Re: [PATCH v2 3/5] KVM: SVM: Manually context switch DEBUGCTL if LBR virtualization is disabled

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 27, 2025, Sean Christopherson wrote:
> On Thu, Feb 27, 2025, Ravi Bangoria wrote:
> > > Somewhat related but independent: CPU automatically clears DEBUGCTL[BTF]
> > > on #DB exception. So, when DEBUGCTL is save/restored by KVM (i.e. when
> > > LBR virtualization is disabled), it's KVM's responsibility to clear
> > > DEBUGCTL[BTF].
> > 
> > Found this with below KUT test.
> > 
> > (I wasn't sure whether I should send a separate series for kernel fix + KUT
> > patch, or you can squash kernel fix in your patch and I shall send only KUT
> > patch. So for now, sending it as a reply here.)
> 
> Actualy, I'll post this along with some other cleanups to the test, and a fix
> for Intel if needed (it _should_ pass on Intel). 

*sigh*

I forgot that KVM doesn't actually support DEBUGCTL_BTF.  VMX drops the flag
entirely, SVM doesn't clear BTF on #DB, the emulator doesn't honor it, it doesn't
play nice KVM_GUESTDBG_SINGLESTEP, and who knows what else.

I could hack in enough support to get it limping, but I most definitely don't want
to do that for an LTS backport.  The only way it has worked in any capacity on AMD
is if the guest happened to enable LBRs at the same time.  So rather than trying
to go straight to a half-baked implementation, I think the least awful option is
to give SVM the same treatment and explicitly squash BTF.  And then bribe someone
to put in the effort to get it fully functional (or at least, as close to fully
functional as we can get it).