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

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Date:   Thu, 26 Jan 2023 21:59:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "zhi.wang.linux@...il.com" <zhi.wang.linux@...il.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure

On Thu, Jan 26, 2023, Huang, Kai wrote:
> On Thu, 2023-01-26 at 17:28 +0000, Sean Christopherson wrote:
> > In other words, once the PTE is zapped/blocked (branch is pruned), it's completely
> > removed from the paging tree and no other tasks can access the branch (page table
> > and its children).  I.e. the only remaining reference to the branch is the pointer
> > handed to the RCU callback.  That means the RCU callback has exclusive access to the
> > branch, i.e. can operate as if it were holding mmu_lock for write.  Furthermore, the
> > RCU callback also doesn't need to flush TLBs because that was again done when
> > pruning the branch.
> > 
> > It's the same idea that KVM already uses for root SPs, the only difference is how
> > KVM determines that there is exactly one entity that holds a reference to the SP.
> 
> Right.  This works fine for normal non-TDX case.  However for TDX unfortunately
> the access to the removed branch (or the removed sub-page-table) isn't that
> "exclusive" as the SEAMCALL to truly zap that branch still needs to hold the
> write lock of the entire Secure EPT tree, so it can still conflict with other
> threads handling new faults.

I thought TDX was smart enough to read-lock only the part of the tree that it's
actually consuming, and write-lock only the part of the tree that it's actually
modifying?

Hrm, but even if TDX takes a read-lock, there's still the problem of it needing
to walk the upper levels, i.e. KVM needs to keep mid-level page tables reachable
until they're fully removed.  Blech.  That should be a non-issue at this time
though, as I don't think KVM will ever REMOVE a page table of a live guest.  I
need to look at the PROMOTE/DEMOTE flows...