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

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Message-ID: <168203178177.2322082.210263549285859631.b4-ty@google.com>
Date:   Thu, 20 Apr 2023 16:05:58 -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>,
        David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH] KVM: x86: Preserve TDP MMU roots until they are
 explicitly invalidated

On Thu, 13 Apr 2023 16:12:51 -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).
> 
> [...]

Applied to kvm-x86 mmu.  In hindsight, I should have speculatively applied this
early on to get more time in -next, but practically speaking I don't think it
will make a difference in the end.

[1/1] KVM: x86: Preserve TDP MMU roots until they are explicitly invalidated
      https://github.com/kvm-x86/linux/commit/920d6bb77d55

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