From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.11]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9A3BD3D0C07; Wed, 8 Jul 2026 19:01:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.175.65.11 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783537268; cv=none; b=AnMp+B0HM/9rtwlfIt1m4NTXvmdI78gCsbUF1LdYjxp/M1HlP+2r6B3NkXOKSZ8IsZQIuJNqMFHb3jBr/NHNVcd+92MKEEomP10Yv44QhFNGiZydQ6/eLFRMyb/jV/Ul1g9GknWBCuJdenbduWLUVAN9aUj1PEnuasYBg3g/YN4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783537268; c=relaxed/simple; bh=JmvmXqvoJhxel/XCqY2ga/B/piNjvTPEr8lOPuxpubA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=aXwPGJGb2zvuU3NOrM95sclFhylRtBAe2OIwQinjut+I8XAU1j+skRnGbgC4UzC2ciiVolNZbiy65MjmV8O2WCmaKMlZyQP7UfSrx1VFqFKdaoL3jAKagV37r1SjJThv9/n+lYvNYO2OhtBBxPlNqSvrn0RxoZ5nDTXyh1wNnAY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com; spf=pass smtp.mailfrom=intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=Ox/qzpCj; arc=none smtp.client-ip=198.175.65.11 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="Ox/qzpCj" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1783537266; x=1815073266; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=JmvmXqvoJhxel/XCqY2ga/B/piNjvTPEr8lOPuxpubA=; b=Ox/qzpCjVd4vPGM6qroNVc30olmwg8ENwV9KqmGs9r473Ct7i0nWdVC2 974ePQiU9yTSMZad3NFxDN9NQ6GDu9CIFbzIHRgcz71Xd/P+qMnjsw0af rvyIjtjdaJB/hSHQtX6DVNyruj/H/u07dMXULslDQq2nYPHhb5WX/eNrC 4K+ARjDk5rjGSUVUNhH+/+qFJzgcm7kYvyWY1wXDHIWFUA07mkgfdZt19 a7/kux3FL16dMZEciOrg4iqNw5cv/JYZY4FVUOiV2FliBUoN5O86aO92+ 6OMuz3D8iVkK5wVWiIvZlz5Kb5OZoSPI4tplgX7hMaCreve8LYkDXHwfG g==; X-CSE-ConnectionGUID: TG1A1FebR5S4E0wed8ScfA== X-CSE-MsgGUID: ESx5ZoUMTYKzUKWcPOdrTA== X-IronPort-AV: E=McAfee;i="6800,10657,11841"; a="94567117" X-IronPort-AV: E=Sophos;i="6.25,153,1779174000"; d="scan'208";a="94567117" Received: from fmviesa008.fm.intel.com ([10.60.135.148]) by orvoesa103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 Jul 2026 12:01:05 -0700 X-CSE-ConnectionGUID: 3obKFQgzQqS3quORMBg99Q== X-CSE-MsgGUID: punpz2zRRdqR7+3AYOReyg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.25,153,1779174000"; d="scan'208";a="251717242" Received: from jmaxwel1-mobl.amr.corp.intel.com (HELO [10.125.111.66]) ([10.125.111.66]) by fmviesa008-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 Jul 2026 12:01:04 -0700 Message-ID: <1d60cebf-4764-4785-bf01-8ca59ab2cc7d@intel.com> Date: Wed, 8 Jul 2026 12:01:01 -0700 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 