PR/57661 CVS commit: src/sys/arch/x86/x86

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

The following reply was made to PR port-amd64/57661; it has been noted by GNATS.

From: "Taylor R Campbell" <riastradh%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/57661 CVS commit: src/sys/arch/x86/x86
Date: Thu, 24 Apr 2025 01:51:21 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Thu Apr 24 01:51:21 UTC 2025
 
 Modified Files:
 	src/sys/arch/x86/x86: identcpu.c
 
 Log Message:
 x86: Reserve space for only the extended CPU state we will use.
 
 CPUID[EAX=0x0d, ECX=0].ECX, i.e., the value of descs[2] after
 x86_cpuid2(0x0d, 0, descs), gives the size in bytes of the extended CPU
 state for all features supported by the hardware in CPUID[EAX=0x0d,
 ECX=0].EAX which can be enabled in XCR0.  However, on i386, it is
 senseless to leave TILECFG and TILEDATA enabled, because they are state
 for Intel AMX instructions which work only in 64-bit mode.
 
 So, instead of querying the hardware's supported features and maximum
 _supported_ extended CPU state size:
 
 1. Query the hardware's supported features.
 2. Enable only those supported by software as well (XCR0_FPU).
 3. Query the hardware's maximum _enabled_ extended CPU state size.
 
 We will also disable TILECFG and TILEDATA on amd64 for now too
 because:
 
 (a) This is not a regression, at least for TILEDATA (and I'm not sure
     any machines ship with TILECFG but not TILEDATA), because the
     size overflowed the PCB page and therefore never worked on amd64
     (PR port-amd64/57661: Crash when booting on Xeon Silver 4416+ in
     KVM/Qemu).
 
 (b) We need a little extra work to properly support reading and
     writing a process's TILECFG and TILEDATA in ptrace(2), and that
     work hasn't been done yet.
 
 While here, write out x86_cpuid2(0x0d, <ecx>, ...) explicitly, rather
 than x86_cpuid(0x0d, ...), to make it clear that ECX must be set --
 otherwise we may get garbage.  (It is, perhaps, an accident that
 x86_cpuid(<eax>, ...) always sets ECX=0, but other CPUID access
 paths, like gcc's <cpuid.h> __cpuid(<eax>, ...), do not, so let's
 make it clear for the reader.)
 
 XXX When we enable TILECFG and TILEDATA in amd64, we should arrange
 to disable them in compat32 processes -- no sense in allocating extra
 space for state they can't use anyway, since the Intel AMX
 instructions work only in 64-bit mode.  However, selectively
 disabling them in some contexts might require hardware support for
 XFD, Extended Feature Disable, which is another kettle of fish to
 deal with.
 
 PR port-amd64/57661: Crash when booting on Xeon Silver 4416+ in
 KVM/Qemu
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.134 -r1.135 src/sys/arch/x86/x86/identcpu.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.