[PATCH] x86: serializing of non-serializing MSR writes

[Jan Beulich <jbeulich@xxxxxxxx>]

Linux commit 25a068b8e9a4e ("x86/apic: Add extra serialization for non-
serializing MSRs") explains why an MFENCE+LFENCE pair is generally
needed ahead of ICR writes in x2APIC mode, and also why at least in
theory such is also needed ahead of TSC_DEADLINE writes. A comment of
our own in send_IPI_mask_x2apic_phys() further explains a condition
under which the LFENCE can be avoided.

Further Linux commit 04c3024560d3 ("x86/barrier: Do not serialize MSR
accesses on AMD") explains that this barrier isn't needed on AMD or
Hygon, and is in fact hampering performance in a measurable way.

Introduce a similarly named helper function, but with a parameter
allowing callers to specify whether a memory access will follow, thus
permitting the LFENCE to be omitted.

Putting an instance in apic_wait_icr_idle() is to be on the safe side.
The one case where it was clearly missing is in send_IPI_shortcut(),
which is also used in x2APIC mode when called from send_IPI_mask().

Function comment shamelessly borrowed (but adapted) from Linux.

Fixes: 5500d265a2a8 ("x86/smp: use APIC ALLBUT destination shorthand when 
possible")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I question the need for a fence ahead of writing TSC_DEADLINE: The Linux
commit message talks about LVT being accessed via MMIO in xAPIC mode,
but that should not be relevant here: It's all the local CPU, so there
ought to not be visibility concerns (much like for a self-IPI no fence
is needed ahead of the ICR write). If that wasn't needed, we could
further use alternatives patching to remove the fence also from
apic_wait_icr_idle() when in xAPIC mode. (And only then would I agree to
have APIC in the feature identifier, like Linux has it.)

A number of apic_write() may better be turned into apic_mem_write(), in
particular e.g. the ones in send_IPI_mask_{flat,phys}(). That way it
would be quite a bit easier to spot paths taken only in xAPIC mode.

The INIT-INIT-SIPI sequence for AP startup doesn't use any barrier, also
not in Linux afaics. I can't explain the lack thereof, though.

--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -1309,6 +1309,12 @@ int reprogram_timer(s_time_t timeout)
 
     if ( tdt_enabled )
     {
+        /*
+         * WRMSR to TSC_DEADLINE is not serializing.  We could pass @timeout
+         * here, but the passed value is preferably compile-time-constant.
+         */
+        weak_wrmsr_fence(false);
+
         wrmsrl(MSR_IA32_TSC_DEADLINE, timeout ? stime2tsc(timeout) : 0);
         return 1;
     }
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -675,8 +675,12 @@ void amd_log_freq(const struct cpuinfo_x
 
 void cf_check early_init_amd(struct cpuinfo_x86 *c)
 {
-       if (c == &boot_cpu_data)
+       if (c == &boot_cpu_data) {
+               /* No fencing needed ahead of certain MSR writes. */
+               setup_force_cpu_cap(X86_FEATURE_NO_WRMSR_FENCE);
+
                amd_init_levelling();
+       }
 
        ctxt_switch_levelling(NULL);
 }
--- a/xen/arch/x86/genapic/x2apic.c
+++ b/xen/arch/x86/genapic/x2apic.c
@@ -97,15 +97,15 @@ static void cf_check send_IPI_mask_x2api
 
     /*
      * Ensure that any synchronisation data written in program order by this
-     * CPU is seen by notified remote CPUs. The WRMSR contained within
-     * apic_icr_write() can otherwise be executed early.
+     * CPU is seen by notified remote CPUs. The WRMSR contained in the loop
+     * below can otherwise be executed early.
      * 
-     * The reason smp_mb() is sufficient here is subtle: the register arguments
+     * The reason MFENCE is sufficient here is subtle: the register arguments
      * to WRMSR must depend on a memory read executed after the barrier. This
      * is guaranteed by cpu_physical_id(), which reads from a global array (and
      * so cannot be hoisted above the barrier even by a clever compiler).
      */
-    smp_mb();
+    weak_wrmsr_fence(true);
 
     local_irq_save(flags);
 
@@ -130,7 +130,7 @@ static void cf_check send_IPI_mask_x2api
     const cpumask_t *cluster_cpus;
     unsigned long flags;
 
-    smp_mb(); /* See above for an explanation. */
+    weak_wrmsr_fence(true); /* See above for an explanation. */
 
     local_irq_save(flags);
 
--- a/xen/arch/x86/include/asm/cpufeatures.h
+++ b/xen/arch/x86/include/asm/cpufeatures.h
@@ -24,7 +24,7 @@ XEN_CPUFEATURE(APERFMPERF,        X86_SY
 XEN_CPUFEATURE(MFENCE_RDTSC,      X86_SYNTH( 9)) /* MFENCE synchronizes RDTSC 
*/
 XEN_CPUFEATURE(XEN_SMEP,          X86_SYNTH(10)) /* SMEP gets used by Xen 
itself */
 XEN_CPUFEATURE(XEN_SMAP,          X86_SYNTH(11)) /* SMAP gets used by Xen 
itself */
-/* Bit 12 unused. */
+XEN_CPUFEATURE(NO_WRMSR_FENCE,    X86_SYNTH(12)) /* No MFENCE{,+LFENCE} ahead 
of certain WRMSR. */
 XEN_CPUFEATURE(IND_THUNK_LFENCE,  X86_SYNTH(13)) /* Use IND_THUNK_LFENCE */
 XEN_CPUFEATURE(IND_THUNK_JMP,     X86_SYNTH(14)) /* Use IND_THUNK_JMP */
 XEN_CPUFEATURE(SC_NO_BRANCH_HARDEN, X86_SYNTH(15)) /* (Disable) Conditional 
branch hardening */
--- a/xen/arch/x86/include/asm/msr.h
+++ b/xen/arch/x86/include/asm/msr.h
@@ -97,6 +97,25 @@ static inline void msr_split(struct cpu_
     regs->rax = (uint32_t)val;
 }
 
+/*
+ * Make previous memory operations globally visible before a WRMSR.  Most
+ * WRMSRs are full serializing instructions themselves and do not require this
+ * barrier.  This may only be required for the TSC_DEADLINE and x2APIC MSRs.
+ *
+ * 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, but callers
+ * can indicate to avoid it when they have a suitable memory access between
+ * the invocation of this function and the WRMSR in question.
+ */
+static inline void weak_wrmsr_fence(bool have_mem_access)
+{
+    alternative("mfence", "", X86_FEATURE_NO_WRMSR_FENCE);
+
+    if ( !have_mem_access )
+        alternative("lfence", "", X86_FEATURE_NO_WRMSR_FENCE);
+}
+
 static inline uint64_t rdtsc(void)
 {
     uint32_t low, high;
--- a/xen/arch/x86/smp.c
+++ b/xen/arch/x86/smp.c
@@ -39,7 +39,10 @@ static unsigned int prepare_ICR2(unsigne
 void apic_wait_icr_idle(void)
 {
     if ( x2apic_enabled )
+    {
+        weak_wrmsr_fence(false);
         return;
+    }
 
     while ( apic_read(APIC_ICR) & APIC_ICR_BUSY )
         cpu_relax();