linux-kernel - Re: [PATCH v2 07/18] KVM: x86/mmu: Do not use guest root level in audit

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Date:   Wed, 23 Feb 2022 17:02:42 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>
Subject: Re: [PATCH v2 07/18] KVM: x86/mmu: Do not use guest root level in
 audit

On Fri, 2022-02-18 at 19:46 +0100, Paolo Bonzini wrote:
> On 2/18/22 19:37, Sean Christopherson wrote:
> > Since I keep bringing it up...
> > 
> > From: Sean Christopherson<seanjc@...gle.com>
> > Date: Fri, 18 Feb 2022 09:43:05 -0800
> > Subject: [PATCH] KVM: x86/mmu: Remove MMU auditing
> > 
> > Remove mmu_audit.c and all its collateral, the auditing code has suffered
> > severe bitrot, ironically partly due to shadow paging being more stable
> > and thus not benefiting as much from auditing, but mostly due to TDP
> > supplanting shadow paging for non-nested guests and shadowing of nested
> > TDP not heavily stressing the logic that is being audited.
> > 
> > Signed-off-by: Sean Christopherson<seanjc@...gle.com>
> 
> Queued, thanks. O:-)

I once kind of played with it.

Note that shadow mmu does have bugs - I can easily crash L1/L2 when
doing repeated migrations when I disable NPT either in L0 or L1,
and when I force the mmu to be always sync (see my strict_mmu patch),
the crashes go away.

mmu audit maybe could have helped with that.

But I won't argue too much about this.

Best regards,
	Maxim Levitsky

> 
> Paolo
>