Messages in this thread | | | Date | Wed, 7 Feb 2018 19:49:33 +0100 | From | Peter Zijlstra <> | Subject | Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f |
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On Wed, Feb 07, 2018 at 07:35:43PM +0100, Borislav Petkov wrote: > On Wed, Feb 07, 2018 at 10:13:35AM -0800, Linus Torvalds wrote: > > Adding more people for this funky warning from the kbuild robot. > > > > Something is confused. UD0 is 0f ff, the bytes after that shouldn't > > matter. But I guess they can be interpreted as modrm bytes, and > > somebody started doing that. > > > > That said, intel only _documents_ UD2 (0f 0b). > > They documented UD0 and UD1 a year ago or so: > > 0F FF /r UD0¹ r32, r/m32 RM Valid Valid Raise invalid opcode exception > 0F B9 /r UD1 r32, r/m32 RM Valid Valid Raise invalid opcode exception. > > and the footnote says > > "1. Some older processors decode the UD0 instruction without a ModR/M > byte. As a result, those processors would deliver an invalid- opcode > exception instead of a fault on instruction fetch when the instruction > with a ModR/M byte (and any implied bytes) would cross a page or segment > boundary." > > So those two take a ModRM byte.
Argh. So the SDM I'm looking at (March 2017) doesn't list UD0 as having a ModR/M byte, it doesn't have that footnote.
> And we chose UD0 for WARN, see arch/x86/include/asm/bug.h for the > reasoning.
Right, we picked UD0 because we _thought_ everybody agreed it being 2 bytes, just like UD2. This is now not true anymore?
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