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

Reinette Chatre
2024-05-01T18:22:37Z

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@intel.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@intel.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@intel.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