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While I can't prove it due to lacking a copy of the old SDM version, I can only assume this placement was a result of what had been described there originally. In any event recent versions of the SDM hve been telling us otherwise. For AMD the situation is more complex: MFENCE is needed ahead and/or after CLFLUSH when the CPU doesn't also support CLFLUSHOPT. (It's "and" in the flush_area_local() case, as we cannot know what the caller's needs are. For cache_writeback() fencing ahead of the flush is sufficient.) Fixes: 623c720fc8da3 ("x86: use CLFLUSHOPT when available") Signed-off-by: Jan Beulich --- TBD: I'd be okay with not touching cache_writeback(), for only being used by VT-d right now. --- v2: Mark AMD behavior as a bug rather than a feature. Retain quotes. --- a/xen/arch/x86/cpu/amd.c +++ b/xen/arch/x86/cpu/amd.c @@ -761,6 +761,13 @@ static void __init noinline detect_bugs( */ if (!cpu_has_nscb) setup_force_cpu_cap(X86_BUG_NULL_SEG); + + /* + * AMD CPUs not supporting CLFLUSHOPT require MFENCE to serialize + * CLFLUSH against other memory accesses. + */ + if (!cpu_has_clflushopt) + setup_force_cpu_cap(X86_BUG_CLFLUSH_MFENCE); } static void init_amd(struct cpuinfo_x86 *c) --- a/xen/arch/x86/flushtlb.c +++ b/xen/arch/x86/flushtlb.c @@ -245,12 +245,15 @@ unsigned int flush_area_local(const void c->x86_clflush_size && c->x86_cache_size && sz && ((sz >> 10) < c->x86_cache_size) ) { - alternative("", "sfence", X86_FEATURE_CLFLUSHOPT); + alternative("", "mfence", X86_BUG_CLFLUSH_MFENCE); for ( i = 0; i < sz; i += c->x86_clflush_size ) alternative_input("ds; clflush %0", "data16 clflush %0", /* clflushopt */ X86_FEATURE_CLFLUSHOPT, "m" (((const char *)va)[i])); + alternative_2("", + "sfence", X86_FEATURE_CLFLUSHOPT, + "mfence", X86_BUG_CLFLUSH_MFENCE); flags &= ~FLUSH_CACHE; } else @@ -274,6 +277,8 @@ void cache_writeback(const void *addr, u unsigned int clflush_size = current_cpu_data.x86_clflush_size ?: 16; const void *end = addr + size; + alternative("", "mfence", X86_BUG_CLFLUSH_MFENCE); + addr -= (unsigned long)addr & (clflush_size - 1); for ( ; addr < end; addr += clflush_size ) { --- a/xen/arch/x86/include/asm/cpufeatures.h +++ b/xen/arch/x86/include/asm/cpufeatures.h @@ -47,6 +47,7 @@ XEN_CPUFEATURE(RET_SIMPLE, X86_SY #define X86_BUG_FPU_PTRS X86_BUG( 0) /* (F)X{SAVE,RSTOR} doesn't save/restore FOP/FIP/FDP. */ #define X86_BUG_NULL_SEG X86_BUG( 1) /* NULL-ing a selector preserves the base and limit. */ +#define X86_BUG_CLFLUSH_MFENCE X86_BUG( 2) /* MFENCE needed to serialize CLFLUSH */ /* Total number of capability words, inc synth and bug words. */ #define NCAPINTS (FSCAPINTS + X86_NR_SYNTH + X86_NR_BUG) /* N 32-bit words worth of info */