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Re: [Qemu-devel] [PATCH v2 17/17] target/riscv: convert to TranslatorOps
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH v2 17/17] target/riscv: convert to TranslatorOps |
Date: |
Tue, 10 Apr 2018 09:05:06 -0500 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 |
On 04/10/2018 07:59 AM, Emilio G. Cota wrote:
> On Tue, Apr 10, 2018 at 11:24:37 +1000, Richard Henderson wrote:
>> On 04/07/2018 04:20 AM, Emilio G. Cota wrote:
>>> + next_page = (ctx->base.pc_first & TARGET_PAGE_MASK) +
>>> TARGET_PAGE_SIZE;
>>> + if (ctx->base.pc_next >= next_page) {
>>
>> This fails for the last page of the address space.
>> Better is
>>
>> page_start = ctx->base.pc_first & TARGET_PAGE_MASK;
>> if (ctx->base.pc_next - page_start >= TARGET_PAGE_SIZE) {
>
> Apart from the variable name change, I fail to see how this (A - B >= C)
> is not equivalent to the above (A => B + C). What am I missing?
Integer overflow. Adding TARGET_PAGE_SIZE might wrap next_page to 0,
which changes the semantics of the conditional; while performing the
subtraction avoids the case of overflow.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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[Qemu-devel] [PATCH v2 12/17] target/s390x: convert to TranslatorOps, Emilio G. Cota, 2018/04/06
[Qemu-devel] [PATCH v2 16/17] target/riscv: convert to DisasContextBase, Emilio G. Cota, 2018/04/06
[Qemu-devel] [PATCH v2 07/17] target/mips: convert to DisasContextBase, Emilio G. Cota, 2018/04/06