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Re: [PATCH v2 3/3] x86/vmx: implement Notify VM Exit



On 6/9/2022 6:09 PM, Roger Pau Monné wrote:
On Thu, Jun 09, 2022 at 03:39:33PM +0800, Xiaoyao Li wrote:
On 6/9/2022 3:04 PM, Tian, Kevin wrote:
+Chenyi/Xiaoyao who worked on the KVM support. Presumably
similar opens have been discussed in KVM hence they have the
right background to comment here.

From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
Sent: Thursday, May 26, 2022 7:12 PM

Under certain conditions guests can get the CPU stuck in an unbounded
loop without the possibility of an interrupt window to occur on
instruction boundary.  This was the case with the scenarios described
in XSA-156.

Make use of the Notify VM Exit mechanism, that will trigger a VM Exit
if no interrupt window occurs for a specified amount of time.  Note
that using the Notify VM Exit avoids having to trap #AC and #DB
exceptions, as Xen is guaranteed to get a VM Exit even if the guest
puts the CPU in a loop without an interrupt window, as such disable
the intercepts if the feature is available and enabled.

Setting the notify VM exit window to 0 is safe because there's a
threshold added by the hardware in order to have a sane window value.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
   - Properly update debug state when using notify VM exit.
   - Reword commit message.
---
This change enables the notify VM exit by default, KVM however doesn't
seem to enable it by default, and there's the following note in the
commit message:

"- There's a possibility, however small, that a notify VM exit happens
     with VM_CONTEXT_INVALID set in exit qualification. In this case, the
     vcpu can no longer run. To avoid killing a well-behaved guest, set
     notify window as -1 to disable this feature by default."

It's not obviously clear to me whether the comment was meant to be:
"There's a possibility, however small, that a notify VM exit _wrongly_
happens with VM_CONTEXT_INVALID".

It's also not clear whether such wrong hardware behavior only affects
a specific set of hardware,

I'm not sure what you mean for a specific set of hardware.

We make it default off in KVM just in case that future silicon wrongly sets
VM_CONTEXT_INVALID bit. Becuase we make the policy that VM cannot continue
running in that case.

For the worst case, if some future silicon happens to have this kind silly
bug, then the existing product kernel all suffer the possibility that their
VM being killed due to the feature is default on.

That's IMO a weird policy.  If there's such behavior in any hardware
platform I would assume Intel would issue an errata, and then we would
just avoid using the feature on affected hardware (like we do with
other hardware features when they have erratas).

If we applied the same logic to all new Intel features we won't use
any of them.  At least in Xen there are already combinations of vmexit
conditions that will lead to the guest being killed.

The reason is that, currently no case will set the VM_CONTEXT_INVALID bit, people in KVM community are cautious on the uncertainty. No one in what case the VM_CONTEXT_INVALID will be in the future.

Anyway, that's only the worry from KVM reviewers.

in a way that we could avoid enabling
notify VM exit there.

There's a discussion in one of the Linux patches that 128K might be
the safer value in order to prevent false positives, but I have no
formal confirmation about this.  Maybe our Intel maintainers can
provide some more feedback on a suitable notify VM exit window
value.

The 128k is the internal threshold for SPR silicon. The internal threshold
is tuned by Intel for each silicon, to make sure it's big enough to avoid
false positive even when user set vmcs.notify_window to 0.

However, it varies for different processor generations.

What is the suitable value is hard to say, it depends on how soon does VMM
want to intercept the VM. Anyway, Intel ensures that even value 0 is safe.

Ideally we need a fixed default value that's guaranteed to work on all
possible hardware that supports the feature, or alternatively a way to
calculate a sane default window based on the hardware platform.

Could we get some wording added to the ISE regarding 0 being a
suitable default value to use because hardware will add a threshold
internally to make the value safe?

We will work with internal architects on this.

Thanks, Roger.




 


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