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Linux 5.0 compat: Disable vector instructions on 5.0+ kernels
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The 5.0 kernel no longer exports the functions we need to do vector
(SSE/SSE2/SSE3/AVX...) instructions.  Disable vector-based checksum
algorithms when building against those kernels.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #8259
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@tonyhutter @behlendorf
tonyhutter authored and behlendorf committed Jan 28, 2019
1 parent ed158b1 commit 0c59329
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Showing 2 changed files with 144 additions and 39 deletions.
41 changes: 32 additions & 9 deletions config/kernel-fpu.m4
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Original file line number Diff line number Diff line change
@@ -1,18 +1,41 @@
dnl #
dnl # Handle differences in kernel FPU code.
dnl #
dnl # 4.2 API change
dnl # asm/i387.h is replaced by asm/fpu/api.h
dnl # Kernel
dnl # 5.0: All kernel fpu functions are GPL only, so we can't use them.
dnl # (nothing defined)
dnl #
dnl # 4.2: Use __kernel_fpu_{begin,end}()
dnl # HAVE_UNDERSCORE_KERNEL_FPU & KERNEL_EXPORTS_X86_FPU
dnl #
dnl # Pre-4.2: Use kernel_fpu_{begin,end}()
dnl # HAVE_KERNEL_FPU & KERNEL_EXPORTS_X86_FPU
dnl #
AC_DEFUN([ZFS_AC_KERNEL_FPU], [
AC_MSG_CHECKING([whether asm/fpu/api.h exists])
AC_MSG_CHECKING([which kernel_fpu function to use])
ZFS_LINUX_TRY_COMPILE([
#include <linux/kernel.h>
#include <asm/fpu/api.h>
#include <asm/i387.h>
#include <asm/xcr.h>
],[
__kernel_fpu_begin();
kernel_fpu_begin();
kernel_fpu_end();
],[
AC_MSG_RESULT(yes)
AC_DEFINE(HAVE_FPU_API_H, 1, [kernel has <asm/fpu/api.h> interface])
AC_MSG_RESULT(kernel_fpu_*)
AC_DEFINE(HAVE_KERNEL_FPU, 1, [kernel has kernel_fpu_* functions])
AC_DEFINE(KERNEL_EXPORTS_X86_FPU, 1, [kernel exports FPU functions])
],[
AC_MSG_RESULT(no)
ZFS_LINUX_TRY_COMPILE([
#include <linux/kernel.h>
#include <asm/fpu/api.h>
],[
__kernel_fpu_begin();
__kernel_fpu_end();
],[
AC_MSG_RESULT(__kernel_fpu_*)
AC_DEFINE(HAVE_UNDERSCORE_KERNEL_FPU, 1, [kernel has __kernel_fpu_* functions])
AC_DEFINE(KERNEL_EXPORTS_X86_FPU, 1, [kernel exports FPU functions])
],[
AC_MSG_RESULT(not exported)
])
])
])
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4 comments on commit 0c59329

@prometheanfire
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Has there been any performance evaluation for this change (just curious as to what to expect). I also wonder if compiling built-in will still have access.

@behlendorf
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Based on previous testing we've done if your system isn't cpu-bound you probably won't see much of a performance impact, what you will see is increased cpu-usage. I'd expect low end systems to be the most impacted.

Now that all the build issues are resolved, we can focus on re-enabling this functionality. As mentioned previously we are exploring several options. As the code currently exists, you'd probably just need to tweak the configure check it a bit to enable this when compiling built in.

@prometheanfire
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ok, that's good at least, the patches relating to adding vector instructions seemed to show much better scrub speeds with them.

@RJVB
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@RJVB RJVB commented on 0c59329 Apr 13, 2019

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Out of equal curiosity: suppose you figure out how to built (locally!) a series 5 kernel so that it exports these symbols again - it looks like you won't have to do anything extra to re-enable the use of vector instructions in ZFS?

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