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Tue, 01 Dec 2020 10:30:45 +0000 Received: from [54.239.6.185] (helo=a483e7b01a66.ant.amazon.com) by xenbits.xenproject.org with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1kk2vl-0007MT-49; Tue, 01 Dec 2020 10:30:45 +0000 X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xen.org; s=20200302mail; h=Content-Transfer-Encoding:Content-Type:In-Reply-To: MIME-Version:Date:Message-ID:From:References:Cc:To:Subject; bh=Dv11DqHyCy/2lJw7+6ik3PRNikc0v30V0ItAaA2NVC8=; b=mYnFuashNVPl7xNQLg+YTHMg6X n+3TIo3m6LhGCgFHunqhzzQCaFXTzIcqxMfBejWmBAVJhNN8eEUWhHCzm3LXf0ZzCu51wr00hm+1j 3SM7OddnXvI3PGaidQft8J+yL2Rmpxl3HOvX/S71xDw+nMJXmT62GClqm6UE/bntf7UQ=; Subject: Re: [PATCH V3 19/23] xen/arm: io: Abstract sign-extension To: Jan Beulich , Oleksandr Cc: "xen-devel@lists.xenproject.org" , Oleksandr Tyshchenko , Stefano Stabellini , Julien Grall , Volodymyr Babchuk References: <1606732298-22107-1-git-send-email-olekstysh@gmail.com> <1606732298-22107-20-git-send-email-olekstysh@gmail.com> <878sai7e1a.fsf@epam.com> <93284ea1-e658-ffff-3223-174d633e38ad@suse.com> From: Julien Grall Message-ID: Date: Tue, 1 Dec 2020 10:30:43 +0000 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <93284ea1-e658-ffff-3223-174d633e38ad@suse.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Hi Jan, On 01/12/2020 07:55, Jan Beulich wrote: > On 01.12.2020 00:27, Oleksandr wrote: >> On 30.11.20 23:03, Volodymyr Babchuk wrote: >>> Oleksandr Tyshchenko writes: >>>> --- a/xen/include/asm-arm/traps.h >>>> +++ b/xen/include/asm-arm/traps.h >>>> @@ -83,6 +83,30 @@ static inline bool VABORT_GEN_BY_GUEST(const struct cpu_user_regs *regs) >>>> (unsigned long)abort_guest_exit_end == regs->pc; >>>> } >>>> >>>> +/* Check whether the sign extension is required and perform it */ >>>> +static inline register_t sign_extend(const struct hsr_dabt dabt, register_t r) >>>> +{ >>>> + uint8_t size = (1 << dabt.size) * 8; >>>> + >>>> + /* >>>> + * Sign extend if required. >>>> + * Note that we expect the read handler to have zeroed the bits >>>> + * outside the requested access size. >>>> + */ >>>> + if ( dabt.sign && (r & (1UL << (size - 1))) ) >>>> + { >>>> + /* >>>> + * We are relying on register_t using the same as >>>> + * an unsigned long in order to keep the 32-bit assembly >>>> + * code smaller. >>>> + */ >>>> + BUILD_BUG_ON(sizeof(register_t) != sizeof(unsigned long)); >>>> + r |= (~0UL) << size; >>> If `size` is 64, you will get undefined behavior there. >> I think, we don't need to worry about undefined behavior here. Having >> size=64 would be possible with doubleword (dabt.size=3). But if "r" >> adjustment gets called (I mean Syndrome Sign Extend bit is set) then >> we deal with byte, halfword or word operations (dabt.size<3). Or I >> missed something? > > At which point please put in a respective ASSERT(), possibly amended > by a brief comment. ASSERT()s are only meant to catch programatic error. However, in this case, the bigger risk is an hardware bug such as advertising a sign extension for either 64-bit (or 32-bit) on Arm64 (resp. Arm32). Actually the Armv8 spec is a bit more blurry when running in AArch32 state because they suggest that the sign extension can be set even for 32-bit access. I think this is a spelling mistake, but it is probably better to be cautious here. Therefore, I would recommend to rework the code so it is only called when len < sizeof(register_t). Cheers, -- Julien Grall