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

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Message-ID: <3b1b1b23-b633-4085-825b-2867f665abc7@intel.com>
Date: Wed, 1 May 2024 11:22:19 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>
CC: 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

Hi Isaku,

On 5/1/2024 11:19 AM, Isaku Yamahata wrote:
> On Wed, May 01, 2024 at 09:54:07AM -0700,
> Reinette Chatre <reinette.chatre@...el.com> wrote:
> 
>> Hi Isaku,
>>
>> On 5/1/2024 8:56 AM, Isaku Yamahata wrote:
>>> On Tue, Apr 30, 2024 at 01:47:07PM -0700,
>>> Reinette Chatre <reinette.chatre@...el.com> wrote:
>>>> 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
>>
>> Did you perhaps intend to write "data[1] should be exit_qual" or would you
>> like to see ndata = 3? I followed existing usages, for example [1] and [2],
>> that have ndata = 2 with "data[1] = vcpu->arch.last_vmentry_cpu".
>>
>>> KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON.
>>> Qemu doesn't assumt ndata = 2. Just report all data within ndata.
>>
>> I am not sure I interpreted your response correctly so I share one possible
>> snippet below as I interpret it. Could you please check where I misinterpreted
>> you? I could also make ndata = 3 to break the existing custom and add
>> "data[2] = vcpu->arch.last_vmentry_cpu" to match existing pattern. What do you
>> think?
>>
> Sorry, I wasn't clear enough. I meant
>   ndata = 3;
>   data[0] = exit_reason.full;
>   data[1] = vcpu->arch.last_vmentry_cpu;
>   data[2] = exit_qual;
> 
> Because I hesitate to change the meaning of data[1] from other usage, I
> appended exit_qual as data[2].

I understand it now. Thank you very much Isaku.

Reinette