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SubjectRe: [RFC PATCH] x86: Remove X86_FEATURE_MFENCE_RDTSC
Date
On 7/4/19 10:46 AM, Josh Poimboeuf wrote:
> AMD and Intel both have serializing lfence (X86_FEATURE_LFENCE_RDTSC).
> They've both had it for a long time, and AMD has had it enabled in Linux
> since Spectre v1 was announced.
>
> Back then, there was a proposal to remove the serializing mfence feature
> bit (X86_FEATURE_MFENCE_RDTSC), since both AMD and Intel have
> serializing lfence. At the time, it was (ahem) speculated that some
> hypervisors might not yet support its removal, so it remained for the
> time being.
>
> Now a year-and-a-half later, it should be safe to remove.

I vaguely remember a concern from a migration point of view, maybe? Adding
Paolo to see if he has any concerns.

Thanks,
Tom

>
> I asked Andrew Cooper about whether it's still needed:
>
> So if you're virtualised, you've got no choice in the matter.  lfence
> is either dispatch-serialising or not on AMD, and you won't be able to
> change it.
>
> Furthermore, you can't accurately tell what state the bit is in, because
> the MSR might not be virtualised at all, or may not reflect the true
> state in hardware.  Worse still, attempting to set the bit may not be
> successful even if there isn't a fault for doing so.
>
> Xen sets the DE_CFG bit unconditionally, as does Linux by the looks of
> things (see MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT).  ISTR other hypervisor
> vendors saying the same, but I don't have any information to hand.
>
> If you are running under a hypervisor which has been updated, then
> lfence will almost certainly be dispatch-serialising in practice, and
> you'll almost certainly see the bit already set in DE_CFG.  If you're
> running under a hypervisor which hasn't been patched since Spectre,
> you've already lost in many more ways.
>
> I'd argue that X86_FEATURE_MFENCE_RDTSC is not worth keeping.
>
> So remove it. This will reduce some code rot, and also make it easier
> to hook barrier_nospec() up to a cmdline disable for performance
> raisins, without having to need an alternative_3() macro.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Pu Wen <puwen@hygon.cn>
> Cc: Borislav Petkov <bp@alien8.de>
> ---
> arch/x86/include/asm/barrier.h | 3 +--
> arch/x86/include/asm/cpufeatures.h | 1 -
> arch/x86/include/asm/msr.h | 3 +--
> arch/x86/kernel/cpu/amd.c | 21 +++------------------
> arch/x86/kernel/cpu/hygon.c | 21 +++------------------
> tools/arch/x86/include/asm/cpufeatures.h | 1 -
> 6 files changed, 8 insertions(+), 42 deletions(-)
>
> diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
> index 84f848c2541a..7f828fe49797 100644
> --- a/arch/x86/include/asm/barrier.h
> +++ b/arch/x86/include/asm/barrier.h
> @@ -49,8 +49,7 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
> #define array_index_mask_nospec array_index_mask_nospec
>
> /* Prevent speculative execution past this barrier. */
> -#define barrier_nospec() alternative_2("", "mfence", X86_FEATURE_MFENCE_RDTSC, \
> - "lfence", X86_FEATURE_LFENCE_RDTSC)
> +#define barrier_nospec() alternative("", "lfence", X86_FEATURE_LFENCE_RDTSC)
>
> #define dma_rmb() barrier()
> #define dma_wmb() barrier()
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 998c2cc08363..9b98edb6b2d3 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -96,7 +96,6 @@
> #define X86_FEATURE_SYSCALL32 ( 3*32+14) /* "" syscall in IA32 userspace */
> #define X86_FEATURE_SYSENTER32 ( 3*32+15) /* "" sysenter in IA32 userspace */
> #define X86_FEATURE_REP_GOOD ( 3*32+16) /* REP microcode works well */
> -#define X86_FEATURE_MFENCE_RDTSC ( 3*32+17) /* "" MFENCE synchronizes RDTSC */
> #define X86_FEATURE_LFENCE_RDTSC ( 3*32+18) /* "" LFENCE synchronizes RDTSC */
> #define X86_FEATURE_ACC_POWER ( 3*32+19) /* AMD Accumulated Power Mechanism */
> #define X86_FEATURE_NOPL ( 3*32+20) /* The NOPL (0F 1F) instructions */
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 5cc3930cb465..86f20d520a07 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -233,8 +233,7 @@ static __always_inline unsigned long long rdtsc_ordered(void)
