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Wed, 11 Sep 2019 08:19:07 +0000 (UTC) Received: from thuth.remote.csb (ovpn-116-192.ams2.redhat.com [10.36.116.192]) by smtp.corp.redhat.com (Postfix) with ESMTP id 1CE9F100194E; Wed, 11 Sep 2019 08:19:05 +0000 (UTC) Subject: Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses To: Andre Przywara References: <20190905171502.215183-1-andre.przywara@arm.com> <20190911091604.380c6df9@donnerap.cambridge.arm.com> From: Thomas Huth Openpgp: preference=signencrypt Autocrypt: addr=thuth@redhat.com; prefer-encrypt=mutual; keydata= mQINBFH7eUwBEACzyOXKU+5Pcs6wNpKzrlJwzRl3VGZt95VCdb+FgoU9g11m7FWcOafrVRwU yYkTm9+7zBUc0sW5AuPGR/dp3pSLX/yFWsA/UB4nJsHqgDvDU7BImSeiTrnpMOTXb7Arw2a2 4CflIyFqjCpfDM4MuTmzTjXq4Uov1giGE9X6viNo1pxyEpd7PanlKNnf4PqEQp06X4IgUacW tSGj6Gcns1bCuHV8OPWLkf4hkRnu8hdL6i60Yxz4E6TqlrpxsfYwLXgEeswPHOA6Mn4Cso9O 0lewVYfFfsmokfAVMKWzOl1Sr0KGI5T9CpmRfAiSHpthhHWnECcJFwl72NTi6kUcUzG4se81 O6n9d/kTj7pzTmBdfwuOZ0YUSqcqs0W+l1NcASSYZQaDoD3/SLk+nqVeCBB4OnYOGhgmIHNW 0CwMRO/GK+20alxzk//V9GmIM2ACElbfF8+Uug3pqiHkVnKqM7W9/S1NH2qmxB6zMiJUHlTH gnVeZX0dgH27mzstcF786uPcdEqS0KJuxh2kk5IvUSL3Qn3ZgmgdxBMyCPciD/1cb7/Ahazr 3ThHQXSHXkH/aDXdfLsKVuwDzHLVSkdSnZdt5HHh75/NFHxwaTlydgfHmFFwodK8y/TjyiGZ zg2Kje38xnz8zKn9iesFBCcONXS7txENTzX0z80WKBhK+XSFJwARAQABtB5UaG9tYXMgSHV0 aCA8dGh1dGhAcmVkaGF0LmNvbT6JAjgEEwECACIFAlVgX6oCGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAAoJEC7Z13T+cC21EbIP/ii9cvT2HHGbFRl8HqGT6+7Wkb+XLMqJBMAIGiQK QIP3xk1HPTsLfVG0ao4hy/oYkGNOP8+ubLnZen6Yq3zAFiMhQ44lvgigDYJo3Ve59gfe99KX EbtB+X95ODARkq0McR6OAsPNJ7gpEUzfkQUUJTXRDQXfG/FX303Gvk+YU0spm2tsIKPl6AmV 1CegDljzjycyfJbk418MQmMu2T82kjrkEofUO2a24ed3VGC0/Uz//XCR2ZTo+vBoBUQl41BD eFFtoCSrzo3yPFS+w5fkH9NT8ChdpSlbNS32NhYQhJtr9zjWyFRf0Zk+T/1P7ECn6gTEkp5k ofFIA4MFBc/fXbaDRtBmPB0N9pqTFApIUI4vuFPPO0JDrII9dLwZ6lO9EKiwuVlvr1wwzsgq zJTPBU3qHaUO4d/8G+gD7AL/6T4zi8Jo/GmjBsnYaTzbm94lf0CjXjsOX3seMhaE6WAZOQQG tZHAO1kAPWpaxne+wtgMKthyPLNwelLf+xzGvrIKvLX6QuLoWMnWldu22z2ICVnLQChlR9d6 WW8QFEpo/FK7omuS8KvvopFcOOdlbFMM8Y/8vBgVMSsK6fsYUhruny/PahprPbYGiNIhKqz7 UvgyZVl4pBFjTaz/SbimTk210vIlkDyy1WuS8Zsn0htv4+jQPgo9rqFE4mipJjy/iboDuQIN BFH7eUwBEAC2nzfUeeI8dv0C4qrfCPze6NkryUflEut9WwHhfXCLjtvCjnoGqFelH/PE9NF4 4VPSCdvD1SSmFVzu6T9qWdcwMSaC+e7G/z0/AhBfqTeosAF5XvKQlAb9ZPkdDr7YN0a1XDfa +NgA+JZB4ROyBZFFAwNHT+HCnyzy0v9Sh3BgJJwfpXHH2l3LfncvV8rgFv0bvdr70U+On2XH 5bApOyW1WpIG5KPJlDdzcQTyptOJ1dnEHfwnABEfzI3dNf63rlxsGouX/NFRRRNqkdClQR3K gCwciaXfZ7ir7fF0u1N2UuLsWA8Ei1JrNypk+MRxhbvdQC4tyZCZ8mVDk+QOK6pyK2f4rMf/ WmqxNTtAVmNuZIwnJdjRMMSs4W4w6N/bRvpqtykSqx7VXcgqtv6eqoDZrNuhGbekQA0sAnCJ VPArerAZGArm63o39me/bRUQeQVSxEBmg66yshF9HkcUPGVeC4B0TPwz+HFcVhheo6hoJjLq knFOPLRj+0h+ZL+D0GenyqD3CyuyeTT5dGcNU9qT74bdSr20k/CklvI7S9yoQje8BeQAHtdV cvO8XCLrpGuw9SgOS7OP5oI26a0548M4KldAY+kqX6XVphEw3/6U1KTf7WxW5zYLTtadjISB X9xsRWSU+Yqs3C7oN5TIPSoj9tXMoxZkCIHWvnqGwZ7JhwARAQABiQIfBBgBAgAJBQJR+3lM AhsMAAoJEC7Z13T+cC21hPAQAIsBL9MdGpdEpvXs9CYrBkd6tS9mbaSWj6XBDfA1AEdQkBOn ZH1Qt7HJesk+qNSnLv6+jP4VwqK5AFMrKJ6IjE7jqgzGxtcZnvSjeDGPF1h2CKZQPpTw890k fy18AvgFHkVk2Oylyexw3aOBsXg6ukN44vIFqPoc+YSU0+0QIdYJp/XFsgWxnFIMYwDpxSHS 5fdDxUjsk3UBHZx+IhFjs2siVZi5wnHIqM7eK9abr2cK2weInTBwXwqVWjsXZ4tq5+jQrwDK cvxIcwXdUTLGxc4/Z/VRH1PZSvfQxdxMGmNTGaXVNfdFZjm4fz0mz+OUi6AHC4CZpwnsliGV ODqwX8Y1zic9viSTbKS01ZNp175POyWViUk9qisPZB7ypfSIVSEULrL347qY/hm9ahhqmn17 Ng255syASv3ehvX7iwWDfzXbA0/TVaqwa1YIkec+/8miicV0zMP9siRcYQkyTqSzaTFBBmqD oiT+z+/E59qj/EKfyce3sbC9XLjXv3mHMrq1tKX4G7IJGnS989E/fg6crv6NHae9Ckm7+lSs IQu4bBP2GxiRQ+NV3iV/KU3ebMRzqIC//DCOxzQNFNJAKldPe/bKZMCxEqtVoRkuJtNdp/5a yXFZ6TfE1hGKrDBYAm4vrnZ4CXFSBDllL59cFFOJCkn4Xboj/aVxxJxF30bn Organization: Red Hat Message-ID: Date: Wed, 11 Sep 2019 10:19:05 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190911091604.380c6df9@donnerap.cambridge.arm.