Re: [PATCH v4 1/4] x86/tdx: Add helper to query maximum TD Quote size

[Peter Fang <peter.fang@intel.com>]

On Mon, Sep 21, 2026 at 03:14:49PM -0700, Dave Hansen wrote:
> On 9/15/26 02:26, Peter Fang wrote:
> > +/**
> > + * tdx_get_max_quote_size() - Get the maximum TD Quote size
> > + *
> > + * Read the maximum size of a TD Quote from a 4-byte TD metadata field. The TDX
> > + * guest driver uses it to size the buffer for Quotes. Older TDX modules do not
> > + * support this field and return an error.
> > + *
> > + * The reported size covers every Quote type supported by the platform,
> > + * including SGX-based Quotes.
> > + *
> > + * A runtime TDX module update does not change the reported size.
> > + *
> > + * Return: Maximum Quote size in bytes on success, or 0 on failure.
> > + */
> > +u32 tdx_get_max_quote_size(void)
> > +{
> > +	u64 val, ret;
> > +
> > +	ret = tdg_vm_rd(TDCS_QUOTE_MAX_SIZE, &val);
> > +
> > +	return ret ? 0 : (u32)val;
> > +}
> > +EXPORT_SYMBOL_GPL(tdx_get_max_quote_size);
> 
> This is an awful lot of stuff I'm not fond of like packed in very few
> lines of code.
> 
> 1. An unscoped, unmentioned, uncommented export
> 2. Ternary form
> 3. Casting
> 4. A kerneldoc comment that is quite verbose and restates the TDX specs
> 5. kerneldocs for something that's not a widely-used API
> 6. Comments that say the same thing as the function name
> 7. Multiple variables declared on one line
> 
> Rather than start trying to cram failure codes into the return code,
> just do:
> 
> /*
>  * Ask the TDX module what the largest possible quote might be.
>  */
> int tdx_get_max_quote_size(u64 *max_quote_size)
> {
> 	int ret = tdg_vm_rd(TDCS_QUOTE_MAX_SIZE, max_quote_size);

I confirmed that no casting is needed here. Even though it's defined as
a 32-bit field, the TDX module should make sure that the upper 32 bits
are zero. So using u64 makes everything cleaner.

> 
> 	/* Old modules do not support this. Tell the caller. */
> 	if (ret)
> 		return -EINVAL;
> 
> 	return 0;
> }
> 
> That's the normal pattern. Or, heck, just return a long and return
> -errnos in there.

I'll rewrite this and trim down the comment. Thanks!