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Date:   Thu, 19 Jan 2023 15:37:38 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Huang, Kai" <kai.huang@intel.com>
Cc:     "zhi.wang.linux@gmail.com" <zhi.wang.linux@gmail.com>,
        "sean.j.christopherson@intel.com" <sean.j.christopherson@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Shahar, Sagi" <sagis@google.com>,
        "isaku.yamahata@gmail.com" <isaku.yamahata@gmail.com>,
        "Aktas, Erdem" <erdemaktas@google.com>,
        "dmatlack@google.com" <dmatlack@google.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@intel.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>
Subject: Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure
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On Thu, Jan 19, 2023, Huang, Kai wrote:
> 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()?

That's effectively what happens by unwinding up the stak with an error code.
Eventually the page fault handler will get the error and retry the guest.

> Basically we treat TDX_OPERAND_BUSY as seamcall_needbreak(), similar to
> rwlock_needbreak().  I haven't thought about details though.

I am strongly opposed to that approach.  I do not want to pollute KVM's MMU code
with a bunch of retry logic and error handling just because the TDX module is
ultra paranoid and hostile to hypervisors.

The problematic scenario of faulting indefinitely on a single instruction should
never happen under normal circumstances, and so KVM should treat such scenarios
as attacks/breakage and pass the buck to userspace.