linux-kernel - Re: [PATCH v19 101/130] KVM: TDX: handle ept violation/misconfig exit

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Date: Tue, 30 Apr 2024 13:47:07 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>, Chao Gao <chao.gao@...el.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<isaku.yamahata@...il.com>, Paolo Bonzini <pbonzini@...hat.com>,
	<erdemaktas@...gle.com>, Sean Christopherson <seanjc@...gle.com>, Sagi Shahar
	<sagis@...gle.com>, Kai Huang <kai.huang@...el.com>, <chen.bo@...el.com>,
	<hang.yuan@...el.com>, <tina.zhang@...el.com>,
	<isaku.yamahata@...ux.intel.com>
Subject: Re: [PATCH v19 101/130] KVM: TDX: handle ept violation/misconfig exit

Hi Isaku,

On 4/3/2024 11:42 AM, Isaku Yamahata wrote:
> On Mon, Apr 01, 2024 at 12:10:58PM +0800,
> Chao Gao <chao.gao@...el.com> wrote:
> 
>>> +static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
>>> +{
>>> +	unsigned long exit_qual;
>>> +
>>> +	if (kvm_is_private_gpa(vcpu->kvm, tdexit_gpa(vcpu))) {
>>> +		/*
>>> +		 * Always treat SEPT violations as write faults.  Ignore the
>>> +		 * EXIT_QUALIFICATION reported by TDX-SEAM for SEPT violations.
>>> +		 * TD private pages are always RWX in the SEPT tables,
>>> +		 * i.e. they're always mapped writable.  Just as importantly,
>>> +		 * treating SEPT violations as write faults is necessary to
>>> +		 * avoid COW allocations, which will cause TDAUGPAGE failures
>>> +		 * due to aliasing a single HPA to multiple GPAs.
>>> +		 */
>>> +#define TDX_SEPT_VIOLATION_EXIT_QUAL	EPT_VIOLATION_ACC_WRITE
>>> +		exit_qual = TDX_SEPT_VIOLATION_EXIT_QUAL;
>>> +	} else {
>>> +		exit_qual = tdexit_exit_qual(vcpu);
>>> +		if (exit_qual & EPT_VIOLATION_ACC_INSTR) {
>>
>> Unless the CPU has a bug, instruction fetch in TD from shared memory causes a
>> #PF. I think you can add a comment for this.
> 
> Yes.
> 
> 
>> Maybe KVM_BUG_ON() is more appropriate as it signifies a potential bug.
> 
> Bug of what component? CPU. If so, I think KVM_EXIT_INTERNAL_ERROR +
> KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON is more appropriate.
> 

Is below what you have in mind?

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 499c6cd9633f..bd30b4c4d710 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1305,11 +1305,18 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
 	} else {
 		exit_qual = tdexit_exit_qual(vcpu);
 		if (exit_qual & EPT_VIOLATION_ACC_INSTR) {
+			/*
+			 * Instruction fetch in TD from shared memory
+			 * causes a #PF.
+			 */
 			pr_warn("kvm: TDX instr fetch to shared GPA = 0x%lx @ RIP = 0x%lx\n",
 				tdexit_gpa(vcpu), kvm_rip_read(vcpu));
-			vcpu->run->exit_reason = KVM_EXIT_EXCEPTION;
-			vcpu->run->ex.exception = PF_VECTOR;
-			vcpu->run->ex.error_code = exit_qual;
+			vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+			vcpu->run->internal.suberror =
+				KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON;
+			vcpu->run->internal.ndata = 2;
+			vcpu->run->internal.data[0] = EXIT_REASON_EPT_VIOLATION;
+			vcpu->run->internal.data[1] = vcpu->arch.last_vmentry_cpu;
 			return 0;
 		}
 	}

Thank you

Reinette