linux-kernel - Re: [PATCH 3/6] KVM: x86: Add dedicated emulator helper for grabbing CPUID.maxphyaddr

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Date:   Tue, 3 Mar 2020 09:48:03 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jan Kiszka <jan.kiszka@...mens.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH 3/6] KVM: x86: Add dedicated emulator helper for grabbing
 CPUID.maxphyaddr

On 02/03/20 20:57, Sean Christopherson wrote:
> Add a helper to retrieve cpuid_maxphyaddr() instead of manually
> calculating the value in the emulator via raw CPUID output.  In addition
> to consolidating logic, this also paves the way toward simplifying
> kvm_cpuid(), whose somewhat confusing return value exists purely to
> support the emulator's maxphyaddr calculation.
> 
> No functional change intended.

I don't think this is a particularly useful change.  Yes, it's not
intuitive but is it more than a matter of documentation (and possibly
moving the check_cr_write snippet into a separate function)?

Paolo

> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/include/asm/kvm_emulate.h |  1 +
>  arch/x86/kvm/emulate.c             | 10 +---------
>  arch/x86/kvm/x86.c                 |  6 ++++++
>  3 files changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> index bf5f5e476f65..ded06515d30f 100644
> --- a/arch/x86/include/asm/kvm_emulate.h
> +++ b/arch/x86/include/asm/kvm_emulate.h
> @@ -222,6 +222,7 @@ struct x86_emulate_ops {
>  
>  	bool (*get_cpuid)(struct x86_emulate_ctxt *ctxt, u32 *eax, u32 *ebx,
>  			  u32 *ecx, u32 *edx, bool check_limit);
> +	int (*get_cpuid_maxphyaddr)(struct x86_emulate_ctxt *ctxt);
>  	bool (*guest_has_long_mode)(struct x86_emulate_ctxt *ctxt);
>  	bool (*guest_has_movbe)(struct x86_emulate_ctxt *ctxt);
>  	bool (*guest_has_fxsr)(struct x86_emulate_ctxt *ctxt);
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index dd19fb3539e0..bf02ed51e90f 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4244,16 +4244,8 @@ static int check_cr_write(struct x86_emulate_ctxt *ctxt)
>  
>  		ctxt->ops->get_msr(ctxt, MSR_EFER, &efer);
>  		if (efer & EFER_LMA) {
> -			u64 maxphyaddr;
> -			u32 eax, ebx, ecx, edx;
> +			int maxphyaddr = ctxt->ops->get_cpuid_maxphyaddr(ctxt);
>  
> -			eax = 0x80000008;
> -			ecx = 0;
> -			if (ctxt->ops->get_cpuid(ctxt, &eax, &ebx, &ecx,
> -						 &edx, false))
> -				maxphyaddr = eax & 0xff;
> -			else
> -				maxphyaddr = 36;
>  			rsvd = rsvd_bits(maxphyaddr, 63);
>  			if (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_PCIDE)
>  				rsvd &= ~X86_CR3_PCID_NOFLUSH;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ddd1d296bd20..5467ee71c25b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6209,6 +6209,11 @@ static bool emulator_get_cpuid(struct x86_emulate_ctxt *ctxt,
>  	return kvm_cpuid(emul_to_vcpu(ctxt), eax, ebx, ecx, edx, check_limit);
>  }
>  
> +static int emulator_get_cpuid_maxphyaddr(struct x86_emulate_ctxt *ctxt)
> +{
> +	return cpuid_maxphyaddr(emul_to_vcpu(ctxt));
> +}
> +
>  static bool emulator_guest_has_long_mode(struct x86_emulate_ctxt *ctxt)
>  {
>  	return guest_cpuid_has(emul_to_vcpu(ctxt), X86_FEATURE_LM);
> @@ -6301,6 +6306,7 @@ static const struct x86_emulate_ops emulate_ops = {
>  	.fix_hypercall       = emulator_fix_hypercall,
>  	.intercept           = emulator_intercept,
>  	.get_cpuid           = emulator_get_cpuid,
> +	.get_cpuid_maxphyaddr= emulator_get_cpuid_maxphyaddr,
>  	.guest_has_long_mode = emulator_guest_has_long_mode,
>  	.guest_has_movbe     = emulator_guest_has_movbe,
>  	.guest_has_fxsr      = emulator_guest_has_fxsr,
>