Re: [PATCH v5 5/6] x86/tdx: Add a helper to query maximum Quote size

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

On Mon, Sep 28, 2026 at 01:32:06PM -0700, Kuppuswamy Sathyanarayanan wrote:
> Hi,
> 
> On 9/28/2026 3:08 AM, Peter Fang wrote:
> > TDX attestation report ("Quote") sizes can grow with newer crypto
> > algorithms, so guests can no longer rely on a fixed-size buffer for the
> > Quote.
> > 
> > The TDX module added a new ABI that reports the largest possible Quote
> > size via a metadata field [1]. Add a helper to query the size instead of
> > exposing tdg_vm_rd() directly, as it can read arbitrary metadata fields.
> > 
> > The reported size covers every Quote type the platform can produce,
> > including SGX-based Quotes.
> > 
> > Thanks to Xu Yilun for suggesting this in an off-list discussion.
> 
> If the patch is suggested by Xu Uilun you can use Suggested-by:

I think tip's preference is to use some prose rather than a tag for
these things [1]. So I'll probably keep the prose unless you have
objections?

[1] https://lore.kernel.org/all/c1c9bddf-1c00-4625-a915-d545acd67fd0@intel.com/

> 
> >  
> > +/*
> > + * Ask the TDX module what the largest possible Quote might be.
> > + */
> > +int tdx_get_max_quote_size(u64 *max_quote_size)
> > +{
> > +	u64 err;
> > +
> > +	err = tdg_vm_rd(TDCS_QUOTE_MAX_SIZE, max_quote_size);
> > +
> > +	/* Old modules do not support this. Tell the caller. */
> > +	if (err)
> > +		return -EINVAL;
> 
> I think -ENODEV or -EOPNOTSUPP is more appropriate return value
> for unsupported case.

Hmm, I don't see either one being a common pattern for guest-side TDX
code. -ENODEV is usually used for some basic operation failure.
-EOPNOTSUPP is not used in the guest. But between the two I think
-EOPNOTSUPP probably fits better. Thanks.

> 
> Also tdg_vm_rd() updates max_quote_size on error case as well.
> 
> You can reset it or use local variable to skip passing incorrect
> value on error case.

That's true, but this won't be a widely used function, and the caller
should check the return value first. So I'd think keeping it simpler
would be better?

BTW on error, max_quote_size will be set to 0. So it's also
deterministic behavior.

>