05/11] x86/virt/tdx: Handle concurrent callers in tdx_pamt_get/put() To: Rick Edgecombe , bp@alien8.de, hpa@zytor.com, kas@kernel.org, kvm@vger.kernel.org, linux-coco@lists.linux.dev, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@redhat.com, nik.borisov@suse.com, pbonzini@redhat.com, seanjc@google.com, tglx@kernel.org, vannapurve@google.com, x86@kernel.org, chao.gao@intel.com, yan.y.zhao@intel.com, kai.huang@intel.com Cc: "Kirill A. Shutemov" References: <20260526023515.288829-1-rick.p.edgecombe@intel.com> <20260526023515.288829-6-rick.p.edgecombe@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; keydata= xsFNBE6HMP0BEADIMA3XYkQfF3dwHlj58Yjsc4E5y5G67cfbt8dvaUq2fx1lR0K9h1bOI6fC oAiUXvGAOxPDsB/P6UEOISPpLl5IuYsSwAeZGkdQ5g6m1xq7AlDJQZddhr/1DC/nMVa/2BoY 2UnKuZuSBu7lgOE193+7Uks3416N2hTkyKUSNkduyoZ9F5twiBhxPJwPtn/wnch6n5RsoXsb ygOEDxLEsSk/7eyFycjE+btUtAWZtx+HseyaGfqkZK0Z9bT1lsaHecmB203xShwCPT49Blxz VOab8668QpaEOdLGhtvrVYVK7x4skyT3nGWcgDCl5/Vp3TWA4K+IofwvXzX2ON/Mj7aQwf5W iC+3nWC7q0uxKwwsddJ0Nu+dpA/UORQWa1NiAftEoSpk5+nUUi0WE+5DRm0H+TXKBWMGNCFn c6+EKg5zQaa8KqymHcOrSXNPmzJuXvDQ8uj2J8XuzCZfK4uy1+YdIr0yyEMI7mdh4KX50LO1 pmowEqDh7dLShTOif/7UtQYrzYq9cPnjU2ZW4qd5Qz2joSGTG9eCXLz5PRe5SqHxv6ljk8mb ApNuY7bOXO/A7T2j5RwXIlcmssqIjBcxsRRoIbpCwWWGjkYjzYCjgsNFL6rt4OL11OUF37wL QcTl7fbCGv53KfKPdYD5hcbguLKi/aCccJK18ZwNjFhqr4MliQARAQABzUVEYXZpZCBDaHJp c3RvcGhlciBIYW5zZW4gKEludGVsIFdvcmsgQWRkcmVzcykgPGRhdmUuaGFuc2VuQGludGVs LmNvbT7CwXgEEwECACIFAlQ+9J0CGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEGg1 lTBwyZKwLZUP/0dnbhDc229u2u6WtK1s1cSd9WsflGXGagkR6liJ4um3XCfYWDHvIdkHYC1t MNcVHFBwmQkawxsYvgO8kXT3SaFZe4ISfB4K4CL2qp4JO+nJdlFUbZI7cz/Td9z8nHjMcWYF IQuTsWOLs/LBMTs+ANumibtw6UkiGVD3dfHJAOPNApjVr+M0P/lVmTeP8w0uVcd2syiaU5jB aht9CYATn+ytFGWZnBEEQFnqcibIaOrmoBLu2b3fKJEd8Jp7NHDSIdrvrMjYynmc6sZKUqH2 I1qOevaa8jUg7wlLJAWGfIqnu85kkqrVOkbNbk4TPub7VOqA6qG5GCNEIv6ZY7HLYd/vAkVY E8Plzq/NwLAuOWxvGrOl7OPuwVeR4hBDfcrNb990MFPpjGgACzAZyjdmYoMu8j3/MAEW4P0z F5+EYJAOZ+z212y1pchNNauehORXgjrNKsZwxwKpPY9qb84E3O9KYpwfATsqOoQ6tTgr+1BR CCwP712H+E9U5HJ0iibN/CDZFVPL1bRerHziuwuQuvE0qWg0+0SChFe9oq0KAwEkVs6ZDMB2 P16MieEEQ6StQRlvy2YBv80L1TMl3T90Bo1UUn6ARXEpcbFE0/aORH/jEXcRteb+vuik5UGY 5TsyLYdPur3TXm7XDBdmmyQVJjnJKYK9AQxj95KlXLVO38lczsFNBFRjzmoBEACyAxbvUEhd GDGNg0JhDdezyTdN8C9BFsdxyTLnSH31NRiyp1QtuxvcqGZjb2trDVuCbIzRrgMZLVgo3upr MIOx1CXEgmn23Zhh0EpdVHM8IKx9Z7V0r+rrpRWFE8/wQZngKYVi49PGoZj50ZEifEJ5qn/H Nsp2+Y+bTUjDdgWMATg9DiFMyv8fvoqgNsNyrrZTnSgoLzdxr89FGHZCoSoAK8gfgFHuO54B