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X-CSE-ConnectionGUID: oLzuGR/LQUudTxwjiq0ucg== X-CSE-MsgGUID: pdwn1rgGTkW8cke+1BwxKQ== X-IronPort-AV: E=McAfee;i="6700,10204,11418"; a="47585228" X-IronPort-AV: E=Sophos;i="6.15,249,1739865600"; d="scan'208";a="47585228" Received: from orviesa005.jf.intel.com ([10.64.159.145]) by orvoesa109.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Apr 2025 09:06:23 -0700 X-CSE-ConnectionGUID: jprk9LZTRiGvaEpFCgNi7g== X-CSE-MsgGUID: 4izxWN+ZSo2csuyrRFsyNA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.15,249,1739865600"; d="scan'208";a="139055788" Received: from sramkris-mobl1.amr.corp.intel.com (HELO [10.124.223.107]) ([10.124.223.107]) by orviesa005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Apr 2025 09:06:21 -0700 Message-ID: Date: Tue, 29 Apr 2025 09:06:19 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 11/14] x86: add KHO support To: Mike Rapoport Cc: Changyuan Lyu , linux-kernel@vger.kernel.org, akpm@linux-foundation.org, anthony.yznaga@oracle.com, arnd@arndb.de, ashish.kalra@amd.com, benh@kernel.crashing.org, bp@alien8.de, catalin.marinas@arm.com, corbet@lwn.net, dave.hansen@linux.intel.com, devicetree@vger.kernel.org, dwmw2@infradead.org, ebiederm@xmission.com, graf@amazon.com, hpa@zytor.com, jgowans@amazon.com, kexec@lists.infradead.org, krzk@kernel.org, linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, luto@kernel.org, mark.rutland@arm.com, mingo@redhat.com, pasha.tatashin@soleen.com, pbonzini@redhat.com, peterz@infradead.org, ptyadav@amazon.de, robh@kernel.org, rostedt@goodmis.org, saravanak@google.com, skinsburskii@linux.microsoft.com, tglx@linutronix.de, thomas.lendacky@amd.com, will@kernel.org, x86@kernel.org References: <20250411053745.1817356-1-changyuanl@google.com> <20250411053745.1817356-12-changyuanl@google.com> <35c58191-f774-40cf-8d66-d1e2aaf11a62@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/29/25 01:06, Mike Rapoport wrote: > On Mon, Apr 28, 2025 at 03:05:55PM -0700, Dave Hansen wrote: >> On 4/10/25 22:37, Changyuan Lyu wrote: >>> From: Alexander Graf >>> >>> +/* >>> + * If KHO is active, only process its scratch areas to ensure we are not >>> + * stepping onto preserved memory. >>> + */ >> >> Same thing on the imperative voice here. >> >> I'm also not fully understanding the comment. Do these "scratch" regions >> basically represent all the memory that's not being handed over? It's >> not obvious. > > Scratch memory represents areas created at the boot of the first kernel and > it's known that scratch areas won't contain any memory that's being handed > over. > >>> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c >>> index 57120f0749cc3..c314212a5ecd5 100644 >>> --- a/arch/x86/kernel/e820.c >>> +++ b/arch/x86/kernel/e820.c >>> @@ -1300,6 +1300,24 @@ void __init e820__memblock_setup(void) >>> memblock_add(entry->addr, entry->size); >>> } >>> >>> + /* >>> + * At this point with KHO we only allocate from scratch memory. >>> + * At the same time, we configure memblock to only allow >>> + * allocations from memory below ISA_END_ADDRESS which is not >>> + * a natural scratch region, because Linux ignores memory below >>> + * ISA_END_ADDRESS at runtime. Beside very few (if any) early >>> + * allocations, we must allocate real-mode trapoline below >> >> trampoline ^ >> >>> + * ISA_END_ADDRESS. >>> + * >>> + * To make sure that we can actually perform allocations during >>> + * this phase, let's mark memory below ISA_END_ADDRESS as scratch >>> + * so we can allocate from there in a scratch-only world. >>> + * >>> + * After real mode trampoline is allocated, we clear scratch >>> + * marking from the memory below ISA_END_ADDRESS >>> + */ >>> + memblock_mark_kho_scratch(0, ISA_END_ADDRESS); >> >> This isn't making a whole ton of sense to me. >> >> Is this *only* to facilitate possible users that need > allocations? If so, please say that. >> >> I _think_ this is trying to say that KHO kernels are special and are >> trying to only allocate from scratch areas. But > allocations are both necessary and not marked by KHO _as_ a scratch area >> which causes a problem. > > Yes :) So, on both of these, could the submitters please add or revise the comments to make it more clear?