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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding :content-language; bh=1VWtBrcKLPcRyiQcf3HkSHTQy7W9yax8VnwC0yLWqL4=; b=jA1ipWoIGCTYsugE0Itg+gz1la3f4e43kHDwfZziViPwSa314GXlJSQR4LsHgmFXHL 4w7nZAvBXrLEVdkVZC9qecjjMmv2vi1C4P04mmmxAzT5iG7RFeLqbZPq+dSxIJuaibM6 y8aPG62b7rPLzZH5JvkXuorAZbvP1dVCqEs/D3/RQZcjBRCsGjaS1ejLAQT9tRonZWDq OE+Kn9pDbLep+2AFMa8A5q/8VI3VvADt+IFY/9HpG6HjQgEQkESx8EhazJTj9P9Hfz8h ecAIPjehEkaBwqFnT3TfhwF8zs9quYWhUdPqSuJ/4SUoQe/3dBfu4TC5fE3VK7wLdEP9 wPmA== X-Gm-Message-State: AOAM532W98r4OUfzkwaVhIL9Fv4KAHaUfqhmoLMLykVGZaivHSw+hIeg mm+M3wyQMn3KU+23u4tK/nk= X-Google-Smtp-Source: ABdhPJyJjrhMArs/3bbPXMkpu011sVs1J0mB7GSLU+p7pzqCy4EGcV/cLZWe51HY/zGD1KCXxFLtPw== X-Received: by 2002:a05:6000:82:: with SMTP id m2mr2089524wrx.314.1606825448753; Tue, 01 Dec 2020 04:24:08 -0800 (PST) Subject: Re: [PATCH V3 19/23] xen/arm: io: Abstract sign-extension To: Julien Grall 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> From: Oleksandr Message-ID: <458dc9d7-6ef7-6591-5212-48363bb56971@gmail.com> Date: Tue, 1 Dec 2020 14:24:07 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US 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? -- Regards, Oleksandr Tyshchenko