linux-kernel - Re: [PATCH v2 2/5] mmu: don't set the present bit unconditionally

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Message-ID: <942237ee-a673-5cbf-0e7e-8e9075c904ec@redhat.com>
Date:	Wed, 13 Jul 2016 10:10:14 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bandan Das <bsd@...hat.com>, kvm@...r.kernel.org
Cc:	guangrong.xiao@...ux.intel.com, kernellwp@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/5] mmu: don't set the present bit unconditionally



On 13/07/2016 00:18, Bandan Das wrote:
> +	/*
> +	 * In the non-EPT case, execonly is not valid and so
> +	 * the following line is equivalent to spte |= PT_PRESENT_MASK.

I think the comment need not mention non-EPT.

> +	 * For the EPT case, shadow_present_mask is 0 if hardware
> +	 * supports it and we honor whatever way the guest set it.

if hardware supports exec-only page table entries.

> +	 * See: FNAME(gpte_access) in paging_tmpl.h
> +	 */
> +	spte |= shadow_present_mask;
>  	if (!speculative)
>  		spte |= shadow_accessed_mask;
>  
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index bc019f7..f2741db 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -181,13 +181,19 @@ no_present:
>  	return true;
>  }
>  
> +/*
> + * For PTTYPE_EPT, a page table can be executable but not readable
> + * on supported processors. Therefore, set_spte does not automatically
> + * set bit 0 if execute only is supported. Here, we repurpose ACC_USER_MASK
> + * to signify readability since it isn't used in the EPT case
> + */
>  static inline unsigned FNAME(gpte_access)(struct kvm_vcpu *vcpu, u64 gpte)
>  {
>  	unsigned access;
>  #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 64a79f2..f73b5dc 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -6366,10 +6366,13 @@ 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(VMX_EPT_READABLE_MASK,
>  			(enable_ept_ad_bits) ? VMX_EPT_ACCESS_BIT : 0ull,
>  			(enable_ept_ad_bits) ? VMX_EPT_DIRTY_BIT : 0ull,
> -			0ull, VMX_EPT_EXECUTABLE_MASK);
> +			0ull, VMX_EPT_EXECUTABLE_MASK,
> +			cpu_has_vmx_ept_execute_only() ?
> +				      0ull : PT_PRESENT_MASK);

Better to use VMX_EPT_READABLE_MASK here, which removes the need for the
BUILD_BUG_ON.

I can do these changes myself.

Paolo

> +		BUILD_BUG_ON(PT_PRESENT_MASK != VMX_EPT_READABLE_MASK);