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

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Message-ID: <30d0cef5-82d5-4325-b149-0e99833b8785@intel.com>
Date: Fri, 22 Nov 2024 10:04:06 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>, kvm@...r.kernel.org,
 pbonzini@...hat.com, seanjc@...gle.com
Cc: isaku.yamahata@...il.com, kai.huang@...el.com,
 linux-kernel@...r.kernel.org, tony.lindgren@...ux.intel.com,
 xiaoyao.li@...el.com, yan.y.zhao@...el.com, x86@...nel.org,
 adrian.hunter@...el.com, Isaku Yamahata <isaku.yamahata@...el.com>,
 Binbin Wu <binbin.wu@...ux.intel.com>, Yuan Yao <yuan.yao@...el.com>
Subject: Re: [RFC PATCH 1/6] x86/virt/tdx: Add SEAMCALL wrappers for TDX KeyID
 management

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.

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.

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.