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SubjectRe: [PATCH v5] x86/speculation/l1tf: Add KConfig for setting the L1D cache flush mode
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 <> wrote:
> >
> > Abhishek Bhardwaj <> 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

Anyway, I think there is a higher level issue you are poking at that might be
better addressed by talking about it directly.


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