linux-kernel - Re: [PATCH v2] KVM: x86: Preserve TDP MMU roots until they are explicitly invalidated

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Date:   Fri, 21 Apr 2023 14:56:07 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        Ben Gardon <bgardon@...gle.com>,
        David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH v2] KVM: x86: Preserve TDP MMU roots until they are
 explicitly invalidated

On Fri, 21 Apr 2023 14:49:46 -0700, Sean Christopherson wrote:
> Preserve TDP MMU roots until they are explicitly invalidated by gifting
> the TDP MMU itself a reference to a root when it is allocated.  Keeping a
> reference in the TDP MMU fixes a flaw where the TDP MMU exhibits terrible
> performance, and can potentially even soft-hang a vCPU, if a vCPU
> frequently unloads its roots, e.g. when KVM is emulating SMI+RSM.
> 
> When KVM emulates something that invalidates _all_ TLB entries, e.g. SMI
> and RSM, KVM unloads all of the vCPUs roots (KVM keeps a small per-vCPU
> cache of previous roots).  Unloading roots is a simple way to ensure KVM
> flushes and synchronizes all roots for the vCPU, as KVM flushes and syncs
> when allocating a "new" root (from the vCPU's perspective).
> 
> [...]

Replaced v1 with this version in kvm-x86 mmu.  I'll omit this from the initial
pull request for 6.4 and submit it separately later on, assuming syzbot doesn't
find more holes in my logic.

David (or anyone else), feel free to provide feedback/reviews, I'll squash
any trivial changes (tags, comment tweaks, etc.) as needed.  I immediately
pushed this to -next purely to get testing on the (hopefully) fixed version.

[1/1] KVM: x86: Preserve TDP MMU roots until they are explicitly invalidated
      https://github.com/kvm-x86/linux/commit/bf4166af027e

--
https://github.com/kvm-x86/linux/tree/next
https://github.com/kvm-x86/linux/tree/fixes