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On Thu, Jun 09, 2022 at 12:09:18PM +0200, 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 > > > > 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 > > > > Signed-off-by: Roger Pau Monné > > > > --- > > > > 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. > > > > > 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? > > Thanks, Roger. >