linux-kernel - Re: [PATCH v3 07/16] x86/virt/tdx: Add tdx_alloc/free

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Message-ID: <71b5325b7aa320e9271871b9cfe68981347391b1.camel@intel.com>
Date: Mon, 29 Sep 2025 17:19:55 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Huang, Kai" <kai.huang@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...hat.com"
	<mingo@...hat.com>, "kas@...nel.org" <kas@...nel.org>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "Annapurve, Vishal"
	<vannapurve@...gle.com>, "Gao, Chao" <chao.gao@...el.com>, "bp@...en8.de"
	<bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 07/16] x86/virt/tdx: Add tdx_alloc/free_page() helpers

On Mon, 2025-09-29 at 15:56 +0800, Yan Zhao wrote:
> > +/*
> > + * The TDX spec treats the registers like an array, as they are ordered
> > + * in the struct. The array size is limited by the number or registers,
> > + * so define the max size it could be for worst case allocations and sanity
> > + * checking.
> > + */
> > +#define MAX_DPAMT_ARG_SIZE (sizeof(struct tdx_module_args) - \
> > +			    offsetof(struct tdx_module_args, rdx))
> The rdx doesn't work for tdh_mem_page_demote(), which copies the pamt pages
> array starting from r12.
> 
> Is there a way to make this more generic?

Sure we could just pass rdx/r12 as an arg. But I'd think to leave it to the
huage pages patches to add that tweak.

> 
> > +/*
> > + * Treat struct the registers like an array that starts at RDX, per
> > + * TDX spec. Do some sanitychecks, and return an indexable type.
> > + */
> > +static u64 *dpamt_args_array_ptr(struct tdx_module_args *args)
> > +{
> > +	WARN_ON_ONCE(tdx_dpamt_entry_pages() > MAX_DPAMT_ARG_SIZE);
> > +
> > +	/*
> > +	 * FORTIFY_SOUCE could inline this and complain when callers copy
> > +	 * across fields, which is exactly what this is supposed to be
> > +	 * used for. Obfuscate it.
> > +	 */
> > +	return (u64 *)((u8 *)args + offsetof(struct tdx_module_args, rdx));
> > +}
> > +
> > +static int alloc_pamt_array(u64 *pa_array)
> > +{
> > +	struct page *page;
> > +
> > +	for (int i = 0; i < tdx_dpamt_entry_pages(); i++) {
> > +		page = alloc_page(GFP_KERNEL);
> > +		if (!page)
> > +			return -ENOMEM;
> When the 1st alloc_page() succeeds but the 2nd alloc_page() fails, the 1st
> page
> must be freed before returning an error. 

Oops, yes. It needs a proper free path here. Thanks.

> 
> > +		pa_array[i] = page_to_phys(page);
> > +	}
> > +
> > +	return 0;
> > +}
> > +
>