Re: [PATCH v19 034/130] KVM: TDX: Get system-wide info about TDX module on initialization

Isaku Yamahata
2024-03-15T16:22:55Z

On Fri, Mar 15, 2024 at 12:09:29PM +1300,
"Huang, Kai" <kai.huang@intel.com> wrote:

>
> > +struct tdx_info {
> > + u64 features0;
> > + u64 attributes_fixed0;
> > + u64 attributes_fixed1;
> > + u64 xfam_fixed0;
> > + u64 xfam_fixed1;
> > +
> > + u16 num_cpuid_config;
> > + /* This must the last member. */
> > + DECLARE_FLEX_ARRAY(struct kvm_tdx_cpuid_config, cpuid_configs);
> > +};
> > +
> > +/* Info about the TDX module. */
> > +static struct tdx_info *tdx_info;
> > +
> > #define TDX_MD_MAP(_fid, _ptr) \
> > { .fid = MD_FIELD_ID_##_fid, \
> > .ptr = (_ptr), }
> > @@ -66,7 +81,7 @@ static size_t tdx_md_element_size(u64 fid)
> > }
> > }
> > -static int __used tdx_md_read(struct tdx_md_map *maps, int nr_maps)
> > +static int tdx_md_read(struct tdx_md_map *maps, int nr_maps)
> > {
> > struct tdx_md_map *m;
> > int ret, i;
> > @@ -84,9 +99,26 @@ static int __used tdx_md_read(struct tdx_md_map *maps, int nr_maps)
> > return 0;
> > }
> > +#define TDX_INFO_MAP(_field_id, _member) \
> > + TD_SYSINFO_MAP(_field_id, struct tdx_info, _member)
> > +
> > static int __init tdx_module_setup(void)
> > {
> > + u16 num_cpuid_config;
> > int ret;
> > + u32 i;
> > +
> > + struct tdx_md_map mds[] = {
> > + TDX_MD_MAP(NUM_CPUID_CONFIG, &num_cpuid_config),
> > + };
> > +
> > + struct tdx_metadata_field_mapping fields[] = {
> > + TDX_INFO_MAP(FEATURES0, features0),
> > + TDX_INFO_MAP(ATTRS_FIXED0, attributes_fixed0),
> > + TDX_INFO_MAP(ATTRS_FIXED1, attributes_fixed1),
> > + TDX_INFO_MAP(XFAM_FIXED0, xfam_fixed0),
> > + TDX_INFO_MAP(XFAM_FIXED1, xfam_fixed1),
> > + };
> > ret = tdx_enable();
> > if (ret) {
> > @@ -94,7 +126,48 @@ static int __init tdx_module_setup(void)
> > return ret;
> > }
> > + ret = tdx_md_read(mds, ARRAY_SIZE(mds));
> > + if (ret)
> > + return ret;
> > +
> > + tdx_info = kzalloc(sizeof(*tdx_info) +
> > + sizeof(*tdx_info->cpuid_configs) * num_cpuid_config,
> > + GFP_KERNEL);
> > + if (!tdx_info)
> > + return -ENOMEM;
> > + tdx_info->num_cpuid_config = num_cpuid_config;
> > +
> > + ret = tdx_sys_metadata_read(fields, ARRAY_SIZE(fields), tdx_info);
> > + if (ret)
> > + goto error_out;
> > +
> > + for (i = 0; i < num_cpuid_config; i++) {
> > + struct kvm_tdx_cpuid_config *c = &tdx_info->cpuid_configs[i];
> > + u64 leaf, eax_ebx, ecx_edx;
> > + struct tdx_md_map cpuids[] = {
> > + TDX_MD_MAP(CPUID_CONFIG_LEAVES + i, &leaf),
> > + TDX_MD_MAP(CPUID_CONFIG_VALUES + i * 2, &eax_ebx),
> > + TDX_MD_MAP(CPUID_CONFIG_VALUES + i * 2 + 1, &ecx_edx),
> > + };
> > +
> > + ret = tdx_md_read(cpuids, ARRAY_SIZE(cpuids));
> > + if (ret)
> > + goto error_out;
> > +
> > + c->leaf = (u32)leaf;
> > + c->sub_leaf = leaf >> 32;
> > + c->eax = (u32)eax_ebx;
> > + c->ebx = eax_ebx >> 32;
> > + c->ecx = (u32)ecx_edx;
> > + c->edx = ecx_edx >> 32;
>
> OK I can see why you don't want to use ...
>
> struct tdx_metadata_field_mapping fields[] = {
> TDX_INFO_MAP(NUM_CPUID_CONFIG, num_cpuid_config),
> };
>
> ... to read num_cpuid_config first, because the memory to hold @tdx_info
> hasn't been allocated, because its size depends on the num_cpuid_config.
>
> And I confess it's because the tdx_sys_metadata_field_read() that got
> exposed in patch ("x86/virt/tdx: Export global metadata read
> infrastructure") only returns 'u64' for all metadata field, and you didn't
> want to use something like this:
>
> u64 num_cpuid_config;
>
> tdx_sys_metadata_field_read(..., &num_cpuid_config);
>
> ...
>
> tdx_info->num_cpuid_config = num_cpuid_config;
>
> Or you can explicitly cast:
>
> tdx_info->num_cpuid_config = (u16)num_cpuid_config;
>
> (I know people may don't like the assigning 'u64' to 'u16', but it seems
> nothing wrong to me, because the way done in (1) below effectively has the
> same result comparing to type case).
>
> But there are other (better) ways to do:
>
> 1) you can introduce a helper as suggested by Xiaoyao in [*]:
>
>
> int tdx_sys_metadata_read_single(u64 field_id,
> int bytes, void *buf)
> {
> return stbuf_read_sys_metadata_field(field_id, 0,
> bytes, buf);
> }
>
> And do:
>
> tdx_sys_metadata_read_single(NUM_CPUID_CONFIG,
> sizeof(num_cpuid_config), &num_cpuid_config);
>
> That's _much_ cleaner than the 'struct tdx_md_map', which only confuses
> people.
>
> But I don't think we need to do this as mentioned above -- we just do type
> cast.
>
> 2) You can just preallocate enough memory. It cannot be larger than 1024B,
> right? You can even just allocate one page. It's just 4K, no one cares.
>
> Then you can do:
>
> struct tdx_metadata_field_mapping tdx_info_fields = {
> ...
> TDX_INFO_MAP(NUM_CPUID_CONFIG, num_cpuid_config),
> };
>
> tdx_sys_metadata_read(tdx_info_fields,
> ARRAY_SIZE(tdx_info_fields, tdx_info);
>
> And then you read the CPUID_CONFIG array one by one using the same 'struct
> tdx_metadata_field_mapping' and tdx_sys_metadata_read():
>
>
> for (i = 0; i < tdx_info->num_cpuid_config; i++) {
> struct tdx_metadata_field_mapping cpuid_fields = {
> TDX_CPUID_CONFIG_MAP(CPUID_CONFIG_LEAVES + i,
> leaf),
> ...
> };
> struct kvm_tdx_cpuid_config *c =
> &tdx_info->cpuid_configs[i];
>
> tdx_sys_metadata_read(cpuid_fields,
> ARRAY_SIZE(cpuid_fields), c);
>
> ....
> }
>
> So stopping having the duplicated 'struct tdx_md_map' and related staff, as
> they are absolutely unnecessary and only confuses people.

Ok, I'll rewrite the code to use tdx_sys_metadata_read() by introducng
tentative struct in a function scope.
--
Isaku Yamahata <isaku.yamahata@intel.com>