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

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Date:   Fri, 20 Jan 2023 00:16:48 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Huang, Kai" <kai.huang@...el.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, Jan 19, 2023, Huang, Kai wrote:
> On Thu, 2023-01-19 at 21:36 +0000, Sean Christopherson wrote:
> > The least invasive idea I have is expand the TDP MMU's concept of "frozen" SPTEs
> > and freeze (a.k.a. lock) the SPTE (KVM's mirror) until the corresponding S-EPT
> > update completes.
> 
> This will introduce another "having-to-wait while SPTE is frozen" problem I
> think, which IIUC means (one way is) you have to do some loop and retry, perhaps
> similar to yield_safe.

Yes, but because the TDP MMU already freezes SPTEs (just for a shorter duration),
I'm 99% sure all of the affected flows already know how to yield/bail when necessary.

The problem with the zero-step mitigation is that it could (theoretically) cause
a "busy" error on literally any accesses, which makes it infeasible for KVM to have
sane behavior.  E.g. freezing SPTEs to avoid the ordering issues isn't necessary
when holding mmu_lock for write, whereas the zero-step madness brings everything
into play.