linux-kernel - Re: [PATCH v2 08/25] x86/virt/tdx: Add SEAMCALL wrappers for TDX page cache management

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Message-ID: <ff549c76-59a3-47f6-b68d-64ef957a7765@intel.com>
Date: Wed, 13 Nov 2024 13:08:22 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>,
 "seanjc@...gle.com" <seanjc@...gle.com>
Cc: "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
 "Yao, Yuan" <yuan.yao@...el.com>, "Huang, Kai" <kai.huang@...el.com>,
 "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
 "Li, Xiaoyao" <xiaoyao.li@...el.com>,
 "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "tony.lindgren@...ux.intel.com" <tony.lindgren@...ux.intel.com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "Zhao, Yan Y" <yan.y.zhao@...el.com>,
 "Chatre, Reinette" <reinette.chatre@...el.com>,
 "Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH v2 08/25] x86/virt/tdx: Add SEAMCALL wrappers for TDX page
 cache management

On 11/13/24 12:51, Edgecombe, Rick P wrote:
> However, some future user of TDH.PHYMEM.PAGE.RECLAIM might want to do something
> else where the enums could add code clarity. But this goes down the road of
> building things that are not needed today.

Here's why the current code is a bit suboptimal:

> +/* TDH.PHYMEM.PAGE.RECLAIM is allowed only when destroying the TD. */
> +static int __tdx_reclaim_page(hpa_t pa)
> +{
...
> +	for (i = TDX_SEAMCALL_RETRIES; i > 0; i--) {
> +		err = tdh_phymem_page_reclaim(pa, &rcx, &rdx, &r8);
...
> +out:
> +	if (WARN_ON_ONCE(err)) {
> +		pr_tdx_error_3(TDH_PHYMEM_PAGE_RECLAIM, err, rcx, rdx, r8);
> +		return -EIO;
> +	}
> +	return 0;
> +}

Let's say I see the error get spit out on the console.  I can't make any
sense out of it from this spot.  I need to go over to the TDX docs or
tdh_phymem_page_reclaim() to look at the *comment* to figure out what
these the registers are named.

The code as proposed has zero self-documenting properties.  It's
actually completely non-self-documenting.  It isn't _any_ better for
readability than just doing:

	struct tdx_module_args args = {};

	for (i = TDX_SEAMCALL_RETRIES; i > 0; i--) {
		args.rcx = pa;
		err = seamcall_ret(TDH_PHYMEM_PAGE_RECLAIM, &args);
		...
	}

	pr_tdx_error_3(TDH_PHYMEM_PAGE_RECLAIM, err,
			args.rcx, args.rdx, args.r8);

Also, this is also showing a lack of naming discipline where things are
named.  The first argument is 'pa' in here but 'page' on the other side:

> +u64 tdh_phymem_page_reclaim(u64 page, u64 *rcx, u64 *rdx, u64 *r8)
> +{
> +	struct tdx_module_args args = {
> +		.rcx = page,

I can't tell you how many recompiles it's cost me when I got lazy about
physical addr vs. virtual addr vs. struct page vs. pfn.

So, yeah, I'd rather not export seamcall_ret(), but I'd rather do that
than have a layer of abstraction that's adding little value while it
also brings obfuscation.