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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <6953f24da03b8ae61ba56b7cf6839d9c8ee44729.camel@intel.com>
Date: Tue, 30 Sep 2025 17:47:19 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Li, Xiaoyao" <xiaoyao.li@...el.com>, "Huang,
 Kai" <kai.huang@...el.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "kas@...nel.org"
	<kas@...nel.org>, "seanjc@...gle.com" <seanjc@...gle.com>, "mingo@...hat.com"
	<mingo@...hat.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "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>
CC: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 07/16] x86/virt/tdx: Add tdx_alloc/free_page() helpers

On Tue, 2025-09-30 at 22:03 +0800, Xiaoyao Li wrote:
> > +/* Bump PAMT refcount for the given page and allocate PAMT memory if needed
> > */
> > +int tdx_pamt_get(struct page *page)
> > +{
> > +	unsigned long hpa = ALIGN_DOWN(page_to_phys(page), PMD_SIZE);
> > +	u64 pamt_pa_array[MAX_DPAMT_ARG_SIZE];
> > +	atomic_t *pamt_refcount;
> > +	u64 tdx_status;
> > +	int ret;
> > +
> > +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> > +		return 0;
> > +
> > +	ret = alloc_pamt_array(pamt_pa_array);
> > +	if (ret)
> > +		return ret;
> > +
> > +	pamt_refcount = tdx_find_pamt_refcount(hpa);
> > +
> > +	scoped_guard(spinlock, &pamt_lock) {
> > +		if (atomic_read(pamt_refcount))
> 
> It's not what I expect the refcount to work (maybe I miss something 
> seriously?)
> 
> My understanding/expectation is that, when refcount is not zero it needs 
> to increment the refcount instead of simply return. And ...

Argh, yes. The following optimization patch should change this to behave
normally with respect the refcount. I should have tested it with the
optimization patches removed, I'll do that next time.

> 
> > +			goto out_free;
> > +
> > +		tdx_status = tdh_phymem_pamt_add(hpa | TDX_PS_2M,
> > pamt_pa_array);
> > +
> > +		if (IS_TDX_SUCCESS(tdx_status)) {
> > +			atomic_inc(pamt_refcount);
> > +		} else {
> > +			pr_err("TDH_PHYMEM_PAMT_ADD failed: %#llx\n",
> > tdx_status);
> > +			goto out_free;
> > +		}
> > +	}
> > +
> > +	return ret;
> > +out_free:
> > +	free_pamt_array(pamt_pa_array);
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(tdx_pamt_get);
> > +
> > +/*
> > + * Drop PAMT refcount for the given page and free PAMT memory if it is no
> > + * longer needed.
> > + */
> > +void tdx_pamt_put(struct page *page)
> > +{
> > +	unsigned long hpa = ALIGN_DOWN(page_to_phys(page), PMD_SIZE);
> > +	u64 pamt_pa_array[MAX_DPAMT_ARG_SIZE];
> > +	atomic_t *pamt_refcount;
> > +	u64 tdx_status;
> > +
> > +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> > +		return;
> > +
> > +	hpa = ALIGN_DOWN(hpa, PMD_SIZE);
> > +
> > +	pamt_refcount = tdx_find_pamt_refcount(hpa);
> > +
> > +	scoped_guard(spinlock, &pamt_lock) {
> > +		if (!atomic_read(pamt_refcount))
> 
> ...
> 
> when refcount > 1, decrease it.
> when refcount is 1, decrease it and remove the PAMT page pair.