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[217.87.95.47]) by smtp.gmail.com with ESMTPSA id u18sm1355827wmj.15.2021.06.22.00.36.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Jun 2021 00:36:33 -0700 (PDT) Subject: Re: [PATCH] KVM: s390: get rid of register asm usage To: Heiko Carstens , Christian Borntraeger , Janosch Frank Cc: David Hildenbrand , Cornelia Huck , Claudio Imbrenda , kvm@vger.kernel.org, linux-s390@vger.kernel.org References: <20210621140356.1210771-1-hca@linux.ibm.com> From: Thomas Huth Message-ID: <7edaf85c-810b-e0f9-5977-6e89270f0709@redhat.com> Date: Tue, 22 Jun 2021 09:36:32 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210621140356.1210771-1-hca@linux.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On 21/06/2021 16.03, Heiko Carstens wrote: > Using register asm statements has been proven to be very error prone, > especially when using code instrumentation where gcc may add function > calls, which clobbers register contents in an unexpected way. > > Therefore get rid of register asm statements in kvm code, even though > there is currently nothing wrong with them. This way we know for sure > that this bug class won't be introduced here. > > Reviewed-by: Christian Borntraeger > Signed-off-by: Heiko Carstens > --- > arch/s390/kvm/kvm-s390.c | 18 +++++++++--------- > 1 file changed, 9 insertions(+), 9 deletions(-) > > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c > index 1296fc10f80c..4b7b24f07790 100644 > --- a/arch/s390/kvm/kvm-s390.c > +++ b/arch/s390/kvm/kvm-s390.c > @@ -329,31 +329,31 @@ static void allow_cpu_feat(unsigned long nr) > > static inline int plo_test_bit(unsigned char nr) > { > - register unsigned long r0 asm("0") = (unsigned long) nr | 0x100; > + unsigned long function = (unsigned long) nr | 0x100; > int cc; > > asm volatile( > + " lgr 0,%[function]\n" > /* Parameter registers are ignored for "test bit" */ > " plo 0,0,0,0(0)\n" > " ipm %0\n" > " srl %0,28\n" > : "=d" (cc) > - : "d" (r0) > - : "cc"); > + : [function] "d" (function) > + : "cc", "0"); > return cc == 0; > } > > static __always_inline void __insn32_query(unsigned int opcode, u8 *query) > { > - register unsigned long r0 asm("0") = 0; /* query function */ > - register unsigned long r1 asm("1") = (unsigned long) query; > - > asm volatile( > - /* Parameter regs are ignored */ > + " lghi 0,0\n" > + " lgr 1,%[query]\n" > + /* Parameter registers are ignored */ > " .insn rrf,%[opc] << 16,2,4,6,0\n" > : > - : "d" (r0), "a" (r1), [opc] "i" (opcode) > - : "cc", "memory"); > + : [query] "d" ((unsigned long)query), [opc] "i" (opcode) Wouldn't it be better to keep the "a" constraint instead of "d" to avoid that the compiler ever passes the "query" value in r0 ? Otherwise the query value might get trashed if it is passed in r0... > + : "cc", "memory", "0", "1"); > } With "a" instead of "d": Reviewed-by: Thomas Huth