM")
> 
> from the tip tree and commit:
> 
>   d1cd3ce90044 ("KVM: MMU: check guest CR3 reserved bits based on its physical address width.")
> 
> from the kvm tree.
> 
> I fixed it up (see below) and can carry the fix as necessary. This
> is now fixed as far as linux-next is concerned, but any non trivial
> conflicts should be mentioned to your upstream maintainer when your tree
> is submitted for merging.  You may also want to consider cooperating
> with the maintainer of the conflicting tree to minimise any particularly
> complex conflicts.
> 

Thomas L., Ingo,

this is completely wrong:

> 
>  static inline u64 rsvd_bits(int s, int e)
>  {
> -	return ((1ULL << (e - s + 1)) - 1) << s;
> +	return __sme_clr(((1ULL << (e - s + 1)) - 1) << s);
>  }
>  

First, rsvd_bits is just a simple function to return some 1 bits.  Applying
a mask based on properties of the host MMU is incorrect.

Second, the masks computed by __reset_rsvds_bits_mask also apply to 
guest page tables, where the C bit is reserved since we don't emulate
SME.

Something like this:

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 2dafd36368cc..e0597d703d72 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4142,16 +4142,24 @@ void
 reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
 {
 	bool uses_nx = context->nx || context->base_role.smep_andnot_wp;
+	struct rsvd_bits_validate *shadow_zero_check;
+	int i;
 
 	/*
 	 * 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,
+        shadow_zero_check = &context->shadow_zero_check;
+	__reset_rsvds_bits_mask(vcpu, shadow_zero_check,
 				boot_cpu_data.x86_phys_bits,
 				context->shadow_root_level, uses_nx,
 				guest_cpuid_has(vcpu, X86_FEATURE_GBPAGES),
 				is_pse(vcpu), true);
+
+	for (i = context->shadow_root_level; --i >= 0; ) {
+		shadow_zero_check->rsvd_bits_mask[i][0] &= ~shadow_me_mask;
+		shadow_zero_check->rsvd_bits_mask[i][1] &= ~shadow_me_mask;
+	}
 }
 EXPORT_SYMBOL_GPL(reset_shadow_zero_bits_mask);

Can you please fix it up?   Please Cc me at [email-masked] too 
because I'll be on vacation next week.

(And thanks Stephen for the heads-up!)

Paolo