linux-kernel - Re: [PATCH v2 16/25] KVM: TDX: Get system-wide info about TDX module on initialization

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Message-ID: <bb60b05d-5ccc-49ab-9a0c-a7f87b0c827c@intel.com>
Date: Thu, 31 Oct 2024 17:23:57 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, pbonzini@...hat.com,
 seanjc@...gle.com
Cc: yan.y.zhao@...el.com, isaku.yamahata@...il.com, kai.huang@...el.com,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 tony.lindgren@...ux.intel.com, reinette.chatre@...el.com,
 Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH v2 16/25] KVM: TDX: Get system-wide info about TDX module
 on initialization

On 10/31/2024 5:09 PM, Binbin Wu wrote:
> 
> 
> 
> On 10/31/2024 3:00 AM, Rick Edgecombe wrote:
> [...]
>> +static u32 tdx_set_guest_phys_addr_bits(const u32 eax, int addr_bits)
>> +{
>> +    return (eax & ~GENMASK(23, 16)) | (addr_bits & 0xff) << 16;
>> +}
>> +
>> +#define KVM_TDX_CPUID_NO_SUBLEAF    ((__u32)-1)
>> +
>> +static void td_init_cpuid_entry2(struct kvm_cpuid_entry2 *entry, 
>> unsigned char idx)
>> +{
>> +    const struct tdx_sys_info_td_conf *td_conf = &tdx_sysinfo->td_conf;
>> +
>> +    entry->function = (u32)td_conf->cpuid_config_leaves[idx];
>> +    entry->index = td_conf->cpuid_config_leaves[idx] >> 32;
>> +    entry->eax = (u32)td_conf->cpuid_config_values[idx][0];
>> +    entry->ebx = td_conf->cpuid_config_values[idx][0] >> 32;
>> +    entry->ecx = (u32)td_conf->cpuid_config_values[idx][1];
>> +    entry->edx = td_conf->cpuid_config_values[idx][1] >> 32;
>> +
>> +    if (entry->index == KVM_TDX_CPUID_NO_SUBLEAF)
>> +        entry->index = 0;
>> +
>> +    /* Work around missing support on old TDX modules */
>> +    if (entry->function == 0x80000008)
>> +        entry->eax = tdx_set_guest_phys_addr_bits(entry->eax, 0xff);
> Is it necessary to set bit 16~23 to 0xff?
> It seems that when userspace wants to retrieve the value, the GPAW will
> be set in tdx_read_cpuid() anyway.

here it is to initialize the configurable CPUID bits that get reported 
to userspace. Though TDX module doesn't allow them to be set in TD_PARAM 
for KVM_TDX_INIT_VM, they get set to 0xff because KVM reuse these bits 
EBX[23:16] as the interface for userspace to configure GPAW of TD guest 
(implemented in setup_tdparams_eptp_controls() in patch 19). That's why 
they need to be set as all-1 to allow userspace to configure.

And the comment above it is wrong and vague. we need to change it to 
something like

	/*
          * Though TDX module doesn't allow the configuration of guest
          * phys addr bits (EBX[23:16]), KVM uses it as the interface for
          * userspace to configure the GPAW. So need to report these bits
          * as configurable to userspace.
          */
>> +}
>> +
>>
> [...]