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X-IronPort-AV: E=McAfee;i="6600,9927,11014"; a="5303737" X-IronPort-AV: E=Sophos;i="6.07,129,1708416000"; d="scan'208";a="5303737" Received: from orviesa004.jf.intel.com ([10.64.159.144]) by orvoesa111.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Mar 2024 16:53:35 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.07,129,1708416000"; d="scan'208";a="17519077" Received: from uagu-mobl.amr.corp.intel.com (HELO [10.209.27.9]) ([10.209.27.9]) by orviesa004-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Mar 2024 16:53:36 -0700 Message-ID: <228473c4-fcda-448e-b859-008687fb052d@intel.com> Date: Fri, 15 Mar 2024 16:53:34 -0700 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v19 007/130] x86/virt/tdx: Export SEAMCALL functions Content-Language: en-US To: Sean Christopherson Cc: Rick P Edgecombe , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Kai Huang , Isaku Yamahata , "x86@kernel.org" , Tina Zhang , Hang Yuan , Bo2 Chen , "sagis@google.com" , "isaku.yamahata@gmail.com" , Erdem Aktas , "pbonzini@redhat.com" References: <8f64043a6c393c017347bf8954d92b84b58603ec.1708933498.git.isaku.yamahata@intel.com> <54ae3bbb-34dc-4b10-a14e-2af9e9240ef1@intel.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/15/24 12:38, Sean Christopherson wrote: >> tdh_mem_page_remove() _should_ just be logically: >> >> * initialize tdx_module_args. Move a few things into place on >> the stack and zero the rest. > The "zero the rest" is what generates the fugly code. The underlying problem is > that the SEAMCALL assembly functions unpack _all_ registers from tdx_module_args. > As a result, tdx_module_args needs to be zeroed to avoid loading registers with > unitialized stack data. It's the "zero the rest" and also the copy: > + if (out) { > + *out = *in; > + ret = seamcall_ret(op, out); > + } else > + ret = seamcall(op, in); Things get a wee bit nicer if you do an out-of-line mempcy() instead of the structure copy. But the really fun part is that 'out' is NULL and the compiler *SHOULD* know it. I'm not actually sure what trips it up. In any case, I think it ends up generating code for both sides of the if/else including the entirely superfluous copy. The two nested while loops (one for TDX_RND_NO_ENTROPY and the other for TDX_ERROR_SEPT_BUSY) also don't make for great code generation. So, sure, the generated code here could be a better. But there's a lot more going on here than just shuffling gunk in and out of the 'struct tdx_module_args', and there's a _lot_ more work to do for one of these than for a plain old kvm_hypercall*(). It might make sense to separate out the "out" functionality into and maybe to uninline _some_ of the helper levels. But after that, there's not a lot of low hanging fruit.