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Wysocki" , Mika Westerberg , Krzysztof =?utf-8?Q?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, Benoit =?iso-8859-1?Q?Gr=E9goire?= , Hui Wang , stable@vger.kernel.org, "Rafael J . Wysocki" Subject: Re: [PATCH v5 1/2] x86/PCI: Ignore E820 reservations for bridge windows on newer systems Message-ID: <20211022012034.GA2703195@bhelgaas> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <73aeec22-2ec7-ff21-5c89-c13f2e90a213@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > 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. > > 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. Bjorn