Re: [PATCH v3 10/21] KVM:x86: Add #CP support in guest exception classification

Yang, Weijiang
2023-07-01T01:58:15Z

On 6/30/2023 11:07 PM, Sean Christopherson wrote:
> On Fri, Jun 30, 2023, Weijiang Yang wrote:
>> On 6/17/2023 2:57 AM, Sean Christopherson wrote:
>>>> Do you mean documentation for #CP as an generic exception or the behavior in
>>>> KVM as this patch shows?
>>> As I pointed out two *years* ago, this entry in the SDM
>>>
>>> — The field's deliver-error-code bit (bit 11) is 1 if each of the following
>>> holds: (1) the interruption type is hardware exception; (2) bit 0
>>> (corresponding to CR0.PE) is set in the CR0 field in the guest-state area;
>>> (3) IA32_VMX_BASIC[56] is read as 0 (see Appendix A.1); and (4) the vector
>>> indicates one of the following exceptions: #DF (vector 8), #TS (10),
>>> #NP (11), #SS (12), #GP (13), #PF (14), or #AC (17).
>>>
>>> needs to read something like
>>>
>>> — The field's deliver-error-code bit (bit 11) is 1 if each of the following
>>> holds: (1) the interruption type is hardware exception; (2) bit 0
>>> (corresponding to CR0.PE) is set in the CR0 field in the guest-state area;
>>> (3) IA32_VMX_BASIC[56] is read as 0 (see Appendix A.1); and (4) the vector
>>> indicates one of the following exceptions: #DF (vector 8), #TS (10),
>>> #NP (11), #SS (12), #GP (13), #PF (14), #AC (17), or #CP (21)[1]
>>>
>>> [1] #CP has an error code if and only if IA32_VMX_CR4_FIXED1 enumerates
>>> support for the 1-setting of CR4.CET.
>> Hi, Sean,
>>
>> I sent above change request to Gil(added in cc), but he shared different
>> opinion on this issue:
> Heh, "opinion".
>
>>  It may make things clearer if we document the statement above (all
>> CET-capable parts enumerate IA32_VMX_BASIC[56] as 1).
>>
>> I will see if we can update future revisions of the SDM to clarify this."
> That would be helpful. Though to be perfectly honest, I simply overlooked the
> existence of IA32_VMX_BASIC[56].
>
> Thanks!
>
>> Then if this is the case,  kvm needs to check IA32_VMX_BASIC[56] before
>> inject exception to nested VM.
>>
>> And this patch could be removed, instead need another patch like below:
>>
>> diff --git a/arch/x86/include/asm/msr-index.h
>> b/arch/x86/include/asm/msr-index.h
>> index ad35355ee43e..6b33aacc8587 100644
>> --- a/arch/x86/include/asm/msr-index.h
>> +++ b/arch/x86/include/asm/msr-index.h
>> @@ -1076,6 +1076,7 @@
>>  #define VMX_BASIC_MEM_TYPE_MASK    0x003c000000000000LLU
>>  #define VMX_BASIC_MEM_TYPE_WB    6LLU
>>  #define VMX_BASIC_INOUT        0x0040000000000000LLU
>> +#define VMX_BASIC_CHECK_ERRCODE    0x0140000000000000LLU
> "Check Error Code" isn't a great description. The flag enumerates that there the
> CPU does *not* perform consistency checks on the error code when injecting hardware
> exceptions.
>
> So something like this?
>
> VMX_BASIC_NO_HW_ERROR_CODE_CC
>
> or maybe
>
> VMX_BASIC_PM_NO_HW_ERROR_CODE_CC
>
> if we want to capture that only protected mode is exempt (I personally prefer
> just VMX_BASIC_NO_HW_ERROR_CODE_CC as "PM" is a bit ambiguous).

I like VMX_BASIC_NO_HW_ERROR_CODE_CC too :-), thanks!

>
>> @@ -2873,7 +2873,8 @@ static int nested_check_vm_entry_controls(struct
>> kvm_vcpu *vcpu,
>>          should_have_error_code =
>>              intr_type == INTR_TYPE_HARD_EXCEPTION && prot_mode &&
>>              x86_exception_has_error_code(vector);
>> -        if (CC(has_error_code != should_have_error_code))
>> +        if (!cpu_has_vmx_basic_check_errcode() &&
>> +            CC(has_error_code != should_have_error_code))
> This is wrong on mutiple fronts:
>
> 1. The new feature flag only excempts hardware exceptions delivered to guests
> with CR0.PE=1. The above will skip the consistency check for all event injection.
>
> 2. KVM needs to check the CPU model that is exposed to L1, not the capabilities
> of the host CPU.
>
> Highlighting the key phrases in the SDM:
>
> The field's deliver-error-code bit (bit 11) is 1 if each of the following holds: (1) the interruption type is
> ^^^^^^^
> hardware exception; (2) bit 0 (corresponding to CR0.PE) is set in the CR0 field in the guest-state area;
> (3) IA32_VMX_BASIC[56] is read as 0 (see Appendix A.1); and (4) the vector indicates one of the following
> exceptions: #DF (vector 8), #TS (10), #NP (11), #SS (12), #GP (13), #PF (14), or #AC (17).
>
> The field's deliver-error-code bit is 0 if any of the following holds: (1) the interruption type is not hardware
> ^^^^^^
> exception; (2) bit 0 is clear in the CR0 field in the guest-state area; or (3) IA32_VMX_BASIC[56] is read as
> 0 and the vector is in one of the following ranges: 0–7, 9, 15, 16, or 18–31.
>
> I think what we want is:
>
> /* VM-entry interruption-info field: deliver error code */
> if (!prot_mode || intr_type != INTR_TYPE_HARD_EXCEPTION ||
> !nested_cpu_has_no_hw_error_code_cc(vcpu)) {
> should_have_error_code =
> intr_type == INTR_TYPE_HARD_EXCEPTION && prot_mode &&
> x86_exception_has_error_code(vector);
> if (CC(has_error_code != should_have_error_code))
> return -EINVAL;
> }

It looks good to me, will take it, thanks a lot!