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b=gMnOqFf/H8lhPoqz4mOg3LitZLgj2KTBI3SqJsUKod+fy5aHT9I/ZXO5klwgBMWo7f3aWo8UvSJGHrwFf11CsHQayVxmvAE+kt5HhzW6E01bpSmlWnHmdoM2r0TDvNp1VXIq0Y7BignpwHsSznJYQo18Ka7RLRlCisck3cUq5ag= Received: from DS0PR12MB8528.namprd12.prod.outlook.com (2603:10b6:8:160::6) by SN7PR12MB8147.namprd12.prod.outlook.com (2603:10b6:806:32e::5) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.7409.46; Mon, 1 Apr 2024 19:41:23 +0000 Received: from DS0PR12MB8528.namprd12.prod.outlook.com ([fe80::e24e:1e8e:1c0d:6e29]) by DS0PR12MB8528.namprd12.prod.outlook.com ([fe80::e24e:1e8e:1c0d:6e29%5]) with mapi id 15.20.7409.042; Mon, 1 Apr 2024 19:41:23 +0000 Message-ID: Date: Mon, 1 Apr 2024 14:41:18 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/6] Implement initial CXL Timeout & Isolation support Content-Language: en-US To: Dan Williams , linux-cxl@vger.kernel.org, linux-pci@vger.kernel.org Cc: dave@stgolabs.net, jonathan.cameron@huawei.com, dave.jiang@intel.com, alison.schofield@intel.com, vishal.l.verma@intel.com, ira.weiny@intel.com, bhelgaas@google.com, linux-mm@kvack.org References: <20240215194048.141411-1-Benjamin.Cheatham@amd.com> <65cea1bc6ac0c_5e9bf294ed@dwillia2-xfh.jf.intel.com.notmuch> <1287adbe-364f-431c-b33f-9d73e7829b6c@amd.com> <66019e1ec2153_2690d29440@dwillia2-mobl3.amr.corp.intel.com.notmuch> From: Ben Cheatham In-Reply-To: <66019e1ec2153_2690d29440@dwillia2-mobl3.amr.corp.intel.com.notmuch> Content-Type: text/plain; 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Responses inline. On 3/25/24 10:54 AM, Dan Williams wrote: > Ben Cheatham wrote: >> Hey Dan, >> >> Sorry that I went radio silent on this, I've been thinking about the approach to take >> from here on out. More below! >> >> On 2/15/24 5:43 PM, Dan Williams wrote: >> >> [snip] >> >>>> The series also introduces a PCIe port bus driver dependency on the CXL >>>> core. I expect to be able to remove that when when my team submits >>>> patches for a future rework of the PCIe port bus driver. >>> >>> We have time to wait for that work to settle. Do not make the job harder >>> in the near term by adding one more dependency to unwind. >>> >> >> For sure, I'll withhold any work I do that touches the port bus driver >> until that's sent upstream. I'll send anything that doesn't touch >> the port driver, i.e. CXL region changes, as RFC as well. >> >> [snip] >> >>> So if someone says, "yes I can tolerate losing a root port at a time and >>> I can tolerate deploying my workloads with userspace memory management, >>> and this is preferable to a reboot", then maybe Linux should entertain >>> CXL Error Isolation. Until such an end use case gains clear uptake it >>> seems too early to worry about plumbing the notification mechanism. >> >> So in my mind the primary use case for this feature is in a >> server/data center environment. Losing a root port and the attached >> memory is definitely preferable to a reboot in this environment, so I >> think that isolation would still be useful even if it isn't >> plug-and-play. > > That is not sufficient. This feature needs an implementation partner to > work through the caveats. > Got it, I'll come back to this if/when I have a customer willing to commit to this. >> I agree with your assessment of what has to happen before we can make >> this feature work. From what I understand these are the main >> requirements for making isolation work: > > Requirement 1 is "Find customer". > >> 1. The memory region can't be onlined as System RAM >> 2. The region needs to be mapped as device-dax >> 3. There need to be safeguards such that someone can't remap the >> region to System RAM with error isolation enabled (added by me) > > No, this policy does not belong in the kernel. > Understood. >> Considering these all have to do with mm, I think that's a good place >> to start. What I'm thinking of starting with is adding a module >> parameter (or config option) to enable isolation for CXL dax regions >> by default, as well as a sysfs option for toggling error isolation for >> the CXL region. When the module parameter is specified, the CXL region >> driver would create the region as a device-dax region by default >> instead of onlining the region as system RAM. The sysfs would allow >> toggling error isolation for CXL regions that are already device-dax. > > No, definitely not going to invent module paramter policy for this. The > policy to not online dax regions is already available. > >> That would cover requirements 1 & 2, but would still allow someone to >> reconfigure the region as a system RAM region with error isolation >> still enabled using sysfs/daxctl. To make sure the this doesn't >> happen, my plan is to have the CXL region driver automatically disable >> error isolation when the underlying memory region is going offline, >> then check to make sure the memory isn't onlined as System RAM before >> re-enabling isolation. > > Why would you ever need to disable isolation? If it is present it at > least nominally allows software to keep running vs hang awaiting a > watchdog-timer reboot. > That's a good point. I think I was just looking at this too long and got a bit lost in the sauce, I'll keep this in mind for if/when I come back to this. >> So, with that in mind, isolation would most likely go from something >> that is enabled by default when compiled in to a feature for >> correctly-configured CXL regions that has to be enabled by the end >> user. If that is sounds good, here's what my roadmap would be going >> forward: > > Again, this needs a customer to weigh the mitigations that the kernel > needs to carry. > >> 1. Enable creating device-dax mapped CXL regions by default > > Already exists. > >> 2. Add support for checking a CXL region meets the requirements >> for enabling isolation (i.e. all devices in >> dax region(s) are device-dax) > > Regions should not know or care if isolation is enabled, the > implications are identical to force unbinding the region driver. > Got it. Thanks, Ben >> 3. Add back in the error isolation enablement and notification >> mechanisms in this patch series > > Not until it is clear that this feature has deployment value.