linux-kernel - Re: [PATCH 05/21] KVM: VMX: Teach EPT violation helper about private mem

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Message-ID: <8761e1b8-4c65-4837-b152-98be86cf220d@intel.com>
Date: Thu, 12 Sep 2024 12:39:36 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>, <seanjc@...gle.com>,
	<pbonzini@...hat.com>, <kvm@...r.kernel.org>
CC: <dmatlack@...gle.com>, <isaku.yamahata@...il.com>, <yan.y.zhao@...el.com>,
	<nik.borisov@...e.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/21] KVM: VMX: Teach EPT violation helper about private
 mem



On 4/09/2024 3:07 pm, Rick Edgecombe wrote:
> Teach EPT violation helper to check shared mask of a GPA to find out
> whether the GPA is for private memory.
> 
> When EPT violation is triggered after TD accessing a private GPA, KVM will
> exit to user space if the corresponding GFN's attribute is not private.
> User space will then update GFN's attribute during its memory conversion
> process. After that, TD will re-access the private GPA and trigger EPT
> violation again. Only with GFN's attribute matches to private, KVM will
> fault in private page, map it in mirrored TDP root, and propagate changes
> to private EPT to resolve the EPT violation.
> 
> Relying on GFN's attribute tracking xarray to determine if a GFN is
> private, as for KVM_X86_SW_PROTECTED_VM, may lead to endless EPT
> violations.
> 
> Co-developed-by: Yan Zhao <yan.y.zhao@...el.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> ---
> TDX MMU part 2 v1:
>   - Split from "KVM: TDX: handle ept violation/misconfig exit"
> ---
>   arch/x86/kvm/vmx/common.h | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/common.h b/arch/x86/kvm/vmx/common.h
> index 78ae39b6cdcd..10aa12d45097 100644
> --- a/arch/x86/kvm/vmx/common.h
> +++ b/arch/x86/kvm/vmx/common.h
> @@ -6,6 +6,12 @@
>   
>   #include "mmu.h"
>   
> +static inline bool kvm_is_private_gpa(struct kvm *kvm, gpa_t gpa)
> +{
> +	/* For TDX the direct mask is the shared mask. */
> +	return !kvm_is_addr_direct(kvm, gpa);
> +}

Does this get used in any other places?  If no I think we can open code 
this in the __vmx_handle_ept_violation().

The reason is I think the name kvm_is_private_gpa() is too generic and 
this is in the header file.  E.g., one can come up with another 
kvm_is_private_gpa() checking the memory attributes to tell whether a 
GPA is private.

Or we rename it to something like

	__vmx_is_faulting_gpa_private()
?

Which clearly says it is checking the *faulting* GPA.

> +
>   static inline int __vmx_handle_ept_violation(struct kvm_vcpu *vcpu, gpa_t gpa,
>   					     unsigned long exit_qualification)
>   {
> @@ -28,6 +34,13 @@ static inline int __vmx_handle_ept_violation(struct kvm_vcpu *vcpu, gpa_t gpa,
>   		error_code |= (exit_qualification & EPT_VIOLATION_GVA_TRANSLATED) ?
>   			      PFERR_GUEST_FINAL_MASK : PFERR_GUEST_PAGE_MASK;
>   
> +	/*
> +	 * Don't rely on GFN's attribute tracking xarray to prevent EPT violation
> +	 * loops.
> +	 */
> +	if (kvm_is_private_gpa(vcpu->kvm, gpa))
> +		error_code |= PFERR_PRIVATE_ACCESS;
> +
>   	return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
>   }
>