Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure

[Sean Christopherson]

On Tue, Jan 17, 2023, Sean Christopherson wrote:
> On Tue, Jan 17, 2023, Zhi Wang wrote:
> > 2) As TDX module doesn't provide contention-and-wait, I guess the following
> > approach might have been discussed when designing this "retry".
> > 
> > KERNEL                          TDX MODULE
> > 
> > SEAMCALL A   ->                 PATH A: Taking locks
> > 
> > SEAMCALL B   ->                 PATH B: Contention on a lock
> > 
> >              <-                 Return "operand busy"
> > 
> > SEAMCALL B   -|
> >               |  <- Wait on a kernel waitqueue
> > SEAMCALL B  <-|
> > 
> > SEAMCALL A   <-                 PATH A: Return
> > 
> > SEAMCALL A   -|
> >               |  <- Wake up the waitqueue
> > SEMACALL A  <-| 
> > 
> > SEAMCALL B  ->                  PATH B: Taking the locks
> > ...
> > 
> > Why not this scheme wasn't chosen?
> 
> AFAIK, I don't think a waitqueue approach as ever been discussed publicly.  Intel
> may have considered the idea internally, but I don't recall anything being proposed
> publically (though it's entirely possible I just missed the discussion).
> 
> Anways, I don't think a waitqueue would be a good fit, at least not for S-EPT
> management, which AFAICT is the only scenario where KVM does the arbitrary "retry
> X times and hope things work".  If the contention occurs due to the TDX Module
> taking an S-EPT lock in VM-Enter, then KVM won't get a chance to do the "Wake up
> the waitqueue" action until the next VM-Exit, which IIUC is well after the TDX
> Module drops the S-EPT lock.  In other words, immediately retrying and then punting
> the problem further up the stack in KVM does seem to be the least awful "solution"
> if there's contention.

Oh, the other important piece I forgot to mention is that dropping mmu_lock deep
in KVM's MMU in order to wait isn't always an option.  Most flows would play nice
with dropping mmu_lock and sleeping, but some paths, e.g. from the mmu_notifier,
(conditionally) disallow sleeping.