<div dir="ltr">I guess I don't see how the two proposals are that different. It's clear that we need an assembler mode that adds nops or prefixes to align jump instructions to ensure that they do not cross cache boundaries. And it seems reasonable to expose that feature as both a command line flag so that existing assembly files can be re-assembled with the mitigation, and as a directive so that it can be explicitly enabled or disabled.<div><br></div><div>I think it would even be reasonable to enable the mitigation by default, and have a flag to disable it for code that cares about jump instruction length, as long as the behavior is all documented, as Eli seems to be saying.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Mar 24, 2020 at 3:55 PM Philip Reames via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p>TLDR - We have a choice to make about assembler support, and a
disagreement about how to move forward. Community input needed.</p>
<p><br>
</p>
<p>Background</p>
<p>Intel has a hardware bug in Skylake and later whose mitigation
requires padding of branches to avoid performance degradation.
Background here:
<a href="https://www.intel.com/content/dam/support/us/en/documents/processors/mitigations-jump-conditional-code-erratum.pdf" target="_blank">https://www.intel.com/content/dam/support/us/en/documents/processors/mitigations-jump-conditional-code-erratum.pdf</a></p>
<p>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. <br>
</p>
<p>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.</p>
<p><br>
</p>
<p>The Choice</p>
<p>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?<br>
</p>
<p>The options as I see them:</p>
<ul>
<li>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.<br>
</li>
<li>Automagic assembler - In this model, the assembler is
responsible for inferring where it is legal to pad without
breaking user expectations. <br>
</li>
</ul>
<p>(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.)<br>
</p>
<p>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?</p>
<p>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. <br>
</p>
<p>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.</p>
<p><br>
</p>
<p>Why am I pushing for a decision now?</p>
<p>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 (<a href="https://reviews.llvm.org/D76398#1930383" target="_blank">https://reviews.llvm.org/D76398#1930383</a>)
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.<br>
</p>
<p><br>
</p>
<p> </p>
<p>Current implementation details</p>
<p>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.<br>
</p>
<p>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. <br>
</p>
<p><br>
</p>
<p>Philip</p>
<p>p.s. For those interested, here's roughly what the last round of
assembler syntax I remember being discussed looked like.</p>
<p>.autopadding<br>
.noautopadding</p>
<p>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...)<br>
</p>
<p>The assembler would provide a command line flag which
conceptually wrapped the whole file in a pair of enable/disable
directives. <br>
</p>
<p>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.)<br>
</p>
<p><br>
</p>
</div>
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