linux-kernel - Re: [PATCH v4 10/12] KVM: x86/mmu: fine-grained gfn zap when guest MTRRs are honored

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Message-ID: <ZPWXXLVlCpccVLvY@yzhao56-desk.sh.intel.com>
Date:   Mon, 4 Sep 2023 16:37:48 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <pbonzini@...hat.com>, <chao.gao@...el.com>, <kai.huang@...el.com>,
        <robert.hoo.linux@...il.com>, <yuan.yao@...ux.intel.com>
Subject: Re: [PATCH v4 10/12] KVM: x86/mmu: fine-grained gfn zap when guest
 MTRRs are honored

On Fri, Aug 25, 2023 at 04:13:35PM -0700, Sean Christopherson wrote:
> On Fri, Jul 14, 2023, Yan Zhao wrote:
> >  void kvm_honors_guest_mtrrs_zap_on_cd_toggle(struct kvm_vcpu *vcpu)
> >  {
> > -	return kvm_mtrr_zap_gfn_range(vcpu, gpa_to_gfn(0), gpa_to_gfn(~0ULL));
> > +	struct kvm_mtrr *mtrr_state = &vcpu->arch.mtrr_state;
> > +	bool mtrr_enabled = mtrr_is_enabled(mtrr_state);
> > +	u8 default_mtrr_type;
> > +	bool cd_ipat;
> > +	u8 cd_type;
> > +
> > +	kvm_honors_guest_mtrrs_get_cd_memtype(vcpu, &cd_type, &cd_ipat);
> > +
> > +	default_mtrr_type = mtrr_enabled ? mtrr_default_type(mtrr_state) :
> > +			    mtrr_disabled_type(vcpu);
> > +
> > +	if (cd_type != default_mtrr_type || cd_ipat)
> > +		return kvm_mtrr_zap_gfn_range(vcpu, gpa_to_gfn(0), gpa_to_gfn(~0ULL));
> 
> Why does this use use the MTRR version but the failure path does not?  Ah, because
> trying to allocate in the failure path will likely fail to allocate memory.  With
> a statically sized array, we can just special case the 0 => -1 case.  Actually,
> we can do that regardless, it just doesn't need a dedicated flag if we use an
> array.
> 
> Using the MTRR version on failure (array is full) means that other vCPUs can see
> that everything is being zapped and go straight to waitin.
Yes, will convert to the way of using statially sized array in next version.

> 
> > +
> > +	/*
> > +	 * If mtrr is not enabled, it will go to zap all above if the default
> 
> Pronouns again.  Maybe this?
> 
> 	/*
> 	 * The default MTRR type has already been checked above, if MTRRs are
> 	 * disabled there are no other MTRR types to consider.
> 	 */
Yes, better :)

> > +	 * type does not equal to cd_type;
> > +	 * Or it has no need to zap if the default type equals to cd_type.
> > +	 */
> > +	if (mtrr_enabled) {
> 
> To save some indentation:
> 
> 	if (!mtrr_enabled)
> 		return;
> 
Got it! 

> > +		if (prepare_zaplist_fixed_mtrr_of_non_type(vcpu, default_mtrr_type))
> > +			goto fail;
> > +
> > +		if (prepare_zaplist_var_mtrr_of_non_type(vcpu, default_mtrr_type))
> > +			goto fail;
> > +
> > +		kvm_zap_or_wait_mtrr_zap_list(vcpu->kvm);
> > +	}
> > +	return;
> > +fail:
> > +	kvm_clear_mtrr_zap_list(vcpu->kvm);
> > +	/* resort to zapping all on failure*/
> > +	kvm_zap_gfn_range(vcpu->kvm, gpa_to_gfn(0), gpa_to_gfn(~0ULL));
> > +	return;
> >  }
> > -- 
> > 2.17.1
> >