linux-kernel - Re: [PATCH] KVM: X86: Make fpu allocation a common function

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Message-ID: <87v9sq46vz.fsf@vitty.brq.redhat.com>
Date:   Tue, 15 Oct 2019 12:53:20 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] KVM: X86: Make fpu allocation a common function

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> On Mon, Oct 14, 2019 at 06:58:49PM +0200, Vitaly Kuznetsov wrote:
>> Xiaoyao Li <xiaoyao.li@...el.com> writes:
>> 
>> > They are duplicated codes to create vcpu.arch.{user,guest}_fpu in VMX
>> > and SVM. Make them common functions.
>> >
>> > No functional change intended.
>> 
>> Would it rather make sense to move this code to
>> kvm_arch_vcpu_create()/kvm_arch_vcpu_destroy() instead?
>
> Does it make sense?  Yes.  Would it actually work?  No.  Well, not without
> other shenanigans.
>
> FPU allocation can't be placed after the call to .create_vcpu() becuase
> it's consumed in kvm_arch_vcpu_init().   FPU allocation can't come before
> .create_vcpu() because the vCPU struct itself hasn't been allocated.

A very theoretical question: why do we have 'struct vcpu' embedded in
vcpu_vmx/vcpu_svm and not the other way around (e.g. in a union)? That
would've allowed us to allocate memory in common code and then fill in
vendor-specific details in .create_vcpu().

-- 
Vitaly