RE: [RFC PATCH 01/15] x86/tdx: Export tdg_vm_rd() for tdx-guest module

[Duan, Zhenzhong]

>-----Original Message-----
>From: Xu Yilun <yilun.xu@xxxxxxxxxxxxxxx>
>Subject: Re: [RFC PATCH 01/15] x86/tdx: Export tdg_vm_rd() for tdx-guest module
>
>On Wed, Sep 30, 2026 at 03:13:40PM -0700, Peter Fang wrote:
>> On Thu, Sep 24, 2026 at 12:10:18PM +0800, Zhenzhong Duan wrote:
>> > Export tdg_vm_rd() to allow the tdx-guest driver module to directly
>> > read TD-scoped metadata fields from the Trust Domain Control Structure
>> > (TDCS) via TDG.VM.RD TDCALL.
>> >
>> > Since tdg_vm_rd() is read-only and cannot cause harm, exporting it
>> > directly is simpler and more flexible. This allows the tdx-guest driver
>> > to read any TDCS field it needs.
>> >
>> > In a following patch, the tdx-guest driver will use tdg_vm_rd() to read
>> > TDCS_CONFIG_FLAGS field directly.
>>
>> Can this be a separate helper? I'm adding another helper for
>> TDCS_QUOTE_MAX_SIZE [1]. I think maybe their patterns should stay consistent.
>>
>> Yilun, any thoughts on this?
>
>There was a helper in the series before this public RFC, but was then
>dropped, something like:
>
>int tdx_get_config_flags(u64 *flags)
>{
>	u64 sret;
>
>	sret = tdg_vm_rd(TDCS_CONFIG_FLAGS, flags);
>	if (sret)
>		return -EIO;
>
>	return 0;
>}
>EXPORT_SYMBOL_FOR_MODULES(tdx_get_config_flags, "tdx-guest");
>
>The concern is how risky is the tdg_vm_rd() export, and how it impacts
>the existing tdg_vm_rd() usage, and the tdh_mng_rd() export which reads
>the same data set on host.
>
>My initial concern about the cons of tdg_vm_rd() export are, the
>SEAMCALL reads any TDCS fields, some of them are writable by tdg_vm_wr()
>and there is no synchronization between them, so seems not a good kAPI.
>Reducing the scope to read-only fields (e.g. TDCS_CONFIG_FLAGS) may be a
>good start.
>
>Now there are 2 cases in flight: tdx_get_max_quote_size() helper in DICE
>and tdg_vm_rd() export here. Maybe we need more cases to see which is
>better but anyway I agree we'd better stay consistent now.

We have below existing use cases which are open-coded.
Maybe need to stay consistent to them?

             tdg_vm_rd(TDCS_TD_CTLS, &controls);
             tdg_vm_rd(TDCS_TOPOLOGY_ENUM_CONFIGURED, &configured);

Thanks
Zhenzhong