> * Thus, use the preferred barrier on the respective CPU, aiming for
> * RDTSCP as the default.
> */
> - asm volatile(ALTERNATIVE_3("rdtsc",
> - "mfence; rdtsc", X86_FEATURE_MFENCE_RDTSC,
> + asm volatile(ALTERNATIVE_2("rdtsc",
> "lfence; rdtsc", X86_FEATURE_LFENCE_RDTSC,
> "rdtscp", X86_FEATURE_RDTSCP)
> : EAX_EDX_RET(val, low, high)
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 8d4e50428b68..3afe07d602dd 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -879,12 +879,8 @@ static void init_amd(struct cpuinfo_x86 *c)
> init_amd_cacheinfo(c);
>
> if (cpu_has(c, X86_FEATURE_XMM2)) {
> - unsigned long long val;
> - int ret;
> -
> /*
> - * A serializing LFENCE has less overhead than MFENCE, so
> - * use it for execution serialization. On families which
> + * Use LFENCE for execution serialization. On families which
> * don't have that MSR, LFENCE is already serializing.
> * msr_set_bit() uses the safe accessors, too, even if the MSR
> * is not present.
> @@ -892,19 +888,8 @@ static void init_amd(struct cpuinfo_x86 *c)
> msr_set_bit(MSR_F10H_DECFG,
> MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT);
>
> - /*
> - * Verify that the MSR write was successful (could be running
> - * under a hypervisor) and only then assume that LFENCE is
> - * serializing.
> - */
> - ret = rdmsrl_safe(MSR_F10H_DECFG, &val);
> - if (!ret && (val & MSR_F10H_DECFG_LFENCE_SERIALIZE)) {
> - /* A serializing LFENCE stops RDTSC speculation */
> - set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC);
> - } else {
> - /* MFENCE stops RDTSC speculation */
> - set_cpu_cap(c, X86_FEATURE_MFENCE_RDTSC);
> - }
> + /* A serializing LFENCE stops RDTSC speculation */
> + set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC);
> }
>
> /*
> diff --git a/arch/x86/kernel/cpu/hygon.c b/arch/x86/kernel/cpu/hygon.c
> index 415621ddb8a2..4e28c1fc8749 100644
> --- a/arch/x86/kernel/cpu/hygon.c
> +++ b/arch/x86/kernel/cpu/hygon.c
> @@ -330,12 +330,8 @@ static void init_hygon(struct cpuinfo_x86 *c)
> init_hygon_cacheinfo(c);
>
> if (cpu_has(c, X86_FEATURE_XMM2)) {
> - unsigned long long val;
> - int ret;
> -
> /*
> - * A serializing LFENCE has less overhead than MFENCE, so
> - * use it for execution serialization. On families which
> + * Use LFENCE for execution serialization. On families which
> * don't have that MSR, LFENCE is already serializing.
> * msr_set_bit() uses the safe accessors, too, even if the MSR
> * is not present.
> @@ -343,19 +339,8 @@ static void init_hygon(struct cpuinfo_x86 *c)
> msr_set_bit(MSR_F10H_DECFG,
> MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT);
>
> - /*
> - * Verify that the MSR write was successful (could be running
> - * under a hypervisor) and only then assume that LFENCE is
> - * serializing.
> - */
> - ret = rdmsrl_safe(MSR_F10H_DECFG, &val);
> - if (!ret && (val & MSR_F10H_DECFG_LFENCE_SERIALIZE)) {
> - /* A serializing LFENCE stops RDTSC speculation */
> - set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC);
> - } else {
> - /* MFENCE stops RDTSC speculation */
> - set_cpu_cap(c, X86_FEATURE_MFENCE_RDTSC);
> - }
> + /* A serializing LFENCE stops RDTSC speculation */
> + set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC);
> }
>
> /*
> diff --git a/tools/arch/x86/include/asm/cpufeatures.h b/tools/arch/x86/include/asm/cpufeatures.h
> index 75f27ee2c263..4cebefd007f1 100644
> --- a/tools/arch/x86/include/asm/cpufeatures.h
> +++ b/tools/arch/x86/include/asm/cpufeatures.h
> @@ -96,7 +96,6 @@
> #define X86_FEATURE_SYSCALL32 ( 3*32+14) /* "" syscall in IA32 userspace */
> #define X86_FEATURE_SYSENTER32 ( 3*32+15) /* "" sysenter in IA32 userspace */
> #define X86_FEATURE_REP_GOOD ( 3*32+16) /* REP microcode works well */
> -#define X86_FEATURE_MFENCE_RDTSC ( 3*32+17) /* "" MFENCE synchronizes RDTSC */
> #define X86_FEATURE_LFENCE_RDTSC ( 3*32+18) /* "" LFENCE synchronizes RDTSC */
> #define X86_FEATURE_ACC_POWER ( 3*32+19) /* AMD Accumulated Power Mechanism */
> #define X86_FEATURE_NOPL ( 3*32+20) /* The NOPL (0F 1F) instructions */
>
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