PR/60426 CVS commit: src

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

The following reply was made to PR kern/60426; it has been noted by GNATS.

From: "Taylor R Campbell" <riastradh%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/60426 CVS commit: src
Date: Fri, 10 Jul 2026 15:11:26 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Fri Jul 10 15:11:25 UTC 2026
 
 Modified Files:
 	src/sys/arch/amd64/amd64: machdep.c netbsd32_machdep.c
 	src/sys/arch/amd64/include: mcontext.h
 	src/sys/arch/i386/i386: machdep.c
 	src/sys/arch/i386/include: mcontext.h
 	src/sys/arch/x86/include: cpu_extended_state.h fpu.h specialreg.h
 	src/sys/arch/x86/x86: fpu.c identcpu.c
 	src/tests/kernel: t_signal_and_fpu.c
 
 Log Message:
 x86: Save and restore all supported extended CPU state on signals.
 
 While here, disable Intel AMX, whose state size (>>8 KiB) exceeds
 MINSIGSTKSZ (8 KiB), until we are ready to safely update the ABI for
 sigaltstack(2).  This isn't a regression: we've never had a release
 with Intel AMX support.
 
 Previously, on signal delivery, we would only save and restore at
 most what FXSAVE does, which is the x87 and SSE registers that always
 exist on amd64.
 
 To save and restore the upper halves of the YMM or ZMM registers
 (AVX/AVX2/AVX512), or the AVX512 registers ZMM16..ZMM31, or the
 enormous AMX state, we need to do more.  And we need to do that even if
 the signal handler doesn't use AVX instructions, because an SSE
 instruction modifying xmmN (e.g., in a vectorized memcpy) will, as a
 side effect, zero the high half of ymmN.
 
 Fortunately, the x86 architecture has an extensible mechanism for
 saving and restoring extended CPU state, the XSAVE/XRSTOR instruction
 family, so we don't need to update this code for every extension --
 just handling the generic XSAVE/XRSTOR mechanism will work for a
 while.
 
 When delivering a signal to a thread that has actually made use of
 these extended registers, we:
 
 1. allocate a separate space on the user's stack for an XSAVE area of
    whatever size is necessary (as long as it fits in the sigaltstack,
    if one was provided), and
 
 2. store a pointer to and the length of the separate XSAVE area in an
    architecturally unused section of the FXSAVE area of the
    mcontext_t, with the _UC_XSAVE bit set.
 
 This way existing applications that have SA_SIGINFO signal handlers
 and expect to find an FXSAVE area in the mcontext_t with x87 and SSE
 register content will continue to work, but if the thread has used
 any state beyond that like AVX registers requiring XSAVE/XRSTOR, on
 return from signal it can restore everything.
 
 Unfortunately, the _size_ that XSAVE needs is variable and might be
 expanded by architectural extensions, such as Intel AMX.  So the
 minimum stack size to store it all might exceed MINSIGSTKSZ.  The
 current MINSIGSTKSZ is plenty for AVX/AVX2/AVX-512.  But before
 raising MINSIGSTKSZ to support larger XSAVE sizes by adding them to
 XCR0_FPU, we need to make sure that older binaries built with the old
 MINSIGSTKSZ continue to work -- of course, they won't be able to use
 (e.g.) Intel AMX state.
 
 To make sure we don't break the ABI accidentally, I've added some
 __CTASSERTs relating XSAVE_MAX_BYTES, the maximum supported XSAVE
 area size, and MINSIGSTKSZ, and a panic at boot if the CPU advertises
 an XSAVE size larger than XSAVE_MAX_BYTES, which would mean either:
 
 (a) the CPU's advertised size for various architecturally defined
     extended CPU state does not match the documentation, or
 
 (b) we have inadvertently bitten off more extended state CPU state
     than we can chew in XCR0_FPU and we have to adjust the ABI and
     add compatibility for userland programs.
 
 Fortunately for getcontext(2), all of the registers in question are
 caller-saves, so it doesn't need to store any extra state.
 
 PR kern/60426: Signal handler corrupts AVX (YMM) registers
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.379 -r1.380 src/sys/arch/amd64/amd64/machdep.c
 cvs rdiff -u -r1.142 -r1.143 src/sys/arch/amd64/amd64/netbsd32_machdep.c
 cvs rdiff -u -r1.24 -r1.25 src/sys/arch/amd64/include/mcontext.h
 cvs rdiff -u -r1.851 -r1.852 src/sys/arch/i386/i386/machdep.c
 cvs rdiff -u -r1.19 -r1.20 src/sys/arch/i386/include/mcontext.h
 cvs rdiff -u -r1.19 -r1.20 src/sys/arch/x86/include/cpu_extended_state.h
 cvs rdiff -u -r1.23 -r1.24 src/sys/arch/x86/include/fpu.h
 cvs rdiff -u -r1.221 -r1.222 src/sys/arch/x86/include/specialreg.h
 cvs rdiff -u -r1.94 -r1.95 src/sys/arch/x86/x86/fpu.c
 cvs rdiff -u -r1.139 -r1.140 src/sys/arch/x86/x86/identcpu.c
 cvs rdiff -u -r1.1 -r1.2 src/tests/kernel/t_signal_and_fpu.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.