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.49]); Wed, 11 Sep 2019 08:19:08 +0000 (UTC) Cc: Paolo Bonzini , kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On 11/09/2019 10.16, Andre Przywara wrote: > On Tue, 10 Sep 2019 20:15:19 +0200 > Thomas Huth wrote: > > Hi, > >> On 05/09/2019 19.15, Andre Przywara wrote: >>> The ARM architecture requires all accesses to device memory to be >>> naturally aligned[1][2]. Normal memory does not have this strict >>> requirement, and in fact many systems do ignore unaligned accesses >>> (by the means of clearing the A bit in SCTLR and accessing normal >>> memory). So the default behaviour of GCC assumes that unaligned accesses >>> are fine, at least if happening on the stack. >>> >>> Now kvm-unit-tests runs some C code with the MMU off, which degrades the >>> whole system memory to device memory. Now every unaligned access will >>> fault, regardless of the A bit. >>> In fact there is at least one place in lib/printf.c where GCC merges >>> two consecutive char* accesses into one "strh" instruction, writing to >>> a potentially unaligned address. >>> This can be reproduced by configuring kvm-unit-tests for kvmtool, but >>> running it on QEMU, which triggers an early printf that exercises this >>> particular code path. >>> >>> Add the -mstrict-align compiler option to the arm64 CFLAGS to fix this >>> problem. Also add the respective -mno-unaligned-access flag for arm. >>> >>> Thanks to Alexandru for helping debugging this. >>> >>> Signed-off-by: Andre Przywara >>> >>> [1] ARMv8 ARM DDI 0487E.a, B2.5.2 >>> [2] ARMv7 ARM DDI 0406C.d, A3.2.1 >>> --- >>> arm/Makefile.arm | 1 + >>> arm/Makefile.arm64 | 1 + >>> 2 files changed, 2 insertions(+) >>> >>> diff --git a/arm/Makefile.arm b/arm/Makefile.arm >>> index a625267..43b4be1 100644 >>> --- a/arm/Makefile.arm >>> +++ b/arm/Makefile.arm >>> @@ -12,6 +12,7 @@ KEEP_FRAME_POINTER := y >>> >>> CFLAGS += $(machine) >>> CFLAGS += -mcpu=$(PROCESSOR) >>> +CFLAGS += -mno-unaligned-access >>> >>> arch_LDFLAGS = -Ttext=40010000 >>> >>> diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64 >>> index 02c24e8..35de5ea 100644 >>> --- a/arm/Makefile.arm64 >>> +++ b/arm/Makefile.arm64 >>> @@ -7,6 +7,7 @@ bits = 64 >>> ldarch = elf64-littleaarch64 >>> >>> arch_LDFLAGS = -pie -n >>> +CFLAGS += -mstrict-align >> >> Instead of adding it to both, Makefile.arm and Makefile.arm64, you could >> also simply add it to Makefile.common instead. > > But the arguments are not the same (admittedly against intuition)? > I thought about defining arch_CFLAGS in both files, then adding that to Makefile.common, but didn't see the advantage over this straightforward approach here. D'oh, never mind, I didn't read the patch properly. I somehow thought that the arguments are the same. It's quite weird that the compiler developers chose different names here... Thomas _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm