RE: [PATCH] i386: Disable BTS and PEBS

[Duan, Zhenzhong]

>-----Original Message-----
>From: Sean Christopherson <seanjc@google.com>
>Sent: Wednesday, July 20, 2022 2:53 AM
>To: Paolo Bonzini <pbonzini@redhat.com>
>Cc: Duan, Zhenzhong <zhenzhong.duan@intel.com>; qemu-
>devel@nongnu.org; mtosatti@redhat.com; likexu@tencent.com; Ma,
>XiangfeiX <xiangfeix.ma@intel.com>
>Subject: Re: [PATCH] i386: Disable BTS and PEBS
>
>On Tue, Jul 19, 2022, Paolo Bonzini wrote:
>> On 7/18/22 22:12, Sean Christopherson wrote:
>> > On Mon, Jul 18, 2022, Paolo Bonzini wrote:
>> > > This needs to be fixed in the kernel because old QEMU/new KVM is
>supported.
>> >
>> > I can't object to adding a quirk for this since KVM is breaking
>> > userspace, but on the KVM side we really need to stop "sanitizing"
>> > userspace inputs unless it puts the host at risk, because inevitably it
>leads to needing a quirk.
>>
>> The problem is not the sanitizing, it's that userspace literally
>> cannot know that this needs to be done because the feature bits are
>> "backwards" (1 = unavailable).
>
>Yes, the bits being inverted contributed to KVM not providing a way for
>userspace to enumerate PEBS and BTS support, but lack of enumeration is a
>seperate issue.
>
>If KVM had simply ignored invalid guest state from the get go, then
>userspace would never have gained a dependency on KVM sanitizing guest
>state.  The fact that KVM didn't enumerate support in any way is an
>orthogonal problem.  To play nice with older userspace, KVM will need to
>add a quirk to restore the sanizting code, but that doesn't solve the
>enumeration issue.  And vice versa, solving the enuemaration problem
>doesn't magically fix old userspace.
Hi,

I didn't clearly understand the boundary of when to use quirk and when to fix 
it directly, appreciate your guide.
My previous understanding for quirk is about backward compatibility, old 
behavior vs. new behavior.
But this issue is more like a regression or bug, and the sanitizing code is 
only in kvm/next branch,
not in kernel upstream yet, why bother to use a quirk?

Thanks
Zhenzhong