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

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Message-ID: <ZiptAV51VcTEhqgY@google.com>
Date: Thu, 25 Apr 2024 07:47:29 -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 Thursday, April 25, 2024 10:10 PM, Sean Christopherson wrote:
> > 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.
> 
> Yeah, the above isn't nice and no need to do bulk rename.
> We could just shorten it here, e.g.:

Works for me.

> > > > @@ -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.
> 
> I thought the two structures perform quite similar jobs and most of the fields in
> kvm_cap, e.g. has_tsc_control, supported_perf_cap, could also be interpreted
> as host values?

No, kvm_caps is filtered and/or generated information, e.g. supported_perf_cap
and supported_xss incorporate host/hardware support, but they also incorporate
KVM's own capabilities.

kvm_host holds pure, unadultered host values.

XSS is a perfect example.  If we shoved the host value in kvm_caps, then we'd
have kvm_caps.supported_xss and kvm_caps.xss, which would be incredibly confusing.
So then we'd need to rename it to kvm_caps.host_xss, which is also confusing,
just less so, and also results in a longer name with no added value.