Re: [PATCH v3 10/21] KVM:x86: Add #CP support in guest exception classification
From: Yang, Weijiang
Date: Fri Jun 30 2023 - 21:58:26 EST
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!