lI8QOfPDG9WDPJ66HCodjTlBEr/Cwq6GruxS5i2Y33YVqxvFvDa1tUtl+iJ2SWKS9kCai2DR 3BwVONJEYSDQaven/EHMlY1q8Vln3lGPsS11vSUK3QcNJjmrgYxH5KsVsf6PNRj9mp8Z1kIG qjRx08+nnyStWC0gZH6NrYyS9rpqH3j+hA2WcI7De51L4Rv9pFwzp161mvtc6eC/GxaiUGuH BNAVP0PY0fqvIC68p3rLIAW3f97uv4ce2RSQ7LbsPsimOeCo/5vgS6YQsj83E+AipPr09Caj 0hloj+hFoqiticNpmsxdWKoOsV0PftcQvBCCYuhKbZV9s5hjt9qn8CE86A5g5KqDf83Fxqm/ vXKgHNFHE5zgXGZnrmaf6resQzbvJHO0Fb0CcIohzrpPaL3YepcLDoCCgElGMGQjdCcSQ+Ci FCRl0Bvyj1YZUql+ZkptgGjikQARAQABwsFfBBgBAgAJBQJUY85qAhsMAAoJEGg1lTBwyZKw l4IQAIKHs/9po4spZDFyfDjunimEhVHqlUt7ggR1Hsl/tkvTSze8pI1P6dGp2XW6AnH1iayn yRcoyT0ZJ+Zmm4xAH1zqKjWplzqdb/dO28qk0bPso8+1oPO8oDhLm1+tY+cOvufXkBTm+whm +AyNTjaCRt6aSMnA/QHVGSJ8grrTJCoACVNhnXg/R0g90g8iV8Q+IBZyDkG0tBThaDdw1B2l asInUTeb9EiVfL/Zjdg5VWiF9LL7iS+9hTeVdR09vThQ/DhVbCNxVk+DtyBHsjOKifrVsYep WpRGBIAu3bK8eXtyvrw1igWTNs2wazJ71+0z2jMzbclKAyRHKU9JdN6Hkkgr2nPb561yjcB8 sIq1pFXKyO+nKy6SZYxOvHxCcjk2fkw6UmPU6/j/nQlj2lfOAgNVKuDLothIxzi8pndB8Jju KktE5HJqUUMXePkAYIxEQ0mMc8Po7tuXdejgPMwgP7x65xtfEqI0RuzbUioFltsp1jUaRwQZ MTsCeQDdjpgHsj+P2ZDeEKCbma4m6Ez/YWs4+zDm1X8uZDkZcfQlD9NldbKDJEXLIjYWo1PH hYepSffIWPyvBMBTW2W5FRjJ4vLRrJSUoEfJuPQ3vW9Y73foyo/qFoURHO48AinGPZ7PC7TF vUaNOTjKedrqHkaOcqB185ahG2had0xnFsDPlx5y In-Reply-To: <20260526023515.288829-6-rick.p.edgecombe@intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/25/26 19:35, Rick Edgecombe wrote: > + scoped_guard(spinlock, &pamt_lock) { > + /* > + * If the pamt page is already added (i.e. refcount >= 1), > + * then just increment the refcount. > + */ > + if (atomic_read(pamt_refcount)) { > + atomic_inc(pamt_refcount); > + goto out_free; > + } I think this pattern is a good fit for atomic_inc_not_zero(). I think it's also fair to say that using an atomic_t here is unnecessary because of the spinlock protecting all accesses. But I also understand that it's here because you want to optimize this path to avoid the spinlock in the future. This is not a hot enough path to care about an atomic_t vs. int. In practice, the performance is going to be bottlenecked on the big spinlock, *not* the atomic_t overhead. I'd just add a changelog blurb to handwave it away for the moment: The pamt_refcount[]s are atomic_t's. They do not strictly need to be because all access is protected by pamt_lock. The overhead of an atomic_t in this situation is minuscule compared to the global lock. Leave the (unnecessary) atomic_t in place to enable future optimization with minimal churn. Does that work for everybody?