Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

From: Stas Sergeev <stsp@list.ru>
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Brian Gerst <brgerst@gmail.com>,
	Chris Metcalf <cmetcalf@mellanox.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Liang Z Li <liang.z.li@intel.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Huang Rui <ray.huang@amd.com>, Jiri Slaby <jslaby@suse.cz>,
	Jonathan Corbet <corbet@lwn.net>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Paul Gortmaker <paul.gortmaker@windriver.com>,
	Vlastimil Babka <vbabka@suse.cz>, Chen Yucong <slaoub@gmail.com>,
	Alexandre Julliard <julliard@winehq.org>,
	Fenghua Yu <fenghua.yu@intel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Shuah Khan <shuah@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	X86 ML <x86@kernel.org>,
	linux-msdos@vger.kernel.org, wine-devel@winehq.org
Subject: Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention
Date: Tue, 14 Mar 2017 00:25:38 +0300	[thread overview]
Message-ID: <6331deea-e9b0-fcfe-b75d-8100f37a615a@list.ru> (raw)
In-Reply-To: <1489190396.131264.47.camel@ranerica-desktop>

11.03.2017 02:59, Ricardo Neri пишет:
> On Fri, 2017-03-10 at 14:33 +0300, Stas Sergeev wrote:
>
>> Why would you need one?
>> Or do you really want to allow these instructions
>> in v86 by the means of emulation? If so - this wasn't
>> clearly stated in the patch description, neither it was
>> properly discussed, it seems.
> It str and sldt can be emulated in vm86 but as Andy mention, the
> behavior sould be the same with and without emulation.
Why would you do that?
I looked up the dosemu2 CPU simulator code that
is used under x86-64. It says this:
---
                                     CODE_FLUSH();
                                     if (REALMODE()) goto illegal_op;
                                     PC += ModRMSim(PC+1, mode) + 1;
                                     error("SLDT not implemented\n");
                                     break;
                                 case 1: /* STR */
                                     /* Store Task Register */
                                     CODE_FLUSH();
                                     if (REALMODE()) goto illegal_op;
                                     PC += ModRMSim(PC+1, mode) + 1;
                                     error("STR not implemented\n");
                                     break;
...
                                 case 0: /* SGDT */
                                     /* Store Global Descriptor Table 
Register */
                                     PC++; PC += ModRM(opc, PC, 
mode|DATA16|MSTORE);
                                     error("SGDT not implemented\n");
                                     break;
                                 case 1: /* SIDT */
                                     /* Store Interrupt Descriptor Table 
Register */
                                     PC++; PC += ModRM(opc, PC, 
mode|DATA16|MSTORE);
                                     error("SIDT not implemented\n");
                                     break;
---

It only implements smsw.
So maybe you can make your code much
simpler and remove the unneeded emulation?
Same is for prot mode. You know the wine's
requirements now - they are very small. And
dosemu doesn't need anything at all but smsw.
And even smsw is very rare.