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Tue, 01 Dec 2020 12:28:19 +0000 Received: from [54.239.6.186] (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 1kk4lW-0003Gh-RT; Tue, 01 Dec 2020 12:28:18 +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=ZSrY3iTXFqVdkNuyUHX4+OrJ/JWlAreJszuINILngnM=; b=H/szbc8m9mjDjUaXhkN+uXJKoP h5xhIFbrSfLynd6zMtAn7HIW1CxlWE6PQ77iIBZxD2MqhluvmI4fkrbZ11YTERBHtPXbxJxGb7Mqp aBXLijOnqitL2cDa8PmUWzWkxVJ004c95Ya1bXspJy9S9sPwTkBBJJrHawEZzu7nWeyE=; Subject: Re: [PATCH V3 19/23] xen/arm: io: Abstract sign-extension To: Oleksandr Cc: Jan Beulich , "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> <932d7826-7e48-aaee-d566-44c384f84e1c@gmail.com> <458dc9d7-6ef7-6591-5212-48363bb56971@gmail.com> From: Julien Grall Message-ID: <8ad8a27e-bbc3-8d9b-04e2-b68de1ff8ef4@xen.org> Date: Tue, 1 Dec 2020 12:28:17 +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: <458dc9d7-6ef7-6591-5212-48363bb56971@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit On 01/12/2020 12:24, Oleksandr wrote: > > On 01.12.20 14:13, Julien Grall wrote: >> Hi Oleksandr, > > Hi Julien. > > >> >>>>>>>> --- 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). >>> >>> I am not sure I understand the recommendation, could you please >>> clarify (also I don't see 'len' being used here). >> >> Sorry I meant 'size'. I think something like: >> >> if ( dabt.sign && (size < sizeof(register_t)) && >>      (r & (1UL << (size - 1)) ) >> { >> } >> >> Another posibility would be: >> >> if ( dabt.sign && (size < sizeof(register_t)) ) >> { >>    /* find whether the sign bit is set and propagate it */ >> } >> >> I have a slight preference for the latter as the "if" is easier to read. >> >> In any case, I think this change should be done in a separate patch (I >> don't mint whether this is done after or before this one). > > ok, I got it, thank you for the clarification. Of course, I will do that > in a separate patch, since the current one is to avoid code duplication > only. BTW, do you have comments on this patch itself? The series is in my TODO list. I will have a look once in a bit :). Cheers, -- Julien Grall