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

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Message-ID: <ZEGh6zrhJX/SN9jp@google.com>
Date:   Thu, 20 Apr 2023 13:34:51 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH] KVM: x86: Preserve TDP MMU roots until they are
 explicitly invalidated

On Thu, Apr 13, 2023 at 04:12:51PM -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.
> 
[...]
> 
> Reported-by: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
> Link: https://lore.kernel.org/all/959c5bce-beb5-b463-7158-33fc4a4f910c@linux.microsoft.com
> Link: https://lkml.kernel.org/r/20220209170020.1775368-1-pbonzini%40redhat.com
> Link: https://lore.kernel.org/all/20230322013731.102955-1-minipli@grsecurity.net
> Cc: David Matlack <dmatlack@...gle.com>
> Cc: Ben Gardon <bgardon@...gle.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: David Matlack <dmatlack@...gle.com>