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d06av22.portsmouth.uk.ibm.com (d06av22.portsmouth.uk.ibm.com [9.149.105.58]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 29C4ttVb59179490 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 12 Oct 2022 04:55:55 GMT Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BA6434C04E; Wed, 12 Oct 2022 04:55:55 +0000 (GMT) Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 9679D4C040; Wed, 12 Oct 2022 04:55:50 +0000 (GMT) Received: from [9.43.80.233] (unknown [9.43.80.233]) by d06av22.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 12 Oct 2022 04:55:50 +0000 (GMT) Message-ID: Date: Wed, 12 Oct 2022 10:25:38 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.2.1 Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support To: Eric DeVolder , Borislav Petkov Cc: linux-kernel@vger.kernel.org, x86@kernel.org, kexec@lists.infradead.org, ebiederm@xmission.com, dyoung@redhat.com, bhe@redhat.com, vgoyal@redhat.com, tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com, hpa@zytor.com, nramas@linux.microsoft.com, thomas.lendacky@amd.com, robh@kernel.org, efault@gmx.de, rppt@kernel.org, david@redhat.com, konrad.wilk@oracle.com, boris.ostrovsky@oracle.com References: <20220909210509.6286-1-eric.devolder@oracle.com> <20220909210509.6286-8-eric.devolder@oracle.com> <7d0697ee-d6e8-dad1-ca77-f2e8104b0b0f@linux.ibm.com> <915f05fd-dbbc-e012-e3d2-e40ca13d4728@oracle.com> Content-Language: en-US From: Sourabh Jain In-Reply-To: <915f05fd-dbbc-e012-e3d2-e40ca13d4728@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: esBB5iKjh4iSYRX_WRSpRsQdhj8GR7EB X-Proofpoint-GUID: FM8bJZAssB732A4HCTs6PSq1k5gYseCg Content-Transfer-Encoding: 8bit X-Proofpoint-UnRewURL: 0 URL was un-rewritten MIME-Version: 1.0 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.895,Hydra:6.0.545,FMLib:17.11.122.1 definitions=2022-10-12_01,2022-10-11_02,2022-06-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxscore=0 suspectscore=0 phishscore=0 adultscore=0 priorityscore=1501 mlxlogscore=999 impostorscore=0 bulkscore=0 malwarescore=0 lowpriorityscore=0 clxscore=1015 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2209130000 definitions=main-2210120029 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/10/22 01:30, Eric DeVolder wrote: > > > On 10/4/22 04:10, Sourabh Jain wrote: >> >> On 30/09/22 21:06, Eric DeVolder wrote: >>> >>> >>> On 9/28/22 11:07, Borislav Petkov wrote: >>>> On Tue, Sep 13, 2022 at 02:12:31PM -0500, Eric DeVolder wrote: >>>>> This topic was discussed previously >>>>> https://lkml.org/lkml/2022/3/3/372. >>>> >>>> Please do not use lkml.org to refer to lkml messages. We have a >>>> perfectly fine archival system at lore.kernel.org. You simply do >>>> >>>> https://lore.kernel.org/r/ >>>> >>>> when you want to point to a previous mail. >>> >>> ok, thanks for pointing that out to me. >>>> >>>>> David points out that terminology is tricky here due to differing >>>>> behaviors. >>>>> And perhaps that is your point in asking for guidance text. It can be >>>>> complicated >>>> >>>> Which means you need an explanation how to use this even more. >>>> >>>> And why is CONFIG_CRASH_MAX_MEMORY_RANGES even a Kconfig item and not >>>> something you discover from the hardware? >>> >>> No, is the short answer. >>> >>>> >>>> Your help text talks about System RAM entries in /proc/iomem which >>>> means >>>> that those entries are present somewhere in the kernel and you can >>>> read >>>> them out and do the proper calculations dynamically instead of >>>> doing the >>>> static CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES thing. >>> >>> The intent is to compute the max size buffer needed to contain a >>> maximum populated elfcorehdr, which is primarily based on the number >>> of CPUs and memory regions. Thus far I (and others involved) have >>> not found a kernel method to determine the maximum number of memory >>> regions possible (if you are aware of one, please let me know!). >>> Thus CONFIG_CRASH_MAX_MEMORY_RANGES was born (rather borrowed from >>> kexec-tools). >>> >>> So no dynamic computation is possible, yet. >>> >>>> >>>>> , but it all comes down to System RAM entries. >>>>> >>>>> I could