From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: AH8x227jYyUKkwoZ1bCszxBI/YzjTInd1RVeHU0sHaMWZFK2xIZ2wDSRPw4GznMjtEY6RHf7OWgW ARC-Seal: i=1; a=rsa-sha256; t=1516980459; cv=none; d=google.com; s=arc-20160816; b=VTD9q/xOhl63Mw4zEtPbKgqrzHAVE6WDPmyI+rHboRX/NJ0tNzth8eJewoN+TdgBsz QzJ9WFEvyw/XDv74N6njfOkBaaqbA9B71CiW9B5UbOM/79KEY8DCB1GwYm+TZUxwwHFz oAMhVDg1tVdJ9/smdQJWNjH6z3SLkerEJ1006VEhNH0t8EfRZ/htdagBzuXLVmW2f6cW xJT/nFwAviM4uiW/Y58dEBW4Y9feCT0if/b0gdwzZk+XB9T594v2hSKSB41jky8DcqzE 1ISxTmp14EoTwkNfphR/MZvQ9d9WCpyfem936w6MAz1JnGSh09ORYKER6205Y7/TVcL2 HvYg== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=content-transfer-encoding:content-language:in-reply-to:mime-version :user-agent:date:message-id:from:references:to:subject :arc-authentication-results; bh=uFTLwoQpmrKmRUrBqQ9Xrq6nVQYx5gqQxbLiBKloVnU=; b=AGrk/7aD54tP6XlS6W2tI3zOkMeU4LfRk/GmeIP4MfFaIFXYDSX/UFrh8azaFmRAkv DWVVSxYI/P85JUWNniltO37O5YsTKecBsbTO+tZ8DU8oXjHFozpa3fLYtw7TDkLDGz0G tCFWA3YXOjfFF1smuwG6wKHY0ZQxRSw+/AM7yIcSJt8BSnxAs6Weks3YlKIvQNMa1qCw G+ENYEa/wGIuKC8b5XcxdGwtiGixxEUA+87NczaiWfHgVFpcf7HBmavoHUv7KZyHSw5x ATM5MhPgbDaA4zeVGqN3PDXmHhmrmy41o0N8V5jYyonI2KU3xDi/PgKAPMAsu+RNeG4W KnQA== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of dave.hansen@intel.com designates 134.134.136.31 as permitted sender) smtp.mailfrom=dave.hansen@intel.com Authentication-Results: mx.google.com; spf=pass (google.com: domain of dave.hansen@intel.com designates 134.134.136.31 as permitted sender) smtp.mailfrom=dave.hansen@intel.com X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.46,417,1511856000"; d="scan'208";a="13700176" Subject: Re: [PATCH v3 5/6] x86/pti: Do not enable PTI on processors which are not vulnerable to Meltdown To: Yves-Alexis Perez , David Woodhouse , arjan@linux.intel.com, tglx@linutronix.de, karahmed@amazon.de, x86@kernel.org, linux-kernel@vger.kernel.org, tim.c.chen@linux.intel.com, bp@alien8.de, peterz@infradead.org, pbonzini@redhat.com, ak@linux.intel.com, torvalds@linux-foundation.org, gregkh@linux-foundation.org, gnomes@lxorguk.ukuu.org.uk References: <1516813025-10794-1-git-send-email-dwmw@amazon.co.uk> <1516813025-10794-6-git-send-email-dwmw@amazon.co.uk> <1516968886.19619.7.camel@debian.org> From: Dave Hansen Message-ID: Date: Fri, 26 Jan 2018 07:27:38 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0 MIME-Version: 1.0 In-Reply-To: <1516968886.19619.7.camel@debian.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1590493766352787258?= X-GMAIL-MSGID: =?utf-8?q?1590669302019348073?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On 01/26/2018 04:14 AM, Yves-Alexis Perez wrote: > I know we'll still be able to manually enable PTI with a command line option, > but it's also a hardening feature which has the nice side effect of emulating > SMEP on CPU which don't support it (e.g the Atom boxes above). For Meltdown-vulnerable systems, it's a no brainer: pti=on. The vulnerability there is just too much. But, if we are going to change the default, IMNHO, we need a clear list of what SMEP emulation mitigates and where. RSB-related Variant 2 stuff on Atom where the kernel speculatively 'ret's back to userspace is certainly a concern. But, there's a lot of other RSB stuffing that's going on that will mitigate that too. Were you thinking of anything concrete? I haven't found anything compelling enough to justify the downsides, especially since things without SMEP tend not to have PCIDs as well.