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

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Date: Wed, 1 May 2024 08:56:20 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: Isaku Yamahata <isaku.yamahata@...el.com>,
	Chao Gao <chao.gao@...el.com>, 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

On Tue, Apr 30, 2024 at 01:47:07PM -0700,
Reinette Chatre <reinette.chatre@...el.com> wrote:

> 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?

Yes. data[0] should be the raw value of exit reason if possible.
data[2] should be exit_qual.  Hmm, I don't find document on data[] for
KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON.
Qemu doesn't assumt ndata = 2. Just report all data within ndata.


> 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
> 
> 
> 

-- 
Isaku Yamahata <isaku.yamahata@...el.com>