linux-kernel - Re: [PATCH v2 12/31] KVM: x86: hyper-v: Introduce kvm_hv_is_tlb_flush

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Date:   Thu, 7 Apr 2022 18:07:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 12/31] KVM: x86: hyper-v: Introduce
 kvm_hv_is_tlb_flush_hcall()

On Thu, Apr 07, 2022, Vitaly Kuznetsov wrote:
> The newly introduced helper checks whether vCPU is performing a
> Hyper-V TLB flush hypercall. This is required to filter out Direct TLB
> flush hypercalls from L2 for processing.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/hyperv.h | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/arch/x86/kvm/hyperv.h b/arch/x86/kvm/hyperv.h
> index 448877b478ef..3687e1e61e0d 100644
> --- a/arch/x86/kvm/hyperv.h
> +++ b/arch/x86/kvm/hyperv.h
> @@ -168,6 +168,30 @@ static inline void kvm_hv_vcpu_empty_flush_tlb(struct kvm_vcpu *vcpu)
>  	tlb_flush_ring = kvm_hv_get_tlb_flush_ring(vcpu);
>  	tlb_flush_ring->read_idx = tlb_flush_ring->write_idx;
>  }
> +
> +static inline bool kvm_hv_is_tlb_flush_hcall(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
> +	u16 code;
> +
> +	if (!hv_vcpu)
> +		return false;
> +
> +#ifdef CONFIG_X86_64
> +	if (is_64_bit_hypercall(vcpu)) {
> +		code = kvm_rcx_read(vcpu) & 0xffff;
> +	} else
> +#endif
> +	{
> +		code = kvm_rax_read(vcpu) & 0xffff;
> +	}

	if (IS_ENABLED(CONFIG_X86_64) && is_64_bit_hypercall(vcpu))
		code = kvm_rcx_read(vcpu) & 0xffff;
	else
		code = kvm_rax_read(vcpu) & 0xffff;

Though I honestly don't see the point, is_64_bit_hypercall() will do the right
thing.

And is the 0xffff really needed?  An implicit cast should work just fine.  If I'm
overlooking something, an explicit would be better, e.g. why not

	code = is_64_bit_hypercall(vcpu) ? kvm_rcx_read(vcpu) :
					   kvm_rax_read(vcpu);

> +
> +	return (code == HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE ||
> +		code == HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST ||
> +		code == HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE_EX ||
> +		code == HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST_EX);
> +}
> +
>  void kvm_hv_vcpu_flush_tlb(struct kvm_vcpu *vcpu);
>  
>  
> -- 
> 2.35.1
>