[xen master] x86/pv: Split pv_hypercall() in two

[patchbot@xxxxxxx]

commit eb7518b89be6df874ec9b2bafadeaaa7c874ea6b
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Oct 4 19:11:45 2021 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Oct 12 16:56:11 2021 +0100

    x86/pv: Split pv_hypercall() in two
    
    The is_pv_32bit_vcpu() conditionals hide four lfences, with two taken on any
    individual path through the function.  There is very little code common
    between compat and native, and context-dependent conditionals predict very
    badly for a period of time after context switch.
    
    Move do_entry_int82() from pv/traps.c into pv/hypercall.c, allowing
    _pv_hypercall() to be static and forced inline.  The delta is:
    
      add/remove: 0/0 grow/shrink: 1/1 up/down: 300/-282 (18)
      Function                                     old     new   delta
      do_entry_int82                                50     350    +300
      pv_hypercall                                 579     297    -282
    
    which is tiny, but the perf implications are large:
    
      Guest | Naples | Milan  | SKX    | CFL-R  |
      ------+--------+--------+--------+--------+
      pv64  |  17.4% |  15.5% |   2.6% |   4.5% |
      pv32  |   1.9% |  10.9% |   1.4% |   2.5% |
    
    These are percentage improvements in raw TSC detlas for a xen_version
    hypercall, with obvious outliers excluded.  Therefore, it is an idealised 
best
    case improvement.
    
    The pv64 path uses `syscall`, while the pv32 path uses `int $0x82` so
    necessarily has higher overhead.  Therefore, dropping the lfences is less 
over
    an overall improvement.
    
    I don't know why the Naples pv32 improvement is so small, but I've double
    checked the numbers and they're consistent.  There's presumably something
    we're doing which is a large overhead in the pipeline.
    
    On the Intel side, both systems are writing to MSR_SPEC_CTRL on
    entry/exit (SKX using the retrofitted microcode implementation, CFL-R using
    the hardware implementation), while SKX is suffering further from XPTI for
    Meltdown protection.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/pv/hypercall.c | 24 +++++++++++++++++++-----
 xen/arch/x86/pv/traps.c     | 11 -----------
 2 files changed, 19 insertions(+), 16 deletions(-)

diff --git a/xen/arch/x86/pv/hypercall.c b/xen/arch/x86/pv/hypercall.c
index 9765e674cf..3579ba905c 100644
--- a/xen/arch/x86/pv/hypercall.c
+++ b/xen/arch/x86/pv/hypercall.c
@@ -23,6 +23,7 @@
 #include <xen/hypercall.h>
 #include <xen/nospec.h>
 #include <xen/trace.h>
+#include <asm/apic.h>
 #include <asm/multicall.h>
 #include <irq_vectors.h>
 
@@ -109,15 +110,15 @@ const pv_hypercall_table_t pv_hypercall_table[] = {
 #undef COMPAT_CALL
 #undef HYPERCALL
 
-void pv_hypercall(struct cpu_user_regs *regs)
+/* Forced inline to cause 'compat' to be evaluated at compile time. */
+static void always_inline
+_pv_hypercall(struct cpu_user_regs *regs, bool compat)
 {
     struct vcpu *curr = current;
-    unsigned long eax;
+    unsigned long eax = compat ? regs->eax : regs->rax;
 
     ASSERT(guest_kernel_mode(curr, regs));
 
-    eax = is_pv_32bit_vcpu(curr) ? regs->eax : regs->rax;
-
     BUILD_BUG_ON(ARRAY_SIZE(pv_hypercall_table) >
                  ARRAY_SIZE(hypercall_args_table));
 
@@ -137,7 +138,7 @@ void pv_hypercall(struct cpu_user_regs *regs)
 
     curr->hcall_preempted = false;
 
-    if ( !is_pv_32bit_vcpu(curr) )
+    if ( !compat )
     {
         unsigned long rdi = regs->rdi;
         unsigned long rsi = regs->rsi;
@@ -348,8 +349,21 @@ void pv_ring1_init_hypercall_page(void *p)
         *(u8  *)(p+ 7) = 0xc3;    /* ret */
     }
 }
+
+void do_entry_int82(struct cpu_user_regs *regs)
+{
+    if ( unlikely(untrusted_msi) )
+        check_for_unexpected_msi((uint8_t)regs->entry_vector);
+
+    _pv_hypercall(regs, true /* compat */);
+}
 #endif
 
+void pv_hypercall(struct cpu_user_regs *regs)
+{
+    _pv_hypercall(regs, false /* native */);
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/arch/x86/pv/traps.c b/xen/arch/x86/pv/traps.c
index 764773c021..1e05a9f1cd 100644
--- a/xen/arch/x86/pv/traps.c
+++ b/xen/arch/x86/pv/traps.c
@@ -24,22 +24,11 @@
 #include <xen/lib.h>
 #include <xen/softirq.h>
 
-#include <asm/apic.h>
 #include <asm/pv/trace.h>
 #include <asm/shared.h>
 #include <asm/traps.h>
 #include <irq_vectors.h>
 
-#ifdef CONFIG_PV32
-void do_entry_int82(struct cpu_user_regs *regs)
-{
-    if ( unlikely(untrusted_msi) )
-        check_for_unexpected_msi((uint8_t)regs->entry_vector);
-
-    pv_hypercall(regs);
-}
-#endif
-
 void pv_inject_event(const struct x86_event *event)
 {
     struct vcpu *curr = current;
--
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