linux-kernel - Re: [PATCH v15 21/23] KVM: MMU: Disable fast path for private memslots

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Date: Fri, 10 May 2024 15:50:26 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Michael Roth <michael.roth@....com>, kvm@...r.kernel.org, linux-coco@...ts.linux.dev, 
	linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org, 
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com, 
	jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com, ardb@...nel.org, 
	vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org, 
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	peterz@...radead.org, srinivas.pandruvada@...ux.intel.com, 
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	vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com, tony.luck@...el.com, 
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	jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com, 
	pankaj.gupta@....com, liam.merwick@...cle.com, papaluri@....com, 
	Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH v15 21/23] KVM: MMU: Disable fast path for private memslots

On Fri, May 10, 2024 at 3:47 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> > +      * Since software-protected VMs don't have a notion of a shared vs.
> > +      * private that's separate from what KVM is tracking, the above
> > +      * KVM_EXIT_MEMORY_FAULT condition wouldn't occur, so avoid the
> > +      * special handling for that case for now.
>
> Very technically, it can occur if userspace _just_ modified the attributes.  And
> as I've said multiple times, at least for now, I want to avoid special casing
> SW-protected VMs unless it is *absolutely* necessary, because their sole purpose
> is to allow testing flows that are impossible to excercise without SNP/TDX hardware.

Yep, it is not like they have to be optimized.

> > +      */
> > +     if (kvm_slot_can_be_private(fault->slot) &&
> > +         !(IS_ENABLED(CONFIG_KVM_SW_PROTECTED_VM) &&
> > +           vcpu->kvm->arch.vm_type == KVM_X86_SW_PROTECTED_VM))
>
> Heh, !(x && y) kills me, I misread this like 4 times.
>
> Anyways, I don't like the heuristic.  It doesn't tie the restriction back to the
> cause in any reasonable way.  Can't this simply be?
>
>         if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)
>                 return false;

You beat me to it by seconds. And it can also be guarded by a check on
kvm->arch.has_private_mem to avoid the attributes lookup.

> Which is much, much more self-explanatory.

Both more self-explanatory and more correct.

Paolo