[PATCH for-4.21 1/2] x86/AMD: avoid REP MOVSB for Zen3/4

[Jan Beulich <jbeulich@xxxxxxxx>]

Along with Zen2 (which doesn't expose ERMS), both families reportedly
suffer from sub-optimal aliasing detection when deciding whether REP MOVSB
can actually be carried out the accelerated way. Therefore we want to
avoid its use in the common case (memset(), copy_page_hot()).

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Question is whether merely avoiding REP MOVSB (but not REP MOVSQ) is going
to be good enough.

--- a/xen/arch/x86/copy_page.S
+++ b/xen/arch/x86/copy_page.S
@@ -57,6 +57,6 @@ END(copy_page_cold)
         .endm
 
 FUNC(copy_page_hot)
-        ALTERNATIVE copy_page_movsq, copy_page_movsb, X86_FEATURE_ERMS
+        ALTERNATIVE copy_page_movsq, copy_page_movsb, X86_FEATURE_XEN_REP_MOVSB
         RET
 END(copy_page_hot)
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -1386,6 +1386,10 @@ static void cf_check init_amd(struct cpu
 
        check_syscfg_dram_mod_en();
 
+       if (c == &boot_cpu_data && cpu_has(c, X86_FEATURE_ERMS)
+           && c->family != 0x19 /* Zen3/4 */)
+               setup_force_cpu_cap(X86_FEATURE_XEN_REP_MOVSB);
+
        amd_log_freq(c);
 }
 
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -684,6 +684,9 @@ static void cf_check init_intel(struct c
         */
        if (c == &boot_cpu_data && c->vfm == INTEL_SKYLAKE_X)
                setup_clear_cpu_cap(X86_FEATURE_CLWB);
+
+       if (c == &boot_cpu_data && cpu_has(c, X86_FEATURE_ERMS))
+               setup_force_cpu_cap(X86_FEATURE_XEN_REP_MOVSB);
 }
 
 const struct cpu_dev __initconst_cf_clobber intel_cpu_dev = {
--- a/xen/arch/x86/include/asm/cpufeatures.h
+++ b/xen/arch/x86/include/asm/cpufeatures.h
@@ -7,7 +7,7 @@
 #define FSCAPINTS FEATURESET_NR_ENTRIES
 
 /* Synthetic words follow the featureset words. */
-#define X86_NR_SYNTH 1
+#define X86_NR_SYNTH 2
 #define X86_SYNTH(x) (FSCAPINTS * 32 + (x))
 
 /* Synthetic features */
@@ -43,6 +43,7 @@ XEN_CPUFEATURE(IBPB_ENTRY_PV,     X86_SY
 XEN_CPUFEATURE(IBPB_ENTRY_HVM,    X86_SYNTH(29)) /* MSR_PRED_CMD used by Xen 
for HVM */
 XEN_CPUFEATURE(USE_VMCALL,        X86_SYNTH(30)) /* Use VMCALL instead of 
VMMCALL */
 XEN_CPUFEATURE(PDX_COMPRESSION,   X86_SYNTH(31)) /* PDX compression */
+XEN_CPUFEATURE(XEN_REP_MOVSB,     X86_SYNTH(32)) /* REP MOVSB used for 
memcpy() and alike. */
 
 /* Bug words follow the synthetic words. */
 #define X86_NR_BUG 1
--- a/xen/arch/x86/memcpy.S
+++ b/xen/arch/x86/memcpy.S
@@ -10,7 +10,7 @@ FUNC(memcpy)
          * precautions were taken).
          */
         ALTERNATIVE "and $7, %edx; shr $3, %rcx", \
-                    STR(rep movsb; RET), X86_FEATURE_ERMS
+                    STR(rep movsb; RET), X86_FEATURE_XEN_REP_MOVSB
         rep movsq
         or      %edx, %ecx
         jz      1f