linux-kernel - Re: [PATCH 10/25] KVM: TDX: Initialize KVM supported capabilities when module setup

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Message-ID: <55366da1-2b9c-4d12-aba7-93c15a1b3b09@suse.com>
Date: Thu, 12 Sep 2024 11:43:08 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, seanjc@...gle.com,
 pbonzini@...hat.com, kvm@...r.kernel.org
Cc: kai.huang@...el.com, isaku.yamahata@...il.com,
 tony.lindgren@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/25] KVM: TDX: Initialize KVM supported capabilities
 when module setup



On 12.09.24 г. 11:37 ч., Xiaoyao Li wrote:
> On 9/12/2024 4:04 PM, Nikolay Borisov wrote:
>>
>>
>> On 5.09.24 г. 16:36 ч., Xiaoyao Li wrote:
>>> On 9/4/2024 7:58 PM, Nikolay Borisov wrote:
>>>>
>>>>
>>>> On 13.08.24 г. 1:48 ч., Rick Edgecombe wrote:
>>>>> From: Xiaoyao Li <xiaoyao.li@...el.com>
>>>>>
>>>>> While TDX module reports a set of capabilities/features that it
>>>>> supports, what KVM currently supports might be a subset of them.
>>>>> E.g., DEBUG and PERFMON are supported by TDX module but currently not
>>>>> supported by KVM.
>>>>>
>>>>> Introduce a new struct kvm_tdx_caps to store KVM's capabilities of 
>>>>> TDX.
>>>>> supported_attrs and suppported_xfam are validated against fixed0/1
>>>>> values enumerated by TDX module. Configurable CPUID bits derive 
>>>>> from TDX
>>>>> module plus applying KVM's capabilities (KVM_GET_SUPPORTED_CPUID),
>>>>> i.e., mask off the bits that are configurable in the view of TDX 
>>>>> module
>>>>> but not supported by KVM yet.
>>>>>
>>>>> KVM_TDX_CPUID_NO_SUBLEAF is the concept from TDX module, switch it 
>>>>> to 0
>>>>> and use KVM_CPUID_FLAG_SIGNIFCANT_INDEX, which are the concept of KVM.
>>>>>
>>>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
>>>>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
>>>>> ---
>>>>> uAPI breakout v1:
>>>>>   - Change setup_kvm_tdx_caps() to use the exported 'struct 
>>>>> tdx_sysinfo'
>>>>>     pointer.
>>>>>   - Change how to copy 'kvm_tdx_cpuid_config' since 'struct 
>>>>> tdx_sysinfo'
>>>>>     doesn't have 'kvm_tdx_cpuid_config'.
>>>>>   - Updates for uAPI changes
>>>>> ---
>>>>>   arch/x86/include/uapi/asm/kvm.h |  2 -
>>>>>   arch/x86/kvm/vmx/tdx.c          | 81 
>>>>> +++++++++++++++++++++++++++++++++
>>>>>   2 files changed, 81 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/include/uapi/asm/kvm.h 
>>>>> b/arch/x86/include/uapi/asm/kvm.h
>>>>> index 47caf508cca7..c9eb2e2f5559 100644
>>>>> --- a/arch/x86/include/uapi/asm/kvm.h
>>>>> +++ b/arch/x86/include/uapi/asm/kvm.h
>>>>> @@ -952,8 +952,6 @@ struct kvm_tdx_cmd {
>>>>>       __u64 hw_error;
>>>>>   };
>>>>> -#define KVM_TDX_CPUID_NO_SUBLEAF    ((__u32)-1)
>>>>> -
>>>>>   struct kvm_tdx_cpuid_config {
>>>>>       __u32 leaf;
>>>>>       __u32 sub_leaf;
>>>>> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
>>>>> index 90b44ebaf864..d89973e554f6 100644
>>>>> --- a/arch/x86/kvm/vmx/tdx.c
>>>>> +++ b/arch/x86/kvm/vmx/tdx.c
>>>>> @@ -31,6 +31,19 @@ static void __used tdx_guest_keyid_free(int keyid)
>>>>>       ida_free(&tdx_guest_keyid_pool, keyid);
>>>>>   }
>>>>> +#define KVM_TDX_CPUID_NO_SUBLEAF    ((__u32)-1)
>>>>> +
>>>>> +struct kvm_tdx_caps {
>>>>> +    u64 supported_attrs;
>>>>> +    u64 supported_xfam;
>>>>> +
>>>>> +    u16 num_cpuid_config;
>>>>> +    /* This must the last member. */
>>>>> +    DECLARE_FLEX_ARRAY(struct kvm_tdx_cpuid_config, cpuid_configs);
>>>>> +};
>>>>> +
>>>>> +static struct kvm_tdx_caps *kvm_tdx_caps;
>>>>> +
>>>>>   static int tdx_get_capabilities(struct kvm_tdx_cmd *cmd)
>>>>>   {
>>>>>       const struct tdx_sysinfo_td_conf *td_conf = 
>>>>> &tdx_sysinfo->td_conf;
>>>>> @@ -131,6 +144,68 @@ int tdx_vm_ioctl(struct kvm *kvm, void __user 
>>>>> *argp)
>>>>>       return r;
>>>>>   }
>>>>> +#define KVM_SUPPORTED_TD_ATTRS (TDX_TD_ATTR_SEPT_VE_DISABLE)
>>>>
>>>> Why isn't TDX_TD_ATTR_DEBUG added as well?
>>>
>>> Because so far KVM doesn't support all the features of a DEBUG TD for 
>>> userspace. e.g., KVM doesn't provide interface for userspace to 
>>> read/write private memory of DEBUG TD.
>>
>> But this means that you can't really run a TDX with SEPT_VE_DISABLE 
>> disabled for debugging purposes, so perhaps it might be necessary to 
>> rethink the condition allowing SEPT_VE_DISABLE to be disabled. Without 
>> the debug flag and SEPT_VE_DISABLE disabled the code refuses to start 
>> the VM, what if one wants to debug some SEPT issue by having an oops 
>> generated inside the vm ?
> 
> sept_ve_disable is allowed to be disable, i.e., set to 0.
> 
> I think there must be some misunderstanding.

There isn't, the current code is:

   201         if (!(td_attr & ATTR_SEPT_VE_DISABLE)) { 

     1                 const char *msg = "TD misconfiguration: 
SEPT_VE_DISABLE attribute must be set.";
     2 

     3                 /* Relax SEPT_VE_DISABLE check for debug TD. */ 

     4                 if (td_attr & ATTR_DEBUG) 

     5                         pr_warn("%s\n", msg); 

     6                 else 

     7                         tdx_panic(msg); 

     8         }


I.e if we disable SEPT_VE_DISABLE without having ATTR_DEBUG it results 
in a panic.

> 
>>>
>>>> <snip>
>>>
>>>
>