On Tue, Mar 31, 2015 at 1:00 PM, Denys Vlasenko <dvlas...@redhat.com> wrote: > After TESTs, use logically correct JZ mnemonic instead of JE > (this doesn't change code). > > Tidy up CMPW insns: > > Modern CPUs are not good with 16-bit operations. > The instructions with 16-bit immediates are especially bad, > on many CPUs they cause length changing prefix stall > in the decoders, costing ~6 cycles to recover. > > Replace CMPWs with CMPLs. > Of these, for form with 8-bit sign-extended immediates > it is a win because they are smaller now > (no 0x66 prefix anymore); > ones with 16-bit immediates are faster. > > @@ -708,7 +708,7 @@ END(sysenter_badsys) > #ifdef CONFIG_X86_ESPFIX32 > movl %ss, %eax > /* see if on espfix stack */ > - cmpw $__ESPFIX_SS, %ax > + cmpl $__ESPFIX_SS, %eax > jne 27f > movl $__KERNEL_DS, %eax > movl %eax, %ds
This is incorrect. 32-bit reads from a segment register are not zero-extended. The upper 16 bits are implementation-defined. Most processors will clear them but it's not guaranteed. -- Brian Gerst -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
