Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock

From: Paolo Bonzini
Date: Wed Oct 16 2019 - 07:26:40 EST

On 16/10/19 13:23, Xiaoyao Li wrote:
> KVM always traps #AC, and only advertises split-lock detection to guest
> when the global variable split_lock_detection_enabled in host is true.
> - If guest enables #AC (CPL3 alignment check or split-lock detection
> enabled), injecting #AC back into guest since it's supposed capable of
> handling it.
> - If guest doesn't enable #AC, KVM reports #AC to userspace (like other
> unexpected exceptions), and we can print a hint in kernel, or let
> userspace (e.g., QEMU) tell the user guest is killed because there is a
> split-lock in guest.
> In this way, malicious guests always get killed by userspace and old
> sane guests cannot survive as well if it causes split-lock. If we do
> want old sane guests work we have to disable the split-lock detection
> (through booting parameter or debugfs) in the host just the same as we
> want to run an old and split-lock generating userspace binary.

Old guests are prevalent enough that enabling split-lock detection by
default would be a big usability issue. And even ignoring that, you
would get the issue you describe below:

> But there is an issue that we advertise split-lock detection to guest
> based on the value of split_lock_detection_enabled to be true in host,
> which can be turned into false dynamically when split-lock happens in
> host kernel.

... which means that supposedly safe guests become unsafe, and that is bad.

> This causes guest's capability changes at run time and I
> don't if there is a better way to inform guest? Maybe we need a pv
> interface?

Even a PV interface would not change the basic fact that a supposedly
safe configuration becomes unsafe.