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

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Date:   Wed, 2 Nov 2022 18:42:54 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 10/31/22 18:37, Sean Christopherson wrote:
>>   	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).
> 

It's possible to create our own asm-offsets.h file using something 
similar to the logic in Kbuild:

#####
# Generate asm-offsets.h

offsets-file := include/generated/asm-offsets.h

always-y += $(offsets-file)
targets += arch/$(SRCARCH)/kernel/asm-offsets.s

arch/$(SRCARCH)/kernel/asm-offsets.s: $(timeconst-file) $(bounds-file)

$(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s FORCE
         $(call filechk,offsets,__ASM_OFFSETS_H__)

Paolo