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Date:   Thu, 19 Jan 2023 23:11:30 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Huang, Kai" <kai.huang@intel.com>
Cc:     "dmatlack@google.com" <dmatlack@google.com>,
        "sean.j.christopherson@intel.com" <sean.j.christopherson@intel.com>,
        "Shahar, Sagi" <sagis@google.com>,
        "isaku.yamahata@gmail.com" <isaku.yamahata@gmail.com>,
        "Aktas, Erdem" <erdemaktas@google.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "zhi.wang.linux@gmail.com" <zhi.wang.linux@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@intel.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 Thu, 2023-01-19 at 21:36 +0000, Sean Christopherson wrote:
> > On Thu, Jan 19, 2023, Huang, Kai wrote:
> > > On Thu, 2023-01-19 at 15:37 +0000, Sean Christopherson wrote:
> > > > 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:
> > > > > > 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.
> > > 
> > > Right.  But IIUC there's legal cases that SEPT SEAMCALL can return BUSY due to
> > > multiple threads trying to read/modify SEPT simultaneously in case of TDP MMU. 
> > > For instance, parallel page faults on different vcpus on private pages.  I
> > > believe this is the main reason to retry.
> > 
> > Um, crud.  I think there's a bigger issue.  KVM always operates on its copy of the
> > S-EPT tables and assumes the the real S-EPT tables will always be synchronized with
> > KVM's mirror.  That assumption doesn't hold true without serializing SEAMCALLs in
> > some way.  E.g. if a SPTE is zapped and mapped at the same time, we can end up with:
> > 
> >   vCPU0                      vCPU1
> >   =====                      =====
> >   mirror[x] = xyz
> >                              old_spte = mirror[x]
> >                              mirror[x] = REMOVED_SPTE
> >                              sept[x] = REMOVED_SPTE
> >   sept[x] = xyz
> 
> IIUC this case cannot happen, as the two steps in the vcpu0 are within read
> lock, which prevents from vcpu1, which holds the write lock during zapping SPTE.

Zapping SPTEs can happen while holding mmu_lock for read, e.g. see the bug fixed
by commit 21a36ac6b6c7 ("KVM: x86/mmu: Re-check under lock that TDP MMU SP hugepage
is disallowed").