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

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Message-ID: <Y2AH6sevOvD/GnKV@google.com>
Date:   Mon, 31 Oct 2022 17:37:46 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        jmattson@...gle.com, jpoimboe@...nel.org
Subject: Re: [PATCH 1/7] KVM: VMX: remove regs argument of __vmx_vcpu_run

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).