linux-kernel - Re: [PATCH v19 027/130] KVM: TDX: Define TDX architectural definitions

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Date: Tue, 16 Apr 2024 12:55:33 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: Isaku Yamahata <isaku.yamahata@...ux.intel.com>, Yan Zhao
	<yan.y.zhao@...el.com>
CC: <isaku.yamahata@...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>, <chen.bo@...el.com>,
	<hang.yuan@...el.com>, <tina.zhang@...el.com>, Sean Christopherson
	<sean.j.christopherson@...el.com>, Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v19 027/130] KVM: TDX: Define TDX architectural
 definitions



On 5/03/2024 9:21 pm, Isaku Yamahata wrote:
> On Fri, Mar 01, 2024 at 03:25:31PM +0800,
> Yan Zhao <yan.y.zhao@...el.com> wrote:
> 
>>> + * TD_PARAMS is provided as an input to TDH_MNG_INIT, the size of which is 1024B.
>>> + */
>>> +#define TDX_MAX_VCPUS	(~(u16)0)
>> This value will be treated as -1 in tdx_vm_init(),
>> 	"kvm->max_vcpus = min(kvm->max_vcpus, TDX_MAX_VCPUS);"
>>
>> This will lead to kvm->max_vcpus being -1 by default.
>> Is this by design or just an error?
>> If it's by design, why not set kvm->max_vcpus = -1 in tdx_vm_init() directly.
>> If an unexpected error, may below is better?
>>
>> #define TDX_MAX_VCPUS   (int)((u16)(~0UL))
>> or
>> #define TDX_MAX_VCPUS 65536
> 
> You're right. I'll use ((int)U16_MAX).
> As TDX 1.5 introduced metadata MAX_VCPUS_PER_TD, I'll update to get the value
> and trim it further. Something following.
> 

[...]

>   
> +	u16 max_vcpus_per_td;
> +

[...]

> -	kvm->max_vcpus = min(kvm->max_vcpus, TDX_MAX_VCPUS);
> +	kvm->max_vcpus = min3(kvm->max_vcpus, tdx_info->max_vcpus_per_td,
> +			     TDX_MAX_VCPUS);
>   

[...]

> -#define TDX_MAX_VCPUS	(~(u16)0)
> +#define TDX_MAX_VCPUS	((int)U16_MAX)

Why do you even need TDX_MAX_VCPUS, given it cannot exceed U16_MAX and 
you will have the 'u16 max_vcpus_per_td' anyway?

IIUC, in KVM_ENABLE_CAP(KVM_CAP_MAX_VCPUS), we can overwrite the 
kvm->max_vcpus to the 'max_vcpus' provided by the userspace, and make 
sure it doesn't exceed tdx_info->max_vcpus_per_td.

Anything I am missing?