Re: [PATCH] x86/TSC: Use RDTSCP

From: Guenter Roeck
Date: Fri Nov 23 2018 - 15:03:14 EST

Hi,

On Mon, Nov 19, 2018 at 07:45:56PM +0100, Borislav Petkov wrote:
> From: Borislav Petkov <bp@xxxxxxx>
>
> Currently, the kernel uses
>
> [LM]FENCE; RDTSC
>
> in the timekeeping code, to guarantee monotonicity of time where the
> *FENCE is selected based on vendor.
>
> Replace that sequence with RDTSCP which is faster or on-par and gives
> the same guarantees.
>
> A microbenchmark on Intel shows that the change is on-par.
>
> On AMD, the change is either on-par with the current LFENCE-prefixed
> RDTSC and some are slightly better with RDTSCP.
>
> The comparison is done with the LFENCE-prefixed RDTSC (and not with the
> MFENCE-prefixed one, as one would normally expect) because all modern
> AMD families make LFENCE serializing and thus avoid the heavy MFENCE by
> effectively enabling X86_FEATURE_LFENCE_RDTSC.
>
> Co-developed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: John Stultz <john.stultz@xxxxxxxxxx>
> Cc: Thomas Lendacky <Thomas.Lendacky@xxxxxxx>
> Cc: x86@xxxxxxxxxx
> ---
> arch/x86/include/asm/msr.h | 15 +++++++++++++--
> 1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 91e4cf189914..f00f2b61d326 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -217,6 +217,8 @@ static __always_inline unsigned long long rdtsc(void)
> */
> static __always_inline unsigned long long rdtsc_ordered(void)
> {
> + DECLARE_ARGS(val, low, high);
> +
> /*
> * The RDTSC instruction is not ordered relative to memory
> * access. The Intel SDM and the AMD APM are both vague on this
> @@ -227,9 +229,18 @@ static __always_inline unsigned long long rdtsc_ordered(void)
> * ordering guarantees as reading from a global memory location
> * that some other imaginary CPU is updating continuously with a
> * time stamp.
> + *
> + * Thus, use the preferred barrier on the respective CPU, aiming for
> + * RDTSCP as the default.
> */
> - barrier_nospec();
> - return rdtsc();
> + asm volatile(ALTERNATIVE_2("mfence; rdtsc",
> + "lfence; rdtsc", X86_FEATURE_LFENCE_RDTSC,
> + "rdtscp", X86_FEATURE_RDTSCP)
> + : EAX_EDX_RET(val, low, high)
> + /* RDTSCP clobbers ECX with MSR_TSC_AUX. */
> + :: "ecx");
> +
> + return EAX_EDX_VAL(val, low, high);
> }
>

This patch results in a crash with certain qemu emulations.

[ 0.756869] hpet0: 3 comparators, 64-bit 100.000000 MHz counter
[ 0.762233] invalid opcode: 0000 [#1] PTI
[ 0.762435] CPU: 0 PID: 1 Comm: swapper Not tainted 4.20.0-rc3-next-20181123 #2
[ 0.762644] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[ 0.762832] EIP: read_tsc+0x4/0x10

To reproduce:

make ARCH=i386 defconfig
echo "CONFIG_HIGHMEM64G=y" >> .config
make ARCH=i386 olddefconfig
make -j30 ARCH=i386

qemu-system-i386 \
-kernel arch/x86/boot/bzImage \
-M q35 -cpu pentium3 -no-reboot -m 256 \
-initrd rootfs.cpio \
--append 'earlycon=uart8250,io,0x3f8,9600n8 rdinit=/sbin/init panic=-1 console=ttyS0 console=tty' \
-nographic -monitor none

initrd or root file system doesn't really matter since the code never
gets there, but it is available from here:
https://github.com/groeck/linux-build-test/blob/master/rootfs/x86/rootfs.cpio.gz
The qemu version does not matter either (I tried with 2.5 and 3.0).

Reverting the patch fixes the problem.

Guenter

---
crash log:

