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

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Date:   Tue, 1 Nov 2022 10:32:04 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Sean Christopherson <seanjc@...gle.com>
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 Mon, Oct 31, 2022 at 05:37:46PM +0000, Sean Christopherson wrote:
> On Fri, Oct 28, 2022, Paolo Bonzini wrote:
> > Registers are reachable through vcpu_vmx, no need to pass
> > a separate pointer to the regs[] array.
> > 
> > No functional change intended.
> > 
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > ---
> >  arch/x86/kernel/asm-offsets.c |  1 +
> >  arch/x86/kvm/vmx/nested.c     |  3 +-
> >  arch/x86/kvm/vmx/vmenter.S    | 58 +++++++++++++++--------------------
> >  arch/x86/kvm/vmx/vmx.c        |  3 +-
> >  arch/x86/kvm/vmx/vmx.h        |  3 +-
> >  5 files changed, 29 insertions(+), 39 deletions(-)
> > 
> > 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?  It already seems to be private.
asm-offsets.c has to "cheat" to get access to it by including
"../kvm/vmx/vmx.h".

So the only concern is exposing the asm offsets, right?  But it seems
highly unlikely any non-KVM code would be using those either.

And, that would be a bug anyway: module code is subject to change and
could always get recompiled.  The kernel proper shouldn't be making any
assumptions about the layouts of module-owned structs.

-- 
Josh