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([2001:b07:6468:f312:a0d1:fc42:c610:f977]) by smtp.gmail.com with ESMTPSA id p8sm32192744wrq.9.2020.08.17.10.19.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 17 Aug 2020 10:19:18 -0700 (PDT) Subject: Re: [PATCH] selftests: kvm: Fix an unexpected failure with newer gcc compiler To: Sean Christopherson , Yang Weijiang Cc: kvm@vger.kernel.org, shuah@kernel.org, peterx@redhat.com, linux-kselftest@vger.kernel.org References: <20200814132105.5122-1-weijiang.yang@intel.com> <20200817164238.GD22407@linux.intel.com> From: Paolo Bonzini Message-ID: <7ad0f9fa-bb57-4c19-475b-3439d7a61bcd@redhat.com> Date: Mon, 17 Aug 2020 19:19:17 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <20200817164238.GD22407@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 17/08/20 18:42, Sean Christopherson wrote: > On Fri, Aug 14, 2020 at 09:21:05PM +0800, Yang Weijiang wrote: >> If debug_regs.c is built with newer gcc, e.g., 8.3.1 on my side, then the generated >> binary looks like over-optimized by gcc: >> >> asm volatile("ss_start: " >> "xor %%rax,%%rax\n\t" >> "cpuid\n\t" >> "movl $0x1a0,%%ecx\n\t" >> "rdmsr\n\t" >> : : : "rax", "ecx"); >> >> is translated to : >> >> 000000000040194e : >> 40194e: 31 c0 xor %eax,%eax <----- rax->eax? >> 401950: 0f a2 cpuid >> 401952: b9 a0 01 00 00 mov $0x1a0,%ecx >> 401957: 0f 32 rdmsr >> >> As you can see rax is replaced with eax in taret binary code. > > It's an optimization. `xor rax, rax` and `xor eax, eax` yield the exact > same result, as writing the lower 32 bits of a GPR in 64-bit mode clears > the upper 32 bits. Using the eax variant avoids the REX prefix and saves > a byte of code. I would have expected that from binutils though, not GCC. > Use `xor %%eax, %%eax`. That should always generate a 2 byte instruction. > Encoding a 64-bit operation would technically be legal, but I doubt any > compiler would do that in practice. Indeed, and in addition the clobbers are incorrect since they miss rbx and rdx. I've sent a patch. Paolo