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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4851ad02ffcsm32648235e9.20.2026.03.05.06.12.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Mar 2026 06:12:00 -0800 (PST) Date: Thu, 5 Mar 2026 14:11:58 +0000 From: David Laight To: Tony W Wang-oc Cc: , , , , , , , , , , David Wang , , , "TimGuo@zhaoxin.com" , , , , , , "CobeChen@zhaoxin.com" Subject: Re: [PATCH] x86/cpu/centaur: Disable X86_FEATURE_FSGSBASE on Zhaoxin C4600 Message-ID: <20260305141158.294ee1e9@pumpkin> In-Reply-To: <70139192-54e5-4a4b-bc96-1fe3ec4f7a0b@zhaoxin.com> References: <20260228173704.62460-1-me@ziyao.cc> <70139192-54e5-4a4b-bc96-1fe3ec4f7a0b@zhaoxin.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 5 Mar 2026 17:03:07 +0800 Tony W Wang-oc wrote: > Thank you for submitting the patch to fix the Zhaoxin CPU issue. > > After internal clarification, we have confirmed that this is an > issue with the ZX-C CPU ucode: > When modifying CR4.FSGSBASE bit 16, the ucode propagates its > value to another MSR register. During execution of FSGSBASE-related > instructions, the hardware actually checks whether this MSR > register's bit is set to determine whether to generate a #UD > exception. > When the CPU enters SMM mode and then returns via RSM, the CR4 > register is restored but the value of CR4.FSGSBASE is not > re-propagated to the MSR register. > As a result, after enabling CR4.FSGSBASE, once the CPU goes > through SMM mode, executing FSGSBASE-related instructions will > trigger a #UD exception. > > This issue exists only on ZX-C CPUs, which have two different > CPU vendor IDs and distinct FMS values. The following patch can > be used to identify ZX-C CPUs and properly handle this issue: > > --- a/arch/x86/kernel/cpu/centaur.c > +++ b/arch/x86/kernel/cpu/centaur.c > @@ -201,6 +201,11 @@ static void init_centaur(struct cpuinfo_x86 *c) > set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC); > #endif > > + if (c->x86 == 6 && c->x86_model == 15 && c->x86_stepping >= 14) { The '>= 14' looks odd to me. It implies it all worked, got broken, and will never be fixed. I'd also add a 1-line comment, something like: /* CR4.FSGSBASE not copied to MSR on return from SMM mode. */ David > + pr_warn_once("CPU has broken FSGSBASE support; clear > FSGSBASE feature\n"); > + setup_clear_cpu_cap(X86_FEATURE_FSGSBASE); > + } > + > init_ia32_feat_ctl(c); > } > > diff --git a/arch/x86/kernel/cpu/zhaoxin.c b/arch/x86/kernel/cpu/zhaoxin.c > index 031379b7d4fa..6a2d6df307ee 100644 > --- a/arch/x86/kernel/cpu/zhaoxin.c > +++ b/arch/x86/kernel/cpu/zhaoxin.c > @@ -89,6 +89,11 @@ static void init_zhaoxin(struct cpuinfo_x86 *c) > set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC); > #endif > > + if (c->x86 == 6 && c->x86_model == 25 && c->x86_stepping <= 3) { > + pr_warn_once("CPU has broken FSGSBASE support; clear > FSGSBASE feature\n"); > + setup_clear_cpu_cap(X86_FEATURE_FSGSBASE); > + } > + > init_ia32_feat_ctl(c); > } > > Sincerely > TonyWWang-oc >