From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Fri, 20 Mar 2015 12:25:03 +0000 Subject: [PATCH v3] arm64: enforce x1|x2|x3 == 0 upon kernel entry as per boot protocol In-Reply-To: References: <20150319104100.GB18473@leverpostej> <1426762852-13699-1-git-send-email-ard.biesheuvel@linaro.org> <20150319133612.GD18473@leverpostej> <20150320114142.GF14766@leverpostej> Message-ID: <20150320122502.GA1474@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Fri, Mar 20, 2015 at 11:45:17AM +0000, Ard Biesheuvel wrote: > On 20 March 2015 at 12:41, Mark Rutland wrote: > >> >> + if (boot_args[1] || boot_args[2] || boot_args[3]) { > >> >> + pr_err("WARNING: boot protocol violation detected (x1 == %llx, x2 == %llx, x3 == %llx)\n", > >> >> + boot_args[1], boot_args[2], boot_args[3]); > >> >> + pr_err("WARNING: your bootloader may fail to load newer kernels\n"); > >> > > >> > If we ever decide to use x1-x3 for something, and try to boot an older > >> > kernel, that warning is going to be a bit misleading. That could matter > >> > for VMs where we're going to see old kernel images for a long time. > >> > > >> > I would like the warning to mention that could be the case. > >> > > >> > It would also be nice if the message were consistently spaced regardless > >> > of the values of x1-x3, so we should zero-pad them (and as that takes a > >> > resonable amount of space, let's give them a line each). > >> > > >> > So could we change the warning to be something like: > >> > > >> > pr_err("WARNING: x1-x3 nonzero in violation of boot protocol:\n" > >> > "\tx1: %016llx\n\tx2: %016llx\n\tx3: %016llx\n" > >> > "This indicates a broken bootloader or old kernel\n", > >> > boot_args[1], boot_args[2], boot_args[3]); > >> > > >> > >> OK, I have applied this change. > >> > >> But I would like to note that we should probably only extend the boot > >> protocol in a way that would not trigger this on older kernels in the > >> first place. > >> I.e., assign a bit in the flags field in the header, which indicates > >> whether some boot protocol enhancement is supported by the kernel > >> being loaded, and only allow x1/x2/x3 to be non-zero if said > >> enhancement defines that. > > > > Good point. > > > > Given that, if you want to restore your original last line, that would > > be fine with me (and my Ack still applies). > > > > I think it's fine to leave it as is Yup, and this is sitting pretty on the arm64 devel branch. Ard: I also pushed a kvm-bounce-page branch for you. Next step would be to merge everything into for-next/core and put your VA changes on top of that. I'd appreciate a sanity check of the current branch first, though! Cheers, Will