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

[Huang, Kai]

On Tue, 2023-01-17 at 21:01 +0000, Sean Christopherson wrote:
> 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.

Could we do something similar to tdp_mmu_iter_cond_resched() but not simple busy
retrying "X times",  at least at those paths that can release mmu_lock()? 
Basically we treat TDX_OPERAND_BUSY as seamcall_needbreak(), similar to
rwlock_needbreak().  I haven't thought about details though.