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Peter Anvin" , Alexander Graf , Boris Ostrovsky , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue Oct 14, 2025 at 6:06 PM UTC, David Kaplan wrote: >> From: Josh Poimboeuf >> > There are several use cases that will benefit from dynamic mitigations= : >> > >> > Use Cases >> > --------- >> > 1. Runtime Policy >> > >> > Some workflows rely on booting a generic kernel before customizing the= system. >> > cloud-init is a popular example of this where a VM is started typicall= y with >> > default settings and then is customized based on a customer-provided >> > configuration file. >> >> I'm not really a fan of this. It adds complexity to some areas that are >> already struggling with too much complexity. >> >> IMO this would need some REALLY strong justification, more than just >> "hey, this makes things more convenient." >> >> The mitigations should be a "set it and forget it" thing. I don't see >> anything here which justifies the considerable maintenance burden this >> would add for all existing and future mitigations. >> > > The problem is there are environments like the one outlined where you can= 't just 'set it and forget it' because the kernel needs it set at boot-time= , but in these environments you don't know how to configure the system unti= l much later in boot. So you end up running with the default settings all = the time, even if you don't need them. And the default settings can have s= ignificant performance impacts in many cases. > > The cloud guys on this thread may be able to offer some additional color = here since I believe that's where you're most likely to have this situation= . > > --David Kaplan There's definitely a desire for more dynamic control at Google too, similar to what Boris said. Getting the config right is tricky and let's not forget it's fundamentally a moving target as David noted. Even without dependence on cloud-init type stuff, it's very useful to be able to change mitigations on a timescale that's faster than we can reboot hosts.=C2=A0