linux-kernel - Re: [PATCH v2 2/2] KVM: x86/mmu: Micro-optimize nEPT's bad memptype/XWR checks

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Date:   Fri, 10 Jan 2020 08:04:53 -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,
        David Laight <David.Laight@...LAB.COM>,
        Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH v2 2/2] KVM: x86/mmu: Micro-optimize nEPT's bad
 memptype/XWR checks

On Fri, Jan 10, 2020 at 12:37:33PM +0100, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@...el.com> writes:
> > --- a/arch/x86/kvm/mmu/paging_tmpl.h
> > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> > @@ -128,6 +128,21 @@ static inline int FNAME(is_present_gpte)(unsigned long pte)
> >  #endif
> >  }
> >  
> > +static bool FNAME(is_bad_mt_xwr)(struct rsvd_bits_validate *rsvd_check, u64 gpte)
> > +{
> > +#if PTTYPE != PTTYPE_EPT
> > +	return false;
> > +#else
> > +	return __is_bad_mt_xwr(rsvd_check, gpte);
> > +#endif
> > +}
> > +
> > +static bool FNAME(is_rsvd_bits_set)(struct kvm_mmu *mmu, u64 gpte, int level)
> > +{
> > +	return __is_rsvd_bits_set(&mmu->guest_rsvd_check, gpte, level) ||
> > +	       FNAME(is_bad_mt_xwr)(&mmu->guest_rsvd_check, gpte);
> > +}
> > +
> 
> Not sure if it would make sense/difference (well, this is famous KVM
> MMU!) but as FNAME(is_bad_mt_xwr)
> 
> has only one call site we could've as well merged it, something like:
> 
> static bool FNAME(is_rsvd_bits_set)(struct kvm_mmu *mmu, u64 gpte, int level)
> {
> #if PTTYPE == PTTYPE_EPT
> 	bool res =  __is_bad_mt_xwr(&mmu->guest_rsvd_check, gpte);
> #else
> 	bool res = false;
> #endif
> 	return __is_rsvd_bits_set(&mmu->guest_rsvd_check, gpte, level) || res;
> }
> 
> but keeping it in-line with __is_rsvd_bits_set()/__is_bad_mt_xwr() in
> mmu.c likely has greater benefits.

Ya, I don't love the code, but it was the least awful of the options I
tried, in that it's the most readable without being too obnoxious.


Similar to your suggestion, but it avoids evaluating __is_bad_mt_xwr() if
reserved bits are set, which is admittedly rare.

	return __is_rsvd_bits_set(&mmu->guest_rsvd_check, gpte, level)
#if PTTYPE == PTTYPE_EPT
	       || __is_bad_mt_xwr(&mmu->guest_rsvd_check, gpte)
#endif
	       ;

Or macrofying the call to keep the call site clean, but IMO this obfuscates
things too much.

	return __is_rsvd_bits_set(&mmu->guest_rsvd_check, gpte, level) ||
	       IS_BAD_MT_XWR(&mmu->guest_rsvd_check, gpte);