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

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Message-ID: <6903d890-c591-4986-8c88-a4b069309033@intel.com>
Date: Fri, 22 Nov 2024 16:08:34 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Sean Christopherson <seanjc@...gle.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 11/22/24 15:55, Sean Christopherson wrote:
> On Fri, Nov 22, 2024, Dave Hansen wrote:
> 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'm not super convinced. page_to_phys(foo) is all it takes

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

That's fair, although it's pervasively conflated across our entire
codebase. But 'struct page' is substantially better than a hpa_t,
phys_addr_t or u64 that can store a full 64-bits of address. Those
conflate a physical address with a physical page, which is *FAR* worse.

>> 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.

It's better than nothing. But I still vastly prefer to have a type that
tells you that something is physically-allocated out of the buddy, RAM,
and page-aligned.

I'd be better to have:

struct tdx_page {
	u64 page_phys_addr;
};

than depend on sparse, IMNHO.

Do you run sparse every time you compile the kernel, btw? ;)