linux-kernel - Re: [PATCH v2 01/25] KVM: x86/mmu: avoid indirect call for get

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Message-ID: <2652c27e-ce8c-eb40-1979-9fe732aa9085@redhat.com>
Date:   Tue, 8 Mar 2022 17:21:09 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        dmatlack@...gle.com
Subject: Re: [PATCH v2 01/25] KVM: x86/mmu: avoid indirect call for get_cr3

On 3/8/22 17:16, Sean Christopherson wrote:
> 
>> +static inline unsigned long kvm_mmu_get_guest_pgd(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
> Wrap the params, no reason to make this line so long.
> 
>> +{
>> +#ifdef CONFIG_RETPOLINE
>> +	if (mmu->get_guest_pgd == kvm_get_guest_cr3)
>> +		return kvm_read_cr3(vcpu);
> This is unnecessarily fragile and confusing at first glance.  Compilers are smart
> enough to generate a non-inline version of functions if they're used for function
> pointers, while still inlining where appropriate.  In other words, just drop
> kvm_get_guest_cr3() entirely, a al get_pdptr => kvm_pdptr_read().

Unfortunately this isn't entirely true.  The function pointer will not 
match between compilation units, in this case between the one that calls 
kvm_mmu_get_guest_pgd and the one that assigned kvm_read_cr3 to the 
function pointer.

Paolo