From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.zytor.com (terminus.zytor.com [198.137.202.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3C04013D53B for ; Wed, 17 Apr 2024 14:59:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.136 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713366000; cv=none; b=cYvDet+00TXA/ANJLOwTW7mdoHWYEBOKHTEqWu4X30WS2eVOFgapvaJsWfm9mSO44bFj2zQxPEkh1wkT37aVuHnzKdqLdMk2XDwAhPX5M0iivAdlOP2WWHob8LnqrUesNt+F+VR5zxkEGlJpdGiJhVLv5LgHxEznmyS8+ZcJwlg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713366000; c=relaxed/simple; bh=ZgrrbaCSJMRtonK7p9OX8MB48F0JI8/7kBVr9BuIrG4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rTSMW+9dIot6RqNWonV9FkfDLERBq5ipw4Hb2FoYcspbNJx7QYJ868uKt3UdMikrGzrBD72dhgczO89mJiCZR87NFkd3ar2rl/RkCRg6qwdzh6UBsPfCPAqtEay7ugfMEet7nzkGFCXvGn4YWNjLdIGoP8AJUFpFborS1Rq/nv0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zytor.com; spf=pass smtp.mailfrom=zytor.com; dkim=pass (2048-bit key) header.d=zytor.com header.i=@zytor.com header.b=QvijAM6O; arc=none smtp.client-ip=198.137.202.136 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zytor.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=zytor.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=zytor.com header.i=@zytor.com header.b="QvijAM6O" Received: from [IPV6:2601:646:8002:4640:7285:c2ff:fefb:fd4] ([IPv6:2601:646:8002:4640:7285:c2ff:fefb:fd4]) (authenticated bits=0) by mail.zytor.com (8.17.2/8.17.1) with ESMTPSA id 43HExUMB4064032 (version=TLSv1.3 cipher=TLS_AES_128_GCM_SHA256 bits=128 verify=NO); Wed, 17 Apr 2024 07:59:30 -0700 DKIM-Filter: OpenDKIM Filter v2.11.0 mail.zytor.com 43HExUMB4064032 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zytor.com; s=2024041401; t=1713365971; bh=VSJmFsr3EBbN1183B4LxpqSIbjOkuFTUUCyoRaRiqHQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=QvijAM6OkDa1B0fd+ovvzDYTcAl/YS0j9Ff9btBrPwoVn3+WSbXqS/PxFEPYQFif7 aeLNzePK6ihThdXJgiUGry6PNUqx7oTBRp3auf/sG05wAq6BZs7up8bILrmXNBMfen hk7F4Jk4duyFZDvTS36yQJL97ELhSIk/gJtkoDzn23Q4drJdcRiTMJzs0kxd2/1Ohm ye7cPqm0Ulc0B/EYpBY9RNcJ8V2jhqgi4dR4SwSP1gp23+eghIh7LgVlrUlpf3y0lP gbCeZsDwF/i+TIJMC93Qilq2pLHcBnIkAmp7cZ6dApNOY4pLvJUf3Aq7foLeA6iGxl 8fQRJ3oyDf0zA== Message-ID: <5dd74948-a951-4a7a-84ac-7e75cc26bb46@zytor.com> Date: Wed, 17 Apr 2024 07:59:25 -0700 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 1/1] x86/fred: Fix INT80 emulation for FRED Content-Language: en-US To: Borislav Petkov , "Xin Li (Intel)" Cc: linux-kernel@vger.kernel.org, luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com, x86@kernel.org References: <20240417063001.3773507-1-xin@zytor.com> <20240417093813.GBZh-YhSQCXgy3OxFI@fat_crate.local> From: "H. Peter Anvin" In-Reply-To: <20240417093813.GBZh-YhSQCXgy3OxFI@fat_crate.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 4/17/24 02:38, Borislav Petkov wrote: > On Tue, Apr 16, 2024 at 11:30:01PM -0700, Xin Li (Intel) wrote: >> 3) The FRED kernel entry handler does *NOT* dispatch INT instructions, >> which is of event type EVENT_TYPE_SWINT, so compared with >> do_int80_emulation(), there is no need to do any user mode check. > > What does that mean? > > An event handler doesn't dispatch INT insns? > > /me is confused. FRED has separate entry flows depending on if the event came from user space or kernel space: asm_fred_entrypoint_user -> fred_entry_from_user asm_fred_entrypoint_kernel -> fred_entry_from_kernel fred_entry_from_kernel does not invoke fred_intx() if the event type is EVENT_TYPE_SWINT, instead it falls through to fred_bad_type(). Perhaps fred_intx() should be renamed fred_intx_user() for additional clarity. (It might also we worth noting in that function that the reason int $0x03 and int $0x04 are dispatched as INT3 and INTO is to be fully user space compatible with legacy IDT, which behaves similarly.) Thus, the int $0x80 code is simply not reachable from kernel space; if kernel code were to invoke int $0x80 or any other INT instruction it will error out before getting to this code. >> +#ifdef CONFIG_X86_FRED >> +/* >> + * A FRED-specific INT80 handler fred_int80_emulation() is required: >> + * >> + * 1) As INT instructions and hardware interrupts are separate event >> + * types, FRED does not preclude the use of vector 0x80 for external >> + * interrupts. As a result the FRED setup code does *NOT* reserve >> + * vector 0x80 and calling int80_is_external() is not merely >> + * suboptimal but actively incorrect: it could cause a system call >> + * to be incorrectly ignored. >> + * >> + * 2) fred_int80_emulation(), only called for handling vector 0x80 of >> + * event type EVENT_TYPE_SWINT, will NEVER be called to handle any >> + * external interrupt (event type EVENT_TYPE_EXTINT). >> + * >> + * 3) The FRED kernel entry handler does *NOT* dispatch INT instructions, >> + * which is of event type EVENT_TYPE_SWINT, so compared with >> + * do_int80_emulation(), there is no need to do any user mode check. >> + * >> + * 4) int80_emulation() does a CLEAR_BRANCH_HISTORY, which is likely >> + * overkill for new x86 CPU implementations that support FRED. >> + * >> + * 5) int $0x80 is the FAST path for 32-bit system calls under FRED. >> + * >> + * A dedicated FRED INT80 handler duplicates quite a bit of the code in >> + * do_int80_emulation(), but it avoids sprinkling more tests and seems >> + * more readable. Just remember that we can always unify common stuff >> + * later if it turns out that it won't diverge anymore, i.e., after the >> + * FRED code settles. >> + */ > > And this is talking about duplication above and that text is duplicated > from the commit message. :) > > I'll zap it when applying. > I suggested putting it into a comment for future reference. Obviously no need to duplicate it in the commit message :) -hpa