linux-kernel - Re: [PATCH 1/7] KVM: VMX: remove regs argument of __vmx_vcpu

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Message-ID: <Y2Ffg5ed5zoijqOB@google.com>
Date:   Tue, 1 Nov 2022 18:03:47 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, jmattson@...gle.com
Subject: Re: [PATCH 1/7] KVM: VMX: remove regs argument of __vmx_vcpu_run

On Tue, Nov 01, 2022, Josh Poimboeuf wrote:
> On Mon, Oct 31, 2022 at 05:37:46PM +0000, Sean Christopherson wrote:
> > > diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
> > > index cb50589a7102..90da275ad223 100644
> > > --- a/arch/x86/kernel/asm-offsets.c
> > > +++ b/arch/x86/kernel/asm-offsets.c
> > > @@ -111,6 +111,7 @@ static void __used common(void)
> > >  
> > >  	if (IS_ENABLED(CONFIG_KVM_INTEL)) {
> > >  		BLANK();
> > > +		OFFSET(VMX_vcpu_arch_regs, vcpu_vmx, vcpu.arch.regs);
> > 
> > Is there an asm-offsets-like solution that doesn't require exposing vcpu_vmx
> > outside of KVM?  We (Google) want to explore loading multiple instances of KVM,
> > i.e. loading multiple versions of kvm.ko at the same time, to allow intra-host
> > migration between versions of KVM to upgrade/rollback KVM without changing the
> > kernel (RFC coming soon-ish).  IIRC, asm-offsets is the only place where I haven't
> > been able to figure out a simple way to avoid exposing KVM's internal structures
> > outside of KVM (so that the structures can change across KVM instances without
> > breaking kernel code).
> 
> Is that really a problem?  Would it even make sense for non-KVM kernel
> code to use 'vcpu_vmx' anyway?

vcpu_vmx itself isn't a problem as non-KVM kernel code _shouldn't_ be using
vcpu_vmx, but I want to go beyond "shouldn't" and make it all-but-impossible for
non-KVM code to reference internal KVM structures/state, e.g. I want to bury all
kvm_host.h headers in kvm/ code instead of exposing them in include/asm/.