Messages in this thread | | | Date | Thu, 08 May 2014 15:27:50 +0300 | From | Nadav Amit <> | Subject | Re: [PATCH 4/5] KVM: x86: Wrong register masking in 64-bit mode |
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On 5/7/14, 6:52 PM, Paolo Bonzini wrote: > Il 07/05/2014 14:32, Nadav Amit ha scritto: >> 32-bit operations are zero extended in 64-bit mode. Currently, the >> code does >> not handle them correctly and keeps the high bits. In 16-bit mode, the >> high >> 32-bits are kept intact. >> >> In addition, although it is not well-documented, when address override >> prefix >> is used with REP-string instruction, RCX high half is zeroed even if >> ECX was >> zero on the first iteration (as if an assignment was performed to ECX). > > Is this true even for REPZ and ZF=0 or REPNZ and ZF=1? > > Paolo The REPZ and REPNZ condition is checked on the end of an iteration (see the REP instruction description on the SDM), so it does not matter. So even REPZ/RENZ would zero RCX high half.
This "feature" is not well-documented, but can be observed. Here is a small code you can try.
--- #include <stdio.h> unsigned long src, dst; int main() { unsigned long long rsi, rdi, rcx; rcx = 0xffffffff00000000ull; rsi = (unsigned long long)&src | 0xffffffff00000000ull; rdi = (unsigned long long)&dst | 0xffffffff00000000ull; printf("before: rsi %llx rdi %llx rcx %llx\n", rsi, rdi ,rcx); asm volatile ( ".byte 0x67\n\t" "repne cmpsd\n\t" : "+S" (rsi), "+D" (rdi), "+c" (rcx) : : "memory", "cc" ); printf("after: rsi %llx rdi %llx rcx %llx\n", rsi, rdi ,rcx); return 0; } ---
Nadav
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