linux-kernel - Re: [PATCH 1/4] KVM: x86: Add a struct to consolidate host values, e.g. EFER, XCR0, etc...

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Date: Thu, 25 Apr 2024 07:10:13 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Wei W Wang <wei.w.wang@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] KVM: x86: Add a struct to consolidate host values,
 e.g. EFER, XCR0, etc...

On Thu, Apr 25, 2024, Wei W Wang wrote:
> On Wednesday, April 24, 2024 6:15 AM, Sean Christopherson wrote:
> > @@ -403,7 +403,7 @@ static void vmx_update_fb_clear_dis(struct kvm_vcpu
> > *vcpu, struct vcpu_vmx *vmx)
> >  	 * and VM-Exit.
> >  	 */
> >  	vmx->disable_fb_clear
> > = !cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF) &&
> > -				(host_arch_capabilities &
> > ARCH_CAP_FB_CLEAR_CTRL) &&
> > +				(kvm_host.arch_capabilities &
> > ARCH_CAP_FB_CLEAR_CTRL) &&
> 
> The line of code appears to be lengthy. It would be preferable to limit it to under
> 80 columns per line.

I agree that staying under 80 is generally preferred, but I find this

	vmx->disable_fb_clear = (kvm_host.arch_capabilities & ARCH_CAP_FB_CLEAR_CTRL) &&
				!boot_cpu_has_bug(X86_BUG_MDS) &&
				!boot_cpu_has_bug(X86_BUG_TAA);

much more readable than this

	vmx->disable_fb_clear = (kvm_host.arch_capabilities &
			 	 ARCH_CAP_FB_CLEAR_CTRL) &&
				!boot_cpu_has_bug(X86_BUG_MDS) &&
				!boot_cpu_has_bug(X86_BUG_TAA);

We should shorten the name to arch_caps, but I don't think that's a net positive,
e.g. unless we do a bulk rename, it'd diverge from several other functions/variables,
and IMO it would be less obvious that the field holds MSR_IA32_ARCH_CAPABILITIES.

> >  				!boot_cpu_has_bug(X86_BUG_MDS) &&
> >  				!boot_cpu_has_bug(X86_BUG_TAA);
> > 

> > @@ -325,11 +332,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
> > gpa_t cr2_or_gpa,
> >  			    int emulation_type, void *insn, int insn_len);
> > fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu);
> > 
> > -extern u64 host_xcr0;
> > -extern u64 host_xss;
> > -extern u64 host_arch_capabilities;
> > -
> >  extern struct kvm_caps kvm_caps;
> > +extern struct kvm_host_values kvm_host;
> 
> Have you considered merging the kvm_host_values and kvm_caps into one unified
> structure?

No really.  I don't see any benefit, only the downside of having to come up with
a name that is intuitive when reading code related to both.

> (If the concern is about naming, we could brainstorm a more encompassing term
> for them)