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X-IronPort-AV: E=McAfee;i="6600,9927,10876"; a="387667408" X-IronPort-AV: E=Sophos;i="6.03,257,1694761200"; d="scan'208";a="387667408" Received: from fmsmga001.fm.intel.com ([10.253.24.23]) by fmsmga103.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 Oct 2023 14:50:43 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10876"; a="903394108" X-IronPort-AV: E=Sophos;i="6.03,257,1694761200"; d="scan'208";a="903394108" Received: from hannahwo-mobl1.amr.corp.intel.com (HELO [10.209.35.60]) ([10.209.35.60]) by fmsmga001-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 Oct 2023 14:48:12 -0700 Message-ID: <9fe968f0-cf38-4abf-b01c-5591c97ec886@intel.com> Date: Fri, 27 Oct 2023 14:50:41 -0700 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 05/50] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled Content-Language: en-US To: Michael Roth , kvm@vger.kernel.org Cc: linux-coco@lists.linux.dev, linux-mm@kvack.org, linux-crypto@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, jroedel@suse.de, thomas.lendacky@amd.com, hpa@zytor.com, ardb@kernel.org, pbonzini@redhat.com, seanjc@google.com, vkuznets@redhat.com, jmattson@google.com, luto@kernel.org, dave.hansen@linux.intel.com, slp@redhat.com, pgonda@google.com, peterz@infradead.org, srinivas.pandruvada@linux.intel.com, rientjes@google.com, dovmurik@linux.ibm.com, tobin@ibm.com, bp@alien8.de, vbabka@suse.cz, kirill@shutemov.name, ak@linux.intel.com, tony.luck@intel.com, marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com, alpergun@google.com, jarkko@kernel.org, ashish.kalra@amd.com, nikunj.dadhania@amd.com, pankaj.gupta@amd.com, liam.merwick@oracle.com, zhi.a.wang@intel.com, Kim Phillips References: <20231016132819.1002933-1-michael.roth@amd.com> <20231016132819.1002933-6-michael.roth@amd.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-2.8 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_LOW, SPF_HELO_NONE,SPF_NONE,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-Greylist: Sender passed SPF test, not delayed by milter-greylist-4.6.4 (snail.vger.email [0.0.0.0]); Fri, 27 Oct 2023 14:50:52 -0700 (PDT) On 10/16/23 06:27, Michael Roth wrote: > Without SEV-SNP, Automatic IBRS protects only the kernel. But when > SEV-SNP is enabled, the Automatic IBRS protection umbrella widens to all > host-side code, including userspace. This protection comes at a cost: > reduced userspace indirect branch performance. > > To avoid this performance loss, don't use Automatic IBRS on SEV-SNP > hosts. Fall back to retpolines instead. Thanks for the updated changelog: Acked-by: Dave Hansen BTW, have you given your hardware folks a hard time about this? It seems _kinda_ silly to be using retpolines when the hardware has a perfectly good IBRS implementation for the kernel. Just please make sure there's a good underlying reason for this behavior and as opposed to being some kind of inadvertent side effect. I assume Auto-IBRS and SEV-SNP are going to be with us for a long time, so it would be nice to have a long term solution here.