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X-CSE-ConnectionGUID: ha4fvSChSAyZQLlQ20kzCg== X-CSE-MsgGUID: KNCRK3eNRCu8YYm7gRhpjQ== X-IronPort-AV: E=McAfee;i="6800,10657,11531"; a="61192911" X-IronPort-AV: E=Sophos;i="6.17,312,1747724400"; d="scan'208";a="61192911" Received: from orviesa007.jf.intel.com ([10.64.159.147]) by orvoesa109.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 26 Sep 2025 09:11:57 -0700 X-CSE-ConnectionGUID: dPQx06EsSp2EIN10J7xiZQ== X-CSE-MsgGUID: oQyf8SwFRMqPukPaf2iwJA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.18,295,1751266800"; d="scan'208";a="177490558" Received: from cmdeoliv-mobl4.amr.corp.intel.com (HELO [10.125.109.59]) ([10.125.109.59]) by orviesa007-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 26 Sep 2025 09:11:55 -0700 Message-ID: <7927271c-61e6-4f90-9127-c855a92fe766@intel.com> Date: Fri, 26 Sep 2025 09:11:54 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 00/16] TDX: Enable Dynamic PAMT To: "Edgecombe, Rick P" , "Zhao, Yan Y" Cc: "Gao, Chao" , "seanjc@google.com" , "Huang, Kai" , "kas@kernel.org" , "Annapurve, Vishal" , "bp@alien8.de" , "linux-kernel@vger.kernel.org" , "mingo@redhat.com" , "dave.hansen@linux.intel.com" , "kvm@vger.kernel.org" , "linux-coco@lists.linux.dev" , "Yamahata, Isaku" , "pbonzini@redhat.com" , "tglx@linutronix.de" , "x86@kernel.org" References: <20250918232224.2202592-1-rick.p.edgecombe@intel.com> <8f772a23-ea7f-40eb-8852-49f5e3e16c15@intel.com> <2b951e427c3f3f06fc310d151b7c9e960c32ec3f.camel@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 9/26/25 09:02, Edgecombe, Rick P wrote: > On Fri, 2025-09-26 at 07:09 -0700, Dave Hansen wrote: >> On 9/25/25 19:28, Yan Zhao wrote: >>>> Lastly, Yan raised some last minute doubts internally about TDX >>>> module locking contention. I’m not sure there is a problem, but >>>> we >>>> can come to an agreement as part of the review. >>> Yes, I found a contention issue that prevents us from dropping the >>> global lock. I've also written a sample test that demonstrates this >>> contention. >> But what is the end result when this contention happens? Does >> everyone livelock? > > You get a TDX_OPERAND_BUSY error code returned. Inside the TDX module > each lock is a try lock. The TDX module tries to take a sequence of > locks and if it meets any contention it will release them all and > return the TDX_OPERAND_BUSY. Some paths in KVM cannot handle failure > and so don't have a way to handle the error. > > So another option to handling this is just to retry until you succeed. > Then you have a very strange spinlock with a heavyweight inner loop. > But since each time the locks are released, some astronomical bad > timing might prevent forward progress. On the KVM side we have avoided > looping. Although, I think we have not exhausted this path. If it can't return failure then the _only_ other option is to spin. Right? I understand the reluctance to have such a nasty spin loop. But other than reworking the KVM code to do the retries at a higher level, is there another option?