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[92.26.98.202]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4405b4f2cfesm2246405e9.19.2025.04.15.15.12.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Apr 2025 15:12:50 -0700 (PDT) Message-ID: Date: Tue, 15 Apr 2025 23:12:49 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v13 09/19] x86: Secure Launch kernel early boot stub To: Sean Christopherson , Ross Philipson Cc: linux-kernel@vger.kernel.org, x86@kernel.org, linux-integrity@vger.kernel.org, linux-doc@vger.kernel.org, linux-crypto@vger.kernel.org, kexec@lists.infradead.org, linux-efi@vger.kernel.org, iommu@lists.linux.dev, dpsmith@apertussolutions.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, dave.hansen@linux.intel.com, ardb@kernel.org, mjg59@srcf.ucam.org, James.Bottomley@hansenpartnership.com, peterhuewe@gmx.de, jarkko@kernel.org, jgg@ziepe.ca, luto@amacapital.net, nivedita@alum.mit.edu, herbert@gondor.apana.org.au, davem@davemloft.net, corbet@lwn.net, ebiederm@xmission.com, dwmw2@infradead.org, baolu.lu@linux.intel.com, kanth.ghatraju@oracle.com, trenchboot-devel@googlegroups.com References: <20250410204149.2576104-1-ross.philipson@oracle.com> <20250410204149.2576104-10-ross.philipson@oracle.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250415_151254_458168_DD5E6B01 X-CRM114-Status: GOOD ( 14.07 ) X-BeenThere: kexec@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "kexec" Errors-To: kexec-bounces+kexec=archiver.kernel.org@lists.infradead.org On 11/04/2025 10:40 pm, Sean Christopherson wrote: > On Thu, Apr 10, 2025, Ross Philipson wrote: >> + * instruction can return for a number of reasons. Test to see if it returned >> + * because the monitor was written to. >> + */ >> + monitor >> + >> +1: >> + mfence >> + mwait >> + movl (%eax), %edx > Why load the value into EDX? At a glance, the value is never consumed. > >> + testl %edx, %edx >> + jz 1b > This usage of MONITOR/MWAIT is flawed. The monitor needs to be re-armed in each > loop, otherwise mwait will be a glorified nop. > > More importantly, the exit condition needs to be checked before monitor, after monitor and before mwait. But yes, the prior logic was definitely wonky. > even on > the first iteration. In the (probably extremely unlikely) scenario that the write > to wake the CPU arrives before MONITOR is executed, this CPU may get stuck waiting > indefinitely. > > E.g. something like: > > > 1: > monitor > cmpl (%eax), 0 $0 Luckily, this will fail to assemble, rather than dereferencing 0. ~Andrew > jnz 2f > mwait > jmp 1b > 2: