Re: [PATCH 00/13] Implement support for IBS virtualization

[Manali Shukla <manali.shukla@amd.com>]

On 9/11/2023 6:02 PM, Manali Shukla wrote:
> On 9/8/2023 7:01 PM, Peter Zijlstra wrote:
>> On Thu, Sep 07, 2023 at 09:19:51PM +0530, Manali Shukla wrote:
>>
>>>> I'm not sure I'm fluent in virt speak (in fact, I'm sure I'm not). Is
>>>> the above saying that a host can never IBS profile a guest?
>>>
>>> Host can profile a guest with IBS if VIBS is disabled for the guest. This is
>>> the default behavior. Host can not profile guest if VIBS is enabled for guest.
>>>
>>>>
>>>> Does the current IBS thing assert perf_event_attr::exclude_guest is set?
>>>
>>> Unlike AMD core pmu, IBS doesn't have Host/Guest filtering capability, thus
>>> perf_event_open() fails if exclude_guest is set for an IBS event.
>>
>> Then you must not allow VIBS if a host cpu-wide IBS counter exists.
>>
>> Also, VIBS reads like it can be (ab)used as a filter.
> 
> I think I get your point: If host IBS with exclude_guest=0 doesn't capture
> guest samples because of VIBS, it is an unintended behavior.
> 
> But if a guest cannot use IBS because a host is using it, that is also
> unacceptable behavior.
> 
> Let me think over it and come back.
> 
> - Manali

Hi Peter,

Apologies for the delay in response. It took me a while to think about
various possible solutions and their feasibility.

Problem with current design is, exclude_guest setting of the host IBS event is
not honored. Essentially, guest samples become invisible to the host IBS even
when exclude_guest=0, when VIBS is enabled on the guest.

Solution 1:
Enforce exclude_guest=1 for host IBS when hw supports VIBS, i.e.

        if (cpuid[VIBS])
                enforce exclude_guest=1
        else
                enforce exclude_guest=0

Disable/enable host IBS at VM Entry/VM Exit if cpuid[VIBS] is set and an active
IBS event exists on that cpu.

The major downside of this approach is, it will break all currently working
scripts which are using perf_event_open() to start an ibs event, since new
kernel will suddenly start failing for IBS events with exclude_guest=0. The
other issue is that the host with cpuid[VIBS] set, would not allow profiling any
guest from the host.

Solution 1.1:
This is an extension to Solution 1. Instead of keying off based on just
cpuid[VIBS] bit, introduce a kvm-amd module parameter and use both:

         if (cpuid[VIBS] && kvm-amd.ko loaded with vibs=1)
                enforce exclude_guest=1
         else
                enforce exclude_guest=0

KVM AMD vibs module parameter determines whether guest will be able to use VIBS
or not.  The kvm-amd.ko should be loaded with vibs=0, if a host wants to profile
guest and the kvm-amd.ko should be loaded with vibs=1, if a guest wants to use
VIBS. However, both are mutually exclusive.

The issue of digressing from current exclude_guest behavior remains with this
solution. Other issues are,
1) If the host IBS is active, with vibs=0, reloading of the kvm-amd.ko with
   vibs=1 will fail until IBS is running on the host.
2) If the host IBS is active, with vibs=1, reloading of the kvm-amd.ko with
   vibs=0 will fail until IBS is running on the host.

Solution 2:
Dynamically disable/enable VIBS per vCPU basis, i.e. when a host IBS is active,
guest will not be able to use VIBS _for that vCPU_. If the host is not using
IBS, VIBS will be enabled at VM Entry.

Although this solution does not digress from existing exclude_guest behavior, it
has its own limitations:
1) VIBS inside the guest is unreliable because host IBS can dynamically change
   VIBS behavior.
2) It works only for SVM and SEV guests, but not for SEV-ES and SEV-SNP guests
   since there is no way to disable VIBS dynamically.

From all the above solutions, we would be more inclined on implementing
solution 1.1.

-Manali