Hi Jonathan, > From: Jonathan Cameron <jonathan.cameron@xxxxxxxxxx> > Sent: Friday, September 15, 2023 4:33 PM > To: Salil Mehta <salil.mehta@xxxxxxxxxx> > Cc: Russell King <linux@xxxxxxxxxxxxxxx>; Rafael J. Wysocki > <rafael@xxxxxxxxxx>; Ard Biesheuvel <ardb@xxxxxxxxxx>; James Morse > <james.morse@xxxxxxx>; linux-pm@xxxxxxxxxxxxxxx; loongarch@xxxxxxxxxxxxxxx; > linux-acpi@xxxxxxxxxxxxxxx; linux-arch@xxxxxxxxxxxxxxx; linux- > kernel@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux- > riscv@xxxxxxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx; Jean- > Philippe Brucker <jean-philippe@xxxxxxxxxx>; jianyong.wu@xxxxxxx; > justin.he@xxxxxxx > Subject: Re: [RFC PATCH v2 27/35] ACPICA: Add new MADT GICC flags fields > [code first?] > > On Fri, 15 Sep 2023 16:17:21 +0100 > Salil Mehta <salil.mehta@xxxxxxxxxx> wrote: > > > Hi Russel, > > Thanks for highlighting your concerns. > > > > > From: Russell King <linux@xxxxxxxxxxxxxxx> > > > Sent: Friday, September 15, 2023 2:43 PM > > > To: Salil Mehta <salil.mehta@xxxxxxxxxx> > > > Cc: Rafael J. Wysocki <rafael@xxxxxxxxxx>; Ard Biesheuvel > > > <ardb@xxxxxxxxxx>; Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>; James > > > Morse <james.morse@xxxxxxx>; linux-pm@xxxxxxxxxxxxxxx; > > > loongarch@xxxxxxxxxxxxxxx; linux-acpi@xxxxxxxxxxxxxxx; linux- > > > arch@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-arm- > > > kernel@xxxxxxxxxxxxxxxxxxx; linux-riscv@xxxxxxxxxxxxxxxxxxx; > > > kvmarm@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx; Jean-Philippe Brucker <jean- > > > philippe@xxxxxxxxxx>; jianyong.wu@xxxxxxx; justin.he@xxxxxxx > > > Subject: Re: [RFC PATCH v2 27/35] ACPICA: Add new MADT GICC flags fields > > > [code first?] > > > > > > On Fri, Sep 15, 2023 at 09:34:46AM +0000, Salil Mehta wrote: > > > > > > Note that the ACPI spec says enabled + online-capable isn't defined. > > > > > > > > > > > > "The information conveyed by this bit depends on the value of the > > > > > > Enabled bit. If the Enabled bit is set, this bit is reserved and > > > > > > must be zero." > > > > > > > > > > > > So, if x86 is doing something with the enabled && online-capable > > > > > > state (other than ignoring the online-capable) then technically it > > > > > > is doing something that the spec doesn't define > > > > > > > > > > And so it is wrong. > > > > > > > > Or maybe, specification has not been updated yet. code-first? > > > > > > What is the point in speculating. If you want to speculate about it, > > > fine, but please don't use speculation as a reason that "oh we need > > > to sort this out before we can merge the patches". > > > > [already replied in other thread but repeating it here] > > > > Sorry, I am not aware but I was suggesting this. Can we have this > > done for ARM first because there is a legitimate use-case. This > > can be done in parallel while other patches are getting reviewed. > > It would be great if they get accepted even in the current form. > > > > > > > This is precisely why engineers are bad at producing products. They > > > like to continually tweak the design, and the design never gets out > > > the door. You need someone who is a project manager to tell engineers > > > when to stop. Without a project manager to do that, eventually the > > > project fades into insignificance because it becomes no longer relevant > > > or has its funding cut. > > > > > > Hotplug VCPU on aarch64 feels exactly like that - it seems to be an > > > engineer project that is just going to for-ever rumble on and never > > > actually see the light of day. > > > > > > Sometimes things are not in single persons control. Yes, it is > > frustrating, I do understand that. > > > > > > > So please - stop speculating and lets get vCPU hotplug *actually* > > > delivered and usable. Even if it's not 100% perfect. > > > > We need to decide what is the criteria of acceptability and it can > > vary across organizations. It depends upon internal requirements. > > The issues what I pointed are, > > > > 1. Legacy OS will not boot on latest platform with hotplug support. > > - Try running older windows on ARM platform with hotplug support. > > - older windows will only see boot cpu with online-capable bit. > > - Will windows use _OSC to check compatibility? > > - We have verified this with older Linux and it only shows 1 CPU. > > 2. Hot(un)plug of cold-booted CPUs. > > - Its use-case is subjective. Maybe you can throw light on this. > > > > With current composition of bits both 1 & 2 cannot be supported > > simultaneously. > > > > It is perfectly okay to live with them while clearly indicating > > what we intend to support or are in process of supporting it. > > But we do need an open discussion about how to proceed. This is > > to avoid surprises later on. > > > > BTW, I am just trying to make every one aware of the problems. > > Step 1 - just allow growing (and shrinking back to initial > enabled cpus). That is fine with current specification and legacy > OS. We only assume CPUs that are hotplugged can later be removed. > That covers most use cases. Yes, we can do that for a moment (at least in qemu) and then not allow unplugging vCPUs which were cold plugged or allow it as a debugging feature but splash a warning. > So what effectively what Russell said. Enable what we can with > the specifications as they stand before getting distracted by > modifying them (again). Yes, agreed. Idea was to clearly highlight them. These can be discussed as part of separate thread in parallel - absolutely! Thanks Salil.
