Re: [RFC PATCH 1/6] x86/virt/tdx: Add SEAMCALL wrappers for TDX KeyID management

Sean Christopherson
2024-11-22T23:55:16Z

On Fri, Nov 22, 2024, Dave Hansen wrote:
> On 11/15/24 12:20, Rick Edgecombe wrote:
> > +struct tdx_td {
> > + hpa_t tdr;
> > + hpa_t *tdcs;
> > +};
>
> This is a step in the right direction because it gives the wrappers some
> more type safety.
>
> But an hpa_t is _barely_ better than a u64. If the 'tdr' is a page,
> then it needs to be _stored_ as a page:
>
> struct page *tdr_page;
>
> Also, please don't forget to spell these things out:
>
> /* TD root structure: */
> struct page *tdr_page;
>
> And the tdcs is an array of pages, right? So it should be:
>
> struct page **tdcs_pages;
>
> Or heck, I _think_ it can theoretically be defined as a variable-length
> array:
>
> struct page *tdcs_pages[];
>
> and use the helpers that we have for that.
>
> Putting it all together, you would have this:
>
> struct tdx_td {
> /* TD root structure: */
> struct page *tdr_page;
>
> int tdcs_nr_pages;
> /* TD control structure: */
> struct page *tdcs_pages[];
> };
>
> That's *MUCH* harder to misuse. It's 100% obvious that you have a
> single page, plus a variable-length array of pages. This is all from
> just looking at the structure definition.

I don't know the full context, but working with "struct page" is a pain when every
user just wants the physical address. KVM SVM had a few cases where pointers were
tracked as "struct page", and it was generally unpleasant to read and work with.

I also don't like conflating the kernel's "struct page" with the architecture's
definition of a 4KiB page.

> You know that 'tdr' is not just some random physical address. It's a
> whole physical page. It's page-aligned. It was allocated, from the
> allocator. It doesn't point to special memory.

Oh, but it does point to special memory. If it *didn't* point at special memory
that is completely opaque and untouchable, then KVM could use a struct overlay,
which would give contextual information and some amount of type safety. E.g.
an equivalent without TDX is "struct vmcs *".

Rather than "struct page", what if we add an address_space (in the Sparse sense),
and a typedef for a TDX pages? Maybe __firmware? E.g.

# define __firmware __attribute__((noderef, address_space(__firmware)))

typedef u64 __firmware *tdx_page_t;

That doesn't give as much compile-time safety, but in some ways it provides more
type safety since KVM (or whatever else cares) would need to make an explicit and
ugly cast to misuse the pointer.

> Ditto for "hpa_t *tdcs". It's not obvious from the data structure that
> it's an array or if it's an array how it got allocated or how large it is.