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

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Message-ID: <38f506575caacd5488f73315b231c3282f893d46.camel@intel.com>
Date:   Thu, 19 Jan 2023 23:08:41 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "zhi.wang.linux@...il.com" <zhi.wang.linux@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure

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.