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(2001-1c00-0c1e-bf00-1054-9d19-e0f0-8214.cable.dynamic.v6.ziggo.nl. [2001:1c00:c1e:bf00:1054:9d19:e0f0:8214]) by smtp.gmail.com with ESMTPSA id ne2sm2519895ejc.44.2021.10.29.01.10.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 29 Oct 2021 01:10:06 -0700 (PDT) Message-ID: <82035130-d810-9f0b-259e-61280de1d81f@redhat.com> Date: Fri, 29 Oct 2021 10:10:05 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH v5 1/2] x86/PCI: Ignore E820 reservations for bridge windows on newer systems Content-Language: en-US From: Hans de Goede To: Bjorn Helgaas Cc: "Rafael J . Wysocki" , Mika Westerberg , =?UTF-8?Q?Krzysztof_Wilczy=c5=84ski?= , Bjorn Helgaas , Myron Stowe , Juha-Pekka Heikkila , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H . Peter Anvin" , linux-acpi@vger.kernel.org, linux-pci@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, =?UTF-8?Q?Benoit_Gr=c3=a9goire?= , Hui Wang , stable@vger.kernel.org, "Rafael J . Wysocki" References: <20211022012034.GA2703195@bhelgaas> <75d1ef5a-13d9-9a67-0139-90b27b084c84@redhat.com> In-Reply-To: <75d1ef5a-13d9-9a67-0139-90b27b084c84@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org Hi Bjorn, On 10/22/21 11:53, Hans de Goede wrote: > Hi Bjorn, > > On 10/22/21 03:20, Bjorn Helgaas wrote: >> On Thu, Oct 21, 2021 at 07:15:57PM +0200, Hans de Goede wrote: >>> On 10/20/21 23:14, Bjorn Helgaas wrote: >>>> On Wed, Oct 20, 2021 at 12:23:26PM +0200, Hans de Goede wrote: >>>>> On 10/19/21 23:52, Bjorn Helgaas wrote: >>>>>> On Thu, Oct 14, 2021 at 08:39:42PM +0200, Hans de Goede wrote: >>>>>>> Some BIOS-es contain a bug where they add addresses which map to system >>>>>>> RAM in the PCI host bridge window returned by the ACPI _CRS method, see >>>>>>> commit 4dc2287c1805 ("x86: avoid E820 regions when allocating address >>>>>>> space"). >>>>>>> >>>>>>> To work around this bug Linux excludes E820 reserved addresses when >>>>>>> allocating addresses from the PCI host bridge window since 2010. >>>>>>> ... >>>> >>>>>> I haven't seen anybody else eager to merge this, so I guess I'll stick >>>>>> my neck out here. >>>>>> >>>>>> I applied this to my for-linus branch for v5.15. >>>>> >>>>> Thank you, and sorry about the build-errors which the lkp >>>>> kernel-test-robot found. >>>>> >>>>> I've just send out a patch which fixes these build-errors >>>>> (verified with both .config-s from the lkp reports). >>>>> Feel free to squash this into the original patch (or keep >>>>> them separate, whatever works for you). >>>> >>>> Thanks, I squashed the fix in. >>>> >>>> HOWEVER, I think it would be fairly risky to push this into v5.15. >>>> We would be relying on the assumption that current machines have all >>>> fixed the BIOS defect that 4dc2287c1805 addressed, and we have little >>>> evidence for that. >>> >>> It is a 10 year old BIOS defect, so hopefully anything from 2018 >>> or later will not have it. >> >> We can hope. AFAIK, Windows allocates space top-down, while Linux >> allocates bottom-up, so I think it's quite possible these defects >> would never be discovered or fixed. In any event, I don't think we >> have much evidence either way. > > Ack. > >>>> I'm not sure there's significant benefit to having this in v5.15. >>>> Yes, the mainline v5.15 kernel would work on the affected machines, >>>> but I suspect most people with those machines are running distro >>>> kernels, not mainline kernels. >>> >>> Fedora and Arch do follow mainline pretty closely and a lot of >>> users are affected by this (see the large number of BugLinks in >>> the commit). >>> >>> I completely understand why you are reluctant to push this out, but >>> your argument about most distros not running mainline kernels also >>> applies to chances of people where this may cause a regression >>> running mainline kernels also being quite small. >> >> True. >> >>>> This issue has been around a long time, so it's not like a regression >>>> that we just introduced. If we fixed these machines and regressed >>>> *other* machines, we'd be worse off than we are now. >>> >>> If we break one machine model and fix a whole bunch of other machines >>> then in my book that is a win. Ideally we would not break anything, >>> but we can only find out if we actually break anything if we ship >>> the change. >> >> I'm definitely not going to try the "fix many, break one" argument on >> Linus. Of course we want to fix systems, but IMO it's far better to >> leave a system broken than it is to break one that used to work. > > Right, what I meant to say with "a win" is a step in the right direction, > we definitely must address any regressions coming from this change as > soon as we learn about them. > >>>> In the meantime, here's another possibility for working around this. >>>> What if we discarded remove_e820_regions() completely, but aligned the >>>> problem _CRS windows a little more? The 4dc2287c1805 case was this: >>>> >>>> BIOS-e820: 00000000bfe4dc00 - 00000000c0000000 (reserved) >>>> pci_root PNP0A03:00: host bridge window [mem 0xbff00000-0xdfffffff] >>>> >>>> where the _CRS window was of size 0x20100000, i.e., 512M + 1M. At >>>> least in this particular case, we could avoid the problem by throwing >>>> away that first 1M and aligning the window to a nice 3G boundary. >>>> Maybe it would be worth giving up a small fraction (less than 0.2% in >>>> this case) of questionable windows like this? >>> >>> The PCI BAR allocation code tries to fall back to the BIOS assigned >>> resource if the allocation fails. That BIOS assigned resource might >>> fall outside of the host bridge window after we round the address. >>> >>> My initial gut instinct here is that this has a bigger chance >>> of breaking things then my change. >>> >>> In the beginning of the thread you said that ideally we would >>> completely stop using the E820 reservations for PCI host bridge >>> windows. Because in hindsight messing with the windows on all >>> machines just to work around a clear BIOS bug in some was not a >>> good idea. >>> >>> This address-rounding/-aligning you now suggest, is again >>> messing with the windows on all machines just to work around >>> a clear BIOS bug in some. At least that is how I see this. >> >> That's true. I assume Red Hat has a bunch of machines and hopefully >> an archive of dmesg logs from them. Those logs should contain good >> E820 and _CRS information, so with a little scripting, maybe we could >> get some idea of what's out there. > > We do have a (large-ish) test-lab, but that contains almost exclusively > servers, where as the original problem was on Dell Precision laptops. > > Also I'm not sure if I can get aggregate data from the lab's machines. > I can reserve time on any model we have to debug specific problems, > but that is targeting one specific model. I'll ask around about this. So I had another idea to get us a whole bunch of dmesg outputs and that is to use the database collected by linux-hardware.org . The dmesg were already individually accessible by selecting a specific model machine, but I asked them if they could do a dump and I just got an email that a dmesg dump is now available here: https://github.com/linuxhw/Dmesg Note be careful with the size of the repository - it will take ~3 gigabytes of network traffic and ~20 gigabytes of space on the drive to checkout it. So if you want dmesg outputs to grep through for e820 / host-bridge-window info, here you go. Regards, Hans