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X-CSE-ConnectionGUID: Q5Yx2Z13R3ScYUGUjgaIOg== X-CSE-MsgGUID: gEx0JZVaQjOSvW6wispuNw== X-IronPort-AV: E=McAfee;i="6800,10657,11697"; a="82203614" X-IronPort-AV: E=Sophos;i="6.21,283,1763452800"; d="scan'208";a="82203614" Received: from orviesa001.jf.intel.com ([10.64.159.141]) by orvoesa103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Feb 2026 09:44:20 -0800 X-CSE-ConnectionGUID: CNwJLfqhQmSfL/wFmjKzww== X-CSE-MsgGUID: xqCxx7TZQY6wgvCnr3+w1Q== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.21,283,1763452800"; d="scan'208";a="249628945" Received: from cjhill-mobl.amr.corp.intel.com (HELO [10.125.108.43]) ([10.125.108.43]) by smtpauth.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Feb 2026 09:44:19 -0800 Message-ID: <655724f8-0098-40ee-a097-ce4c0249933d@intel.com> Date: Tue, 10 Feb 2026 09:44:16 -0800 Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v5 16/45] x86/virt/tdx: Add tdx_alloc/free_control_page() helpers To: Sean Christopherson , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, Kiryl Shutsemau , Paolo Bonzini Cc: linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev, kvm@vger.kernel.org, Kai Huang , Rick Edgecombe , Yan Zhao , Vishal Annapurve , Ackerley Tng , Sagi Shahar , Binbin Wu , Xiaoyao Li , Isaku Yamahata References: <20260129011517.3545883-1-seanjc@google.com> <20260129011517.3545883-17-seanjc@google.com> Content-Language: en-US From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 1/28/26 17:14, Sean Christopherson wrote: > +static void tdx_pamt_put(struct page *page) > +{ > + u64 pamt_pa_array[MAX_NR_DPAMT_ARGS]; > + atomic_t *pamt_refcount; > + u64 tdx_status; > + > + if (!tdx_supports_dynamic_pamt(&tdx_sysinfo)) > + return; > + > + pamt_refcount = tdx_find_pamt_refcount(page_to_pfn(page)); > + > + scoped_guard(spinlock, &pamt_lock) { > + /* > + * If the there are more than 1 references on the pamt page, > + * don't remove it yet. Just decrement the refcount. > + */ > + if (atomic_read(pamt_refcount) > 1) { > + atomic_dec(pamt_refcount); > + return; > + } > + > + /* Try to remove the pamt page and take the refcount 1->0. */ > + tdx_status = tdh_phymem_pamt_remove(page, pamt_pa_array); > + > + /* > + * Don't free pamt_pa_array as it could hold garbage when > + * tdh_phymem_pamt_remove() fails. Don't panic/BUG_ON(), as > + * there is no risk of data corruption, but do yell loudly as > + * failure indicates a kernel bug, memory is being leaked, and > + * the dangling PAMT entry may cause future operations to fail. > + */ > + if (WARN_ON_ONCE(!IS_TDX_SUCCESS(tdx_status))) > + return; > + > + atomic_dec(pamt_refcount); > + } > + > + /* > + * pamt_pa_array is populated up to tdx_dpamt_entry_pages() by the TDX > + * module with pages, or remains zero inited. free_pamt_array() can > + * handle either case. Just pass it unconditionally. > + */ > + free_pamt_array(pamt_pa_array); > +} This looks funky. Right now, this is: spin_lock(pamt_lock) atomic_inc/dec(fine-grained-refcount) tdcall_blah_blah() spin_unlock(pamt_lock) Where it *always* acquires the global lock when DPAMT is supported. Couldn't we optimize it so that it only acquires it when it has to keep the refcount stable at zero? Roughly: slow_path = atomic_dec_and_lock(fine-grained-refcount, pamt_lock) if (!slow_path) goto out; // fine-grained-refcount==0 and must stay that way with // pamt_lock held. Remove the DPAMT pages: tdh_phymem_pamt_remove(page, pamt_pa_array) out: spin_unlock(pamt_lock) On the acquire side, you do: fast_path = atomic_inc_not_zero(fine-grained-refcount) if (fast_path) return; // slow path: spin_lock(pamt_lock) // Was the race lost with another 0=>1 increment? if (atomic_read(fine-grained-refcount) > 0) goto out_inc tdh_phymem_pamt_add(page, pamt_pa_array) // Inc after the TDCALL so another thread won't race ahead of us // and try to use a non-existent PAMT entry out_inc: atomic_inc(fine-grained-refcount) spin_unlock(pamt_lock) Then, at least only the 0=>1 and 1=>0 transitions need the global lock. The fast paths only touch the refcount which isn't shared nearly as much as the global lock. BTW, this probably still needs to be spin_lock_irq(), not what I wrote above, but that's not a big deal to add. I've stared at this for a bit and don't see any holes. Does anyone else see any?