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

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On 10/16/2019 7:26 PM, Paolo Bonzini wrote:
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:

Right, whether enabling split-lock detection is made by the administrator. The administrator is supposed to know the consequence of enabling it. Enabling it means don't want any split-lock happens in userspace, of course VMM softwares are under control.

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.

I don't catch you about the unsafe?

If host disables split-lock detection dynamically, then the MST_TEST_CTL.split_lock is clear in the hardware and we can use the PV interface to notify the guest so that guest knows it loses the capability of split-lock detection. In this case, I think safety is meaningless for both host and guest.

Paolo




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