Hey folks,
On Mon, Jan 15, 2024 at 06:26:56PM -0500, Sasha Levin wrote:
From: "Borislav Petkov (AMD)" bp@alien8.de
[ Upstream commit 04c3024560d3a14acd18d0a51a1d0a89d29b7eb5 ]
AMD does not have the requirement for a synchronization barrier when acccessing a certain group of MSRs. Do not incur that unnecessary penalty there.
Erwan just mentioned that this one is not in 6.1 and in 5.15. And I have mails about it getting picked up by AUTOSEL.
Did the AI reconsider in the meantime?
:-P
Thx.
Two autosel messages are:
https://lore.kernel.org/r/20240115232718.209642-11-sashal@kernel.org https://lore.kernel.org/r/20240115232611.209265-13-sashal@kernel.org
Leaving in the rest for reference.
There will be a CPUID bit which explicitly states that a MFENCE is not needed. Once that bit is added to the APM, this will be extended with it.
While at it, move to processor.h to avoid include hell. Untangling that file properly is a matter for another day.
Some notes on the performance aspect of why this is relevant, courtesy of Kishon VijayAbraham Kishon.VijayAbraham@amd.com:
On a AMD Zen4 system with 96 cores, a modified ipi-bench[1] on a VM shows x2AVIC IPI rate is 3% to 4% lower than AVIC IPI rate. The ipi-bench is modified so that the IPIs are sent between two vCPUs in the same CCX. This also requires to pin the vCPU to a physical core to prevent any latencies. This simulates the use case of pinning vCPUs to the thread of a single CCX to avoid interrupt IPI latency.
In order to avoid run-to-run variance (for both x2AVIC and AVIC), the below configurations are done:
Disable Power States in BIOS (to prevent the system from going to lower power state)
Run the system at fixed frequency 2500MHz (to prevent the system from increasing the frequency when the load is more)
With the above configuration:
*) Performance measured using ipi-bench for AVIC: Average Latency: 1124.98ns [Time to send IPI from one vCPU to another vCPU]
Cumulative throughput: 42.6759M/s [Total number of IPIs sent in a second from 48 vCPUs simultaneously]
*) Performance measured using ipi-bench for x2AVIC: Average Latency: 1172.42ns [Time to send IPI from one vCPU to another vCPU]
Cumulative throughput: 40.9432M/s [Total number of IPIs sent in a second from 48 vCPUs simultaneously]
From above, x2AVIC latency is ~4% more than AVIC. However, the expectation is x2AVIC performance to be better or equivalent to AVIC. Upon analyzing the perf captures, it is observed significant time is spent in weak_wrmsr_fence() invoked by x2apic_send_IPI().
With the fix to skip weak_wrmsr_fence()
*) Performance measured using ipi-bench for x2AVIC: Average Latency: 1117.44ns [Time to send IPI from one vCPU to another vCPU]
Cumulative throughput: 42.9608M/s [Total number of IPIs sent in a second from 48 vCPUs simultaneously]
Comparing the performance of x2AVIC with and without the fix, it can be seen the performance improves by ~4%.
Performance captured using an unmodified ipi-bench using the 'mesh-ipi' option with and without weak_wrmsr_fence() on a Zen4 system also showed significant performance improvement without weak_wrmsr_fence(). The 'mesh-ipi' option ignores CCX or CCD and just picks random vCPU.
Average throughput (10 iterations) with weak_wrmsr_fence(), Cumulative throughput: 4933374 IPI/s
Average throughput (10 iterations) without weak_wrmsr_fence(), Cumulative throughput: 6355156 IPI/s
[1] https://github.com/bytedance/kvm-utils/tree/master/microbenchmark/ipi-bench
Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Link: https://lore.kernel.org/r/20230622095212.20940-1-bp@alien8.de Signed-off-by: Sasha Levin sashal@kernel.org
arch/x86/include/asm/barrier.h | 18 ------------------ arch/x86/include/asm/cpufeatures.h | 2 +- arch/x86/include/asm/processor.h | 18 ++++++++++++++++++ arch/x86/kernel/cpu/amd.c | 3 +++ arch/x86/kernel/cpu/common.c | 7 +++++++ arch/x86/kernel/cpu/hygon.c | 3 +++ 6 files changed, 32 insertions(+), 19 deletions(-)
diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h index 3ba772a69cc8..dab2db15a8c4 100644 --- a/arch/x86/include/asm/barrier.h +++ b/arch/x86/include/asm/barrier.h @@ -81,22 +81,4 @@ do { \ #include <asm-generic/barrier.h> -/*
- Make previous memory operations globally visible before
- a WRMSR.
