linux-kernel - Re: [PATCH v2 1/6] KVM: x86/mmu: add a new mmu zap helper to indicate memtype changes

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Message-ID: <ZHaSYqOWDluRmIAd@yzhao56-desk.sh.intel.com>
Date:   Wed, 31 May 2023 08:18:42 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Chao Gao <chao.gao@...el.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <pbonzini@...hat.com>
Subject: Re: [PATCH v2 1/6] KVM: x86/mmu: add a new mmu zap helper to
 indicate memtype changes

On Tue, May 30, 2023 at 04:51:25PM -0700, Sean Christopherson wrote:
> On Tue, May 30, 2023, Yan Zhao wrote:
> > On Thu, May 25, 2023 at 08:54:15AM -0700, Sean Christopherson wrote:
> > And I combined the __kvm_mmu_honors_guest_mtrrs() into
> > kvm_mmu_honors_guest_mtrrs(). Not sure if you like it :)
> 
> I would prefer to provide a separater inner helper, mainly so that the common
> case callers don't need to pass %false.  I don't love passing bools, but it's
> tolerable for a one-off use of an inner helper.

Ok. Will follow this way.
It's reasonable :)

> > +/*
> > + * Returns if KVM honors guest MTRRs
> > + * @override_vm_has_noncoherent_dma: Allow caller to override non-coherent DMA
> > + *                                   status returned from
> > + *                                   kvm_arch_has_noncoherent_dma()
> > + */
> > +bool kvm_mmu_honors_guest_mtrrs(struct kvm *kvm,
> > +                               bool override_vm_has_noncoherent_dma)
> > +{
> > +       bool noncoherent_dma = override_vm_has_noncoherent_dma ? true :
> > +                              kvm_arch_has_noncoherent_dma(kvm);
> 
> The "override" name is confusing, e.g. it won't be clear when it's safe/correct
> for a new caller to override kvm_arch_has_noncoherent_dma().  If we go with a
> single helper, I could live with:
> 
> bool kvm_mmu_honors_guest_mtrrs(struct kvm *kvm, bool stopping_noncoherent_dma)
> {
> 	bool noncoherent_dma = stopping_noncoherent_dma ||
> 			       kvm_arch_has_noncoherent_dma(kvm);
> 
> 	...
> }
> 
> but that makes it awkward to use common code for start+stop assignment, and as
> above there are three "normal" callers that would have to pass magic %false
> values regardless of the name.