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Fri, 04 Apr 2025 15:57:01 -0700 (PDT) Message-ID: <86903805-569a-41d5-93d8-df8169e61cef@suse.com> Date: Sat, 5 Apr 2025 01:56:59 +0300 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: [PATCH v3 3/6] x86/bugs: Fix RSB clearing in indirect_branch_prediction_barrier() To: Josh Poimboeuf Cc: x86@kernel.org, linux-kernel@vger.kernel.org, amit@kernel.org, kvm@vger.kernel.org, amit.shah@amd.com, thomas.lendacky@amd.com, bp@alien8.de, tglx@linutronix.de, peterz@infradead.org, pawan.kumar.gupta@linux.intel.com, corbet@lwn.net, mingo@redhat.com, dave.hansen@linux.intel.com, hpa@zytor.com, seanjc@google.com, pbonzini@redhat.com, daniel.sneddon@linux.intel.com, kai.huang@intel.com, sandipan.das@amd.com, boris.ostrovsky@oracle.com, Babu.Moger@amd.com, david.kaplan@amd.com, dwmw@amazon.co.uk, andrew.cooper3@citrix.com References: <27fe2029a2ef8bc0909e53e7e4c3f5b437242627.1743617897.git.jpoimboe@kernel.org> From: Nikolay Borisov Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 4.04.25 г. 18:17 ч., Josh Poimboeuf wrote: > On Fri, Apr 04, 2025 at 05:45:37PM +0300, Nikolay Borisov wrote: >> >> >> On 2.04.25 г. 21:19 ч., Josh Poimboeuf wrote: >>> IBPB is expected to clear the RSB. However, if X86_BUG_IBPB_NO_RET is >>> set, that doesn't happen. Make indirect_branch_prediction_barrier() >>> take that into account by calling __write_ibpb() which already does the >>> right thing. >> >> I find this changelog somewhat dubious. So zen < 4 basically have >> IBPB_NO_RET, your patch 2 in this series makes using SBPB for cores which >> have SRSO_NO or if the mitigation is disabled. So if you have a core which >> is zen <4 and doesn't use SBPB then what happens? > > I'm afraid I don't understand the question. In that case write_ibpb() > uses IBPB and manually clears the RSB. > Actually isn't this patch a noop. The old code simply wrote the value of x86_pred_cmd to the IA32-PRED_CMD register iff FEATURE_IBPB was set. So x86_pred_cmd might contain either PRED_CMD_IBPB or PRED_CMD_SBPB, meaning the correct value was written. With your change you now call __write_ibpb() which does effectively the same thing.