- MFENCE makes writes visible, but only affects load/store
- instructions. WRMSR is unfortunately not a load/store
- instruction and is unaffected by MFENCE. The LFENCE ensures
- that the WRMSR is not reordered.
- Most WRMSRs are full serializing instructions themselves and
- do not require this barrier. This is only required for the
- IA32_TSC_DEADLINE and X2APIC MSRs.
- */
-static inline void weak_wrmsr_fence(void) -{
- asm volatile("mfence; lfence" : : : "memory");
-}
#endif /* _ASM_X86_BARRIER_H */ diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h index d6089072ee41..dd6f714c215e 100644 --- a/arch/x86/include/asm/cpufeatures.h +++ b/arch/x86/include/asm/cpufeatures.h @@ -305,10 +305,10 @@ #define X86_FEATURE_MSR_TSX_CTRL (11*32+20) /* "" MSR IA32_TSX_CTRL (Intel) implemented */
#define X86_FEATURE_SRSO (11*32+24) /* "" AMD BTB untrain RETs */ #define X86_FEATURE_SRSO_ALIAS (11*32+25) /* "" AMD BTB untrain RETs through aliasing */ #define X86_FEATURE_IBPB_ON_VMEXIT (11*32+26) /* "" Issue an IBPB only on VMEXIT */ +#define X86_FEATURE_APIC_MSRS_FENCE (11*32+27) /* "" IA32_TSC_DEADLINE and X2APIC MSRs need fencing */ /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */ #define X86_FEATURE_AVX_VNNI (12*32+ 4) /* AVX VNNI instructions */ diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index bbbf27cfe701..52329752dd4f 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -861,4 +861,22 @@ enum mds_mitigations { extern bool gds_ucode_mitigated(void); +/*
- Make previous memory operations globally visible before
- a WRMSR.
- MFENCE makes writes visible, but only affects load/store
- instructions. WRMSR is unfortunately not a load/store
- instruction and is unaffected by MFENCE. The LFENCE ensures
- that the WRMSR is not reordered.
- Most WRMSRs are full serializing instructions themselves and
- do not require this barrier. This is only required for the
- IA32_TSC_DEADLINE and X2APIC MSRs.
- */
+static inline void weak_wrmsr_fence(void) +{
- alternative("mfence; lfence", "", ALT_NOT(X86_FEATURE_APIC_MSRS_FENCE));
+}
#endif /* _ASM_X86_PROCESSOR_H */ diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c index dba7fe7ecea9..06186b7d0ed0 100644 --- a/arch/x86/kernel/cpu/amd.c +++ b/arch/x86/kernel/cpu/amd.c @@ -1158,6 +1158,9 @@ static void init_amd(struct cpuinfo_x86 *c) if (!cpu_has(c, X86_FEATURE_HYPERVISOR) && cpu_has_amd_erratum(c, amd_erratum_1485)) msr_set_bit(MSR_ZEN4_BP_CFG, MSR_ZEN4_BP_CFG_SHARED_BTB_FIX_BIT);
- /* AMD CPUs don't need fencing after x2APIC/TSC_DEADLINE MSR writes. */
- clear_cpu_cap(c, X86_FEATURE_APIC_MSRS_FENCE);
} #ifdef CONFIG_X86_32 diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index 01c4f8f45b83..1e870f6dd51c 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1687,6 +1687,13 @@ static void identify_cpu(struct cpuinfo_x86 *c) c->apicid = apic->phys_pkg_id(c->initial_apicid, 0); #endif
- /*
* Set default APIC and TSC_DEADLINE MSR fencing flag. AMD and* Hygon will clear it in ->c_init() below.*/- set_cpu_cap(c, X86_FEATURE_APIC_MSRS_FENCE);
- /*
- Vendor-specific initialization. In this section we
- canonicalize the feature flags, meaning if there are
diff --git a/arch/x86/kernel/cpu/hygon.c b/arch/x86/kernel/cpu/hygon.c index 9e8380bd4fb9..8a80d5343f3a 100644 --- a/arch/x86/kernel/cpu/hygon.c +++ b/arch/x86/kernel/cpu/hygon.c @@ -347,6 +347,9 @@ static void init_hygon(struct cpuinfo_x86 *c) set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS); check_null_seg_clears_base(c);
- /* Hygon CPUs don't need fencing after x2APIC/TSC_DEADLINE MSR writes. */
- clear_cpu_cap(c, X86_FEATURE_APIC_MSRS_FENCE);
} static void cpu_detect_tlb_hygon(struct cpuinfo_x86 *c) -- 2.43.0