From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mailbox.box.xen0n.name (mail.xen0n.name [115.28.160.31]) by sourceware.org (Postfix) with ESMTPS id 5E35E385840F; Wed, 22 Feb 2023 09:35:52 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 5E35E385840F Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=xen0n.name Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=xen0n.name DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=xen0n.name; s=mail; t=1677058544; bh=i67nSuihb9OKJCCz7bXLcWSt3OPJPxNhTVTQLKhrH8g=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=LuR3RNNibKqAg7NhtgBmsdliLtOeM3erpSE7lJDjDMx1pV6yHoZeyBiSxP19+Go56 ewNHZgqP5iUAqZhyNACYpcT4HGaE31Xfohna2xQE6LHIEs0Gr0WgrtM44g9aNupRhZ z6rPrxHNWbbHs3FObECc9pTxOi97I0uZx+rvg8wI= Received: from [100.100.57.122] (unknown [58.34.185.106]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by mailbox.box.xen0n.name (Postfix) with ESMTPSA id A2A74600DA; Wed, 22 Feb 2023 17:35:44 +0800 (CST) Message-ID: Date: Wed, 22 Feb 2023 17:35:44 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.8.0 Subject: Re: [PATCH] LoongArch: Change the value of macro TRY_EMPTY_VM_SPACE from 0x8000000000 to 0x1000000000. Content-Language: en-US To: Lulu Cheng , Xi Ruoyao , gcc-patches@gcc.gnu.org, pinskia@gcc.gnu.org, richard.sandiford@arm.com Cc: xuchenghua@loongson.cn References: <20230221072001.480858-1-chenglulu@loongson.cn> <6333af35-1490-6d03-b385-22b6f41abe49@loongson.cn> From: WANG Xuerui In-Reply-To: <6333af35-1490-6d03-b385-22b6f41abe49@loongson.cn> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-2.8 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_BARRACUDACENTRAL,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 2023/2/22 17:30, Lulu Cheng wrote: > > 在 2023/2/21 下午9:56, WANG Xuerui 写道: >> Hi, >> >> On 2023/2/21 21:03, Lulu Cheng wrote: >>> >>> 在 2023/2/21 下午3:41, Xi Ruoyao 写道: >>>> On Tue, 2023-02-21 at 15:20 +0800, Lulu Cheng wrote: >>>>> Like la264 only has 40 effective bits of virtual address space. >>>> I'm OK with the change.  But the VA length is configurable building >>>> the >>>> kernel.  Is there any specific reason LA264 has to use the 40-bit >>>> configuration, or should we reword the commit message like "for >>>> supporting the configuration with less page table level or smaller >>>> page >>>> size"? >>> >>> I consulted with my colleagues who are working on the kernel, >>> >>> it looks like this: >>> >>> The la264 chip desgn is physically 40-bit virtual address. >>> >>> User mode and kernel mode each account for half: >>> >>> User mode :    0x0-0x7f ffff ffff >>> >>> Kernel mode:  0xffff ff80 0000 0000 -0xffff ffff ffff ffff >>> >>> The high bit is the sign extension of bit39. >>> >> Looking at the comments around the TRY_EMPTY_VM_SPACE definitions, >> they all indicate that the guessed range should be "likely free" -- >> that implies "usable". Given the common VM allocation behavior, we >> want TRY_EMPTY_VM_SPACE to point at a reasonably high place in the VA >> so it's "likely free". >> >> So IMO the point is, will there be any LoongArch HW in the >> foreseeable future, with less than maybe 40 bits of VA? If the answer >> is "no" then a TRY_EMPTY_VM_SPACE near the 40-bit VA ceiling would be >> appropriate. Otherwise you may have to choose a value near or even >> below a 32-bit VA's upper limit: according to the ISA manual Volume >> 1, Section 2.1.5, "typical VALEN is in the range of [40, 48]"; also >> see Section 5.2.3, RDVA can be as large as 8, so the actual VA space >> could theoretically be as narrow as 40-8=32 bits. > > > Yes, I agree with your point of view this is looking for a "likely > free" virtual memory space. > > > But if I want to support chips with less than 40-bit virtual > addresses, then the value of this macro needs to be set small. > > I think setting this value small will increase the probability of > virtual address mapping failure. Not exactly; in case the TYPE_EMPTY_VM_SPACE address happen to be occupied, the mmap will still return something else that's nonzero (consult mmap's man page for details), and will not just blow the process up straight away. But... > > Chips with less than 40-bit virtual address space are small chips for > embedded use. > > The purpose of pch is to make compilation faster, but I think we > rarely compile on embedded systems. So this situation may not be > within our consideration. > Everything makes more sense with this context. Now put these justification into the commit message (after a little bit of rewording maybe) and I think we're good to go then ;-)