[llvm-dev] Status of Intel JCC Mitigations and Next Steps

Tue Mar 24 20:20:46 PDT 2020

Changing the length of a sequence of assembly instructions will break someone’s code at some point.  The length of a sequence of instructions is known, in general, and people will write code to take advantage of that. For example, I’ve seen assembly code using something like Duff’s device, except that instead of using a jump table, it just computed the destination as “base+n*caselength”.  Given that, I don’t think it’s reasonable to hide this mechanism from user control.

We definitely should not have any undocumented or unpredictable behavior in the assembler.  The actual instruction bytes matter.  That said, I’m not sure there’s a strong line between “automagic” and “explicit”, as long as the rules are documented.

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Philip Reames via llvm-dev
Sent: Tuesday, March 24, 2020 3:55 PM
To: llvm-dev <llvm-dev at lists.llvm.org>
Cc: Luo, Yuanke <yuanke.luo at intel.com>; Zhang, Annita <annita.zhang at intel.com>; Craig Topper <craig.topper at intel.com>
Subject: [EXT] [llvm-dev] Status of Intel JCC Mitigations and Next Steps

TLDR - We have a choice to make about assembler support, and a disagreement about how to move forward.  Community input needed.

Background

Intel has a hardware bug in Skylake and later whose mitigation requires padding of branches to avoid performance degradation.  Background here: https://www.intel.com/content/dam/support/us/en/documents/processors/mitigations-jump-conditional-code-erratum.pdf

We now have in tree support for alignment of such branches via nop padding, and limited support for padding existing instructions with either prefixes or larger immediate values.  This has survived several days of dedicated testing and appears to be reasonably robust.  The padding support applies both to branch alignment for the mitigation, but also normal align directives.

The original patches proposed a somewhat different approach than we've ended up taking - primarily because of memory overhead concerns.  However, there was also a deeper disagreement on the original review threads (D70157 and others) which was never settled, and we seem to be at a point where this needs attention.  In short, the question is how assembler support should be handled.

The Choice

The problematic use case comes when assembling user provided .s files.  (Instead of the more restricted output of the compiler.)  Our basic choice is do we want to force a new directive syntax (and thus a source code change to use the new feature), or attempt to automatically infer where it's safe to insert padding?

The options as I see them:

  *   Assembler directives w/explicit opt in - In this model, assembler input is assumed to only enable padding in regions where it is safe to do so.
  *   Automagic assembler - In this model, the assembler is responsible for inferring where it is legal to pad without breaking user expectations.

(I'll stop and disclaim that I'm strongly in favor of the former.  I've tried to describe the pros/cons of each, but my perspective is definitely biased.)

The difference between the two is a huge amount of complexity, and a very fundamental correctness risk.  The basic problem is that assemblers have to handle unconstrained inputs, and IMO, the semantics of assembler as used in practice is so under specified that it's really hard to infer semantics in any useful way.  As a couple of examples, is the fault behavior of an instruction well defined?  Is the label near an instruction used by the signal handler?  Is that data byte just before an instruction actually decoded as part of the instruction?

The benefit of the later option is that existing assembly files can be used without modification.  This is a huge advantage in terms of ease of mitigation for existing code bases.  It's also the approach the original patch sets for GCC took.

In the original review thread(s), I had taken the position that we should reject the automagic assembler based on the correctness concerns mentioned.  I had thought the consensus in the review was clearly in that direction as well, but this has recently come up again.  Given that, I wanted to open it to a wider audience.

Why am I pushing for a decision now?

There are two major reasons.  First, there have recently been a couple of patches posted and landed (D76176, and D76052) building towards the automagic assembler variant.  And second, I've started getting review comments (https://reviews.llvm.org/D76398#1930383) which block forward progress on generalized padding support assuming the automagic interpretation.  Implementing the automatic assembler variant for prefix and immediate padding adds substantial complexity and I would very much like not to bother with if I don't have to.

Current implementation details

We have support in the integrated assembler only for autopadding suppression.  This allows a LLVM based compiler to effectively apply padding selectively.  In particular, we've instrumented lowering from MI to MC (X86MCInstLowering.cpp) to selectively disable padding around constructs which are thought to be problematic.  We do not have an agreed upon syntax for this in assembler; the code that got checked in is modeled closely around the last seriously discussed variant (see below).  This support is able to use all of the padding variants: nop, prefix, and immediate.

We also have limited support in the assembler for not inserting nops between fragments where doing so would break known idioms.  The list of such idioms is, IMO, ad hoc.  This assembler support does not include prefix or immediate padding.

Philip

p.s. For those interested, here's roughly what the last round of assembler syntax I remember being discussed looked like.

.autopadding
.noautopadding

These two directives would respectively enable and disable automatic padding of instructions within the region defined.  It's presumed to be legal to insert nops between instructions, modify encodings, or otherwise adjust offsets of instruction boundaries within the region to achieve target specific desired alignments.  Similarly, it's presumed not to be legal to change relative offsets outside an explicitly enabled region.  (Except for existing cases - e.g. relaxation of branches, etc...)

The assembler would provide a command line flag which conceptually wrapped the whole file in a pair of enable/disable directives.

We'd previously discussed a variant with push/pop semantics and more fine grained control over alignment requests, but I believe we decided that was overkill in the end.  (I walked away with that impression based on the integrated assembler work at least.)

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