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[92.26.98.202]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a6d0f1d91bsm3142259f8f.42.2025.06.20.16.29.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Jun 2025 16:29:41 -0700 (PDT) Message-ID: <83e4ffb4-26e9-4fc9-90ad-256a1613ba7d@citrix.com> Date: Sat, 21 Jun 2025 00:29:40 +0100 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: [PATCHv6 07/16] x86/vsyscall: Reorganize the #PF emulation code To: Sohil Mehta , dave.hansen@intel.com Cc: acme@redhat.com, aik@amd.com, akpm@linux-foundation.org, alexander.shishkin@linux.intel.com, ardb@kernel.org, ast@kernel.org, bp@alien8.de, brijesh.singh@amd.com, changbin.du@huawei.com, christophe.leroy@csgroup.eu, corbet@lwn.net, daniel.sneddon@linux.intel.com, dave.hansen@linux.intel.com, ebiggers@google.com, geert+renesas@glider.be, houtao1@huawei.com, hpa@zytor.com, jgg@ziepe.ca, jgross@suse.com, jpoimboe@kernel.org, kai.huang@intel.com, kees@kernel.org, kirill.shutemov@linux.intel.com, leitao@debian.org, linux-doc@vger.kernel.org, linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux@rasmusvillemoes.dk, luto@kernel.org, mcgrof@kernel.org, mhiramat@kernel.org, michael.roth@amd.com, mingo@kernel.org, mingo@redhat.com, namhyung@kernel.org, paulmck@kernel.org, pawan.kumar.gupta@linux.intel.com, peterz@infradead.org, rick.p.edgecombe@intel.com, rppt@kernel.org, sandipan.das@amd.com, shijie@os.amperecomputing.com, tglx@linutronix.de, tj@kernel.org, tony.luck@intel.com, vegard.nossum@oracle.com, x86@kernel.org, xin3.li@intel.com, xiongwei.song@windriver.com, ytcoode@gmail.com References: <9d351d80-66fe-486f-bdb3-370859dc47cc@intel.com> <262c0fd2-ac66-4ce7-903f-4062f1fe1d6e@citrix.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 21/06/2025 12:18 am, Sohil Mehta wrote: > On 6/20/2025 4:08 PM, Andrew Cooper wrote: >>> But, the resulting code is wonky. It needs to do something more like this: >>> >>> if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER) >>> return false; >>> >>> if (error_code & X86_PF_INSTR)) >>> return __emulate_vsyscall(regs, address); >> To do this, LASS needs a proper interlink against NX || SMEP. >> >> If neither NX nor SMEP are active, the CPU does not report X86_PF_INSTR, >> meaning that fetches are reported as plain reads. >> >> This leads to some fun corner cases in SMAP and now LASS too for virt. > Maybe I am missing something, but LASS works pre-paging so it wouldn't > generate a PF, right? Oh right, yes.  This is a preexisting bug in vsyscall #PF handling. It simply became obvious with Dave's suggested rearrangement. ~Andrew