linux-kernel - Re: [RFC 07/55] KVM: arm/arm64: Add virtual EL2 state emulation framework

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Date:   Fri, 02 Jun 2017 15:25:26 -0400
From:   Bandan Das <bsd@...hat.com>
To:     Christoffer Dall <cdall@...aro.org>
Cc:     Jintack Lim <jintack@...columbia.edu>, christoffer.dall@...aro.org,
        marc.zyngier@....com, pbonzini@...hat.com, rkrcmar@...hat.com,
        linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, vladimir.murzin@....com,
        suzuki.poulose@....com, mark.rutland@....com, james.morse@....com,
        lorenzo.pieralisi@....com, kevin.brodsky@....com,
        wcohen@...hat.com, shankerd@...eaurora.org, geoff@...radead.org,
        andre.przywara@....com, eric.auger@...hat.com,
        anna-maria@...utronix.de, shihwei@...columbia.edu,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 07/55] KVM: arm/arm64: Add virtual EL2 state emulation framework

Christoffer Dall <cdall@...aro.org> writes:

> On Fri, Jun 02, 2017 at 01:36:23PM -0400, Bandan Das wrote:
>> Christoffer Dall <cdall@...aro.org> writes:
>> 
>> > On Thu, Jun 01, 2017 at 04:05:49PM -0400, Bandan Das wrote:
>> >> Jintack Lim <jintack@...columbia.edu> writes:
>> >> ...
>> >> > +/**
>> >> > + * kvm_arm_setup_shadow_state -- prepare shadow state based on emulated mode
>> >> > + * @vcpu: The VCPU pointer
>> >> > + */
>> >> > +void kvm_arm_setup_shadow_state(struct kvm_vcpu *vcpu)
>> >> > +{
>> >> > +	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
>> >> > +
>> >> > +	ctxt->hw_pstate = *vcpu_cpsr(vcpu);
>> >> > +	ctxt->hw_sys_regs = ctxt->sys_regs;
>> >> > +	ctxt->hw_sp_el1 = ctxt->gp_regs.sp_el1;
>> >> > +}
>> >> > +
>> >> > +/**
>> >> > + * kvm_arm_restore_shadow_state -- write back shadow state from guest
>> >> > + * @vcpu: The VCPU pointer
>> >> > + */
>> >> > +void kvm_arm_restore_shadow_state(struct kvm_vcpu *vcpu)
>> >> > +{
>> >> > +	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
>> >> > +
>> >> > +	*vcpu_cpsr(vcpu) = ctxt->hw_pstate;
>> >> > +	ctxt->gp_regs.sp_el1 = ctxt->hw_sp_el1;
>> >> > +}
>> >> > +
>> >> > +void kvm_arm_init_cpu_context(kvm_cpu_context_t *cpu_ctxt)
>> >> > +{
>> >> > +	cpu_ctxt->hw_sys_regs = &cpu_ctxt->sys_regs[0];
>> >> > +}
>> >> 
>> >> 
>> >> IIUC, the *_shadow_state() functions will set hw_* pointers to
>> >> either point to the "real" state or the shadow state to manage L2 ?
>> >> Maybe, it might make sense to make these function names a little more
>> >> generic since they are not dealing with setting the shadow state
>> >> alone.
>> >> 
>> >
>> > The notion of 'shadow state' is borrowed from shadow page tables, in
>> > which you always load some 'shadow copy' of the 'real value' into the
>> > hardware, so the shadow state is the one that's used for execution by
>> > the hardware.
>> >
>> > The shadow state may be the same as the VCPU's EL1 state, for example,
>> > or it may be a modified version of the VCPU's EL2 state, for example.
>> 
>> Yes, it can be the same. Although, as you said above, "shadow" conventionally
>> refers to the latter.
>
> That's not what I said.  I said shadow is the thing you use in the
> hardware, which may be the same, and may be something different.  The
> important point being, that it is what gets used by the hardware, and
> that it's decoupled, not necessarily different, from the virtual
> state.

I was referring to your first paragraph. And conventionally, in the context of
shadow page tables, it is always different.

>> When it's pointing to EL1 state, it's not really
>> shadow state anymore.
>> 
>
> You can argue it both ways, in the end, all that's important is whether
> or not it's clear what the functions do.
>
>> > If you have better suggestions for naming, we're open to that though.
>> >
>> 
>> Oh nothing specifically, I just felt like "shadow" in the function name
>> could be confusing. Borrowing from kvm_arm_init_cpu_context(), 
>> how about kvm_arm_setup/restore_cpu_context()  ?
>
> I have no objection to these names.
>
>> 
>> BTW, on a separate note, we might as well get away with the typedef and
>> call struct kvm_cpu_context directly.
>> 
> I don't think it's worth changing the code just for that, but if you
> feel it's a significant cleanup, you can send a patch with a good
> argument for why it's worth changing in the commit message.

Sure! The cleanup is not part of the series but sticking to either one
of them in this patch is. As for the argument, typedefs for structs are
discouraged as part of the coding style.

> Thanks,
> -Christoffer