perhaps offer an overly simplified example such that for >>>>> 1GiB block >>>>> size, for example, the CRASH_MAX_MEMORY_RANGES of 32768 would >>>>> allow for 32TiB >>>>> of memory? >>>> >>>> Yes, and stick it somewhere in Documentation/admin-guide/kdump/ and >>>> refer to it in that help text so that people can find it and read >>>> how to >>>> use your new option. >>>> >>> ok >>> >>>>> The kbuf.bufsz value is obtained via a call to >>>>> prepare_elf_headers(); I can >>>>> not initialize it at its declaration. >>>> >>>> Sorry, I meant this: >>>> >>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c >>>> index 8fc7d678ac72..ee6fd9f1b2b9 100644 >>>> --- a/arch/x86/kernel/crash.c >>>> +++ b/arch/x86/kernel/crash.c >>>> @@ -395,8 +395,9 @@ int crash_load_segments(struct kimage *image) >>>>       if (ret) >>>>           return ret; >>>>   -    image->elf_headers = kbuf.buffer; >>>> -    image->elf_headers_sz = kbuf.bufsz; >>>> +    image->elf_headers    = kbuf.buffer; >>>> +    image->elf_headers_sz    = kbuf.bufsz; >>>> +    kbuf.memsz        = kbuf.bufsz; >>>>     #if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG) >>>>       /* Ensure elfcorehdr segment large enough for hotplug changes */ >>>> @@ -407,9 +408,8 @@ int crash_load_segments(struct kimage *image) >>>>       image->elf_headers_sz = kbuf.memsz; >>>>       image->elfcorehdr_index = image->nr_segments; >>>>       image->elfcorehdr_index_valid = true; >>>> -#else >>>> -    kbuf.memsz = kbuf.bufsz; >>>>   #endif >>>> + >>>>       kbuf.buf_align = ELF_CORE_HEADER_ALIGN; >>>>       kbuf.mem = KEXEC_BUF_MEM_UNKNOWN; >>>>       ret = kexec_add_buffer(&kbuf); >>>> >>> ok >>> >>>>> I'm at a loss as to what to do differently here. You've raised >>>>> this issue >>>>> before and I went back and looked at the suggestions then and I >>>>> don't see >>>>> how that applies to this situation. How is this situation >>>>> different than the >>>>> #ifdef CONFIG_KEXEC_FILE that immediately preceeds it? >>>> >>>> See the diff at the end. I'm not saying this is how you should do it >>>> but it should give you a better idea. The logic being, the functions >>>> in the .c file don't really need ifdeffery around them - you're adding >>>> 1-2 functions and crash.c is not that big - so they can be built in >>>> unconditionally. You'd need the ifdeffery *in the header only* when >>>> crash.c is not being built. >>> ok; I've overlooked that scenario. >>>> >>>> But I've done it with ifdeffery in the .c file now because yes, the >>>> kexec code is a minefield of ifdeffery. Hell, there's ifdeffery >>>> even in >>>> the headers for structs. Ifdeffery you don't really need. Someone >>>> should >>>> clean that up and simplify this immensely. >>> >>> ok >>> >>>> >>>>> Currently there is a concurrent effort for PPC support by Sourabh >>>>> Jain, and in that effort arch_map_crash_pages() is using __va(paddr). >>>> >>>> Why? >>>> >>>>> I do not know the nuances between kmap_local_page() and __va() to >>>>> answer the question. >>>> >>>> kmap_local_page() is a generic interface and it should work on any >>>> arch. >>>> >>>> And it is documented even: >>>> >>>> $ git grep kmap_local_page Documentation/ >>>> >>>>> If kmap_local_page() works for all archs, then I'm happy to drop >>>>> these >>>>> arch_ variants and use it directly. >>>> >>>> Yes, pls do. >>> >>> I'll check with Sourabh to see if PPC can work with kmap_local_page(). >> I think kmap_local_page do support on  PowerPC. But can you explain >> why we need this >> function here, aren't the reserve memory already available to use? > > On x86, attempts to access the elfcorehdr without mapping it did not > work (resulted > in a fault). > > Let me know if using kmap_local_page() in place of __va() in > arch_map_crash_pages(). > If it does, then I can eliminate arch_un/map_crash_pages() and use > kmap_local_page() > directly. Hello Eric, Atleast on ppc64 we have direct mapping available and hence just by doing page shift on physical address (__va) we can get valid virtual address on powerpc. In short we don't have to generate mapping again to access reserved region. Regardless let's go with kdump_local_page API, it is supported on powerpc. Thanks, Sourabh Jain