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

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Message-ID: <Z0EZ4gt2J8hVJz4x@google.com>
Date: Fri, 22 Nov 2024 15:55:14 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>, kvm@...r.kernel.org, pbonzini@...hat.com, 
	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 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.