...
[ 0.756366] HPET: 3 timers in total, 0 timers will be used for per-cpu timer
[ 0.756718] hpet0: at MMIO 0xfed00000, IRQs 2, 8, 0
[ 0.756869] hpet0: 3 comparators, 64-bit 100.000000 MHz counter
[ 0.762233] invalid opcode: 0000 [#1] PTI
[ 0.762435] CPU: 0 PID: 1 Comm: swapper Not tainted 4.20.0-rc3-next-20181123
#2
[ 0.762644] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
Ubuntu-1.8.2-1ubuntu1 04/01/2014
[ 0.762832] EIP: read_tsc+0x4/0x10
[ 0.762832] Code: 00 01 00 eb 89 90 55 89 e5 5d c3 90 90 90 90 90 90 90 90 90 90 90 55 a1 44 5a 8b c5 89 e5 5d c3 8d b6 00 00 00 00 55 89 e5 57 <0f> ae f0b
[ 0.762832] EAX: c58062c0 EBX: c58062c0 ECX: 00000008 EDX: c4c1f0a0
[ 0.762832] ESI: c58168c0 EDI: 00200086 EBP: cb495ed4 ESP: cb495ed0
[ 0.762832] DS: 007b ES: 007b FS: 0000 GS: 00e0 SS: 0068 EFLAGS: 00200002
[ 0.762832] CR0: 80050033 CR2: 00000000 CR3: 0597a000 CR4: 000006f0
[ 0.762832] Call Trace:
[ 0.762832] tk_setup_internals.constprop.10+0x3d/0x260
[ 0.762832] timekeeping_notify+0x56/0xc0
[ 0.762832] __clocksource_select+0xf3/0x150
[ 0.762832] ? boot_override_clock+0x42/0x42
[ 0.762832] clocksource_done_booting+0x2d/0x3b
[ 0.762832] do_one_initcall+0x68/0x15e
[ 0.762832] ? set_debug_rodata+0xf/0xf
[ 0.762832] kernel_init_freeable+0xec/0x18b
[ 0.762832] ? rest_init+0x80/0x80
[ 0.762832] kernel_init+0x8/0xf0
[ 0.762832] ret_from_fork+0x2e/0x38
[ 0.762832] Modules linked in:
[ 0.762832] ---[ end trace ba4ba4587aec46c9 ]---
[ 0.762832] EIP: read_tsc+0x4/0x10
[ 0.762832] Code: 00 01 00 eb 89 90 55 89 e5 5d c3 90 90 90 90 90 90 90 90 90 90 90 55 a1 44 5a 8b c5 89 e5 5d c3 8d b6 00 00 00 00 55 89 e5 57 <0f> ae f0b
[ 0.762832] EAX: c58062c0 EBX: c58062c0 ECX: 00000008 EDX: c4c1f0a0
[ 0.762832] ESI: c58168c0 EDI: 00200086 EBP: cb495ed4 ESP: c597f47c
[ 0.762832] DS: 007b ES: 007b FS: 0000 GS: 00e0 SS: 0068 EFLAGS: 00200002
[ 0.762832] CR0: 80050033 CR2: 00000000 CR3: 0597a000 CR4: 000006f0

---
bisect log:

# bad: [8c9733fd9806c71e7f2313a280f98cb3051f93df] Add linux-next specific files for 20181123
# good: [9ff01193a20d391e8dbce4403dd5ef87c7eaaca6] Linux 4.20-rc3
git bisect start 'HEAD' 'v4.20-rc3'
# good: [34c2101b4f765edf1b91c2837da9c60fbf9f6912] Merge remote-tracking branch 'spi-nor/spi-nor/next'
git bisect good 34c2101b4f765edf1b91c2837da9c60fbf9f6912
# good: [15367a0657fc8027ff3466bf0202bde9f270259b] Merge remote-tracking branch 'kgdb/kgdb-next'
git bisect good 15367a0657fc8027ff3466bf0202bde9f270259b
# bad: [d29686ab179c34c5dbaac067a9effbeeb6a8073e] Merge remote-tracking branch 'soundwire/next'
git bisect bad d29686ab179c34c5dbaac067a9effbeeb6a8073e
# bad: [7cd63670817c236ccaf21ffe9d7b4921afeab130] Merge remote-tracking branch 'tip/auto-latest'
git bisect bad 7cd63670817c236ccaf21ffe9d7b4921afeab130
# good: [f9c8ebdcb052214954136426990e13a88e45e906] Merge remote-tracking branch 'audit/next'
git bisect good f9c8ebdcb052214954136426990e13a88e45e906
# good: [4142a8a10e7673fa71ff4cb5b4693c4dda299f7c] Merge remote-tracking branch 'spi/for-next'
git bisect good 4142a8a10e7673fa71ff4cb5b4693c4dda299f7c
# good: [fc922a4c7c72131f6c65e4af4e36ff25f3af1485] Merge branch 'x86/cpu'
git bisect good fc922a4c7c72131f6c65e4af4e36ff25f3af1485
# good: [906d11c1ff007ad18e2f3477cca04c437ce19c64] Merge branch 'x86/microcode'
git bisect good 906d11c1ff007ad18e2f3477cca04c437ce19c64
# good: [d836e1d42739a86738dd92a1b7ab6729b2485cba] Merge branch 'x86/platform'
git bisect good d836e1d42739a86738dd92a1b7ab6729b2485cba
# bad: [191bba684b7459842e72b52718ec61f8bdbd8499] Merge branch 'x86/timers'
git bisect bad 191bba684b7459842e72b52718ec61f8bdbd8499
# good: [f9cc0921aa4edd9bd4dd6943d5e38f69a301e8ee] Merge branch 'x86/pti'
git bisect good f9cc0921aa4edd9bd4dd6943d5e38f69a301e8ee
# bad: [2e94061096c5c3aa6c3fe3ec2bec176c1f9c1b07] x86/TSC: Use RDTSCP
git bisect bad 2e94061096c5c3aa6c3fe3ec2bec176c1f9c1b07
# good: [a786ef152cdcfebc923a67f63c7815806eefcf81] x86/tsc: Make calibration refinement more robust
git bisect good a786ef152cdcfebc923a67f63c7815806eefcf81
# first bad commit: [2e94061096c5c3aa6c3fe3ec2bec176c1f9c1b07] x86/TSC: Use RDTSCP