linux-kernel - Re: [PATCH 16/61] KVM: x86: Encapsulate CPUID entries and metadata in struct

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Date:   Mon, 24 Feb 2020 13:55:51 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/61] KVM: x86: Encapsulate CPUID entries and metadata
 in struct

On Fri, Feb 21, 2020 at 03:58:47PM +0100, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@...el.com> writes:
> 
> > @@ -539,7 +549,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
> >  		entry->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
> >  
> >  		for (i = 1, max_idx = entry->eax & 0xff; i < max_idx; ++i) {
> > -			if (!do_host_cpuid(&entry[i], nent, maxnent, function, 0))
> > +			entry = do_host_cpuid(array, 2, 0);
> 
> I'd change this to 
>                         entry = do_host_cpuid(array, function, 0);
> 
> to match other call sites.

Done.  That did look weird, no idea why I decided to hardcode only this one.

> > +			if (!entry)
> >  				goto out;
> >  		}
> >  		break;
> > @@ -802,22 +814,22 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
> >  	return r;
> >  }
> >  
> > -static int do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 func,
> > -			 int *nent, int maxnent, unsigned int type)
> > +static int do_cpuid_func(struct kvm_cpuid_array *array, u32 func,
> > +			 unsigned int type)
> >  {
> > -	if (*nent >= maxnent)
> > +	if (array->nent >= array->maxnent)
> >  		return -E2BIG;
> >  
> >  	if (type == KVM_GET_EMULATED_CPUID)
> > -		return __do_cpuid_func_emulated(entry, func, nent, maxnent);
> > +		return __do_cpuid_func_emulated(array, func);
> 
> Would it make sense to move 'if (array->nent >= array->maxnent)' check
> to __do_cpuid_func_emulated() to match do_host_cpuid()?

I considered doing exactly that.  IIRC, I opted not to because at this
point in the series, the initial call to do_host_cpuid() is something like
halfway down the massive __do_cpuid_func(), and eliminating the early check
didn't feel quite right, e.g. there is a fair amount of unnecessary code
that runs before hitting the first do_host_cpuid().

What if I add a patch towards the end of the series to move this check into
__do_cpuid_func_emulated(), i.e. after __do_cpuid_func() has been trimmed
down to size and the early check really is superfluous.