On Thu, Jul 09, 2020 at 12:42:57PM -0700, Doug Anderson wrote: > Hi, > > On Thu, Jul 9, 2020 at 3:51 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > > > Abhishek Bhardwaj <abhishekbh@xxxxxxxxxx> writes: > > > This change adds a new kernel configuration that sets the l1d cache > > > flush setting at compile time rather than at run time. > > > > > > The reasons for this change are as follows - > > > > > > - Kernel command line arguments are getting unwieldy. These parameters > > > are not a scalable way to set the kernel config. They're intended as a > > > super limited way for the bootloader to pass info to the kernel and > > > also as a way for end users who are not compiling the kernel themselves > > > to tweak the kernel behavior. > > > > > > - Also, if a user wants this setting from the start. It's a definite > > > smell that it deserves to be a compile time thing rather than adding > > > extra code plus whatever miniscule time at runtime to pass an > > > extra argument. > > > > > > - Finally, it doesn't preclude the runtime / kernel command line way. > > > Users are free to use those as well. > > > > TBH, I don't see why this is a good idea. > > > > 1) I'm not following your argumentation that the command line option is > > a poor Kconfig replacement. The L1TF mode is a boot time (module > > load time) decision and the command line parameter is there to > > override the carefully chosen and sensible default behaviour. > > When you say that the default behavior is carefully chosen and > sensible, are you saying that (in your opinion) there would never be a > good reason for someone distributing a kernel to others to change the > default? Certainly I agree that having the kernel command line > parameter is nice to allow someone to override whatever the person > building the kernel chose, but IMO it's not a good way to change the > default built-in to the kernel. > > The current plan (as I understand it) is that we'd like to ship > Chromebook kernels with this option changed from the default that's > there now. In your opinion, is that a sane thing to do? > > > > 2) You can add the desired mode to the compiled in (partial) kernel > > command line today. > > This might be easier on x86 than it is on ARM. ARM (and ARM64) > kernels only have two modes: kernel provides cmdline and bootloader > provides cmdline. There are out-of-mainline ANDROID patches to > address this but nothing in mainline. > > The patch we're discussing now is x86-only so it's not such a huge > deal, but the fact that combining the kernel and bootloader > commandline never landed in mainline for arm/arm64 means that this > isn't a super common/expected thing to do. > > > > 3) Boot loaders are well capable of handling large kernel command lines > > and the extra time spend for reading the parameter does not matter > > at all. > > Long command lines can still be a bit of a chore for humans to deal > with. Many times I've needed to look at "/proc/cmdline" and make > sense of it. The longer the command line is and the more cruft > stuffed into it the more of a chore it is. Yes, this is just one > thing to put in the command line, but if 10 different drivers all have > their "one thing" to put there it gets really long. If 100 different > drivers all want their one config option there it gets really really > long. IMO the command line should be a last resort place to put > things and should just contain: This takes me back to my years doing android kernel work for Intel, I'm glad those are over. Yes, the android kernel command lines got hideous, I think we even had patches to make the cmdline buffer bigger than the default was. >From a practical point of view the command line was part of the boot image and cryptography protected so it was a handy way to securely communicate parameters from the platform to the kernel, drivers and even just user mode. It got pretty ugly but, it worked (mostly). What I don't get is why pick on l1tf in isolation? There are a bunch of command line parameters similar to l1tf. Would a more general option make sense? Anyway, I think there is a higher level issue you are poking at that might be better addressed by talking about it directly. --mark
