linux-kernel - Re: [PATCH v3 07/34] x86/hyperv: Introduce HV_MAX_SPARSE_VCPU_BANKS/HV_VCPUS_PER_SPARSE

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Message-ID: <20220425154721.xunncuuuzs55nwc7@liuwe-devbox-debian-v2>
Date:   Mon, 25 Apr 2022 15:47:21 +0000
From:   Wei Liu <wei.liu@...nel.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Michael Kelley <mikelley@...rosoft.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        Wei Liu <wei.liu@...nel.org>
Subject: Re: [PATCH v3 07/34] x86/hyperv: Introduce
 HV_MAX_SPARSE_VCPU_BANKS/HV_VCPUS_PER_SPARSE_BANK constants

On Thu, Apr 14, 2022 at 03:19:46PM +0200, Vitaly Kuznetsov wrote:
> It may not come clear from where the magical '64' value used in
> __cpumask_to_vpset() come from. Moreover, '64' means both the maximum
> sparse bank number as well as the number of vCPUs per bank. Add defines
> to make things clear. These defines are also going to be used by KVM.
> 
> No functional change.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  include/asm-generic/hyperv-tlfs.h |  5 +++++
>  include/asm-generic/mshyperv.h    | 11 ++++++-----
>  2 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/include/asm-generic/hyperv-tlfs.h b/include/asm-generic/hyperv-tlfs.h
> index fdce7a4cfc6f..020ca9bdbb79 100644
> --- a/include/asm-generic/hyperv-tlfs.h
> +++ b/include/asm-generic/hyperv-tlfs.h
> @@ -399,6 +399,11 @@ struct hv_vpset {
>  	u64 bank_contents[];
>  } __packed;
>  
> +/* The maximum number of sparse vCPU banks which can be encoded by 'struct hv_vpset' */
> +#define HV_MAX_SPARSE_VCPU_BANKS (64)
> +/* The number of vCPUs in one sparse bank */
> +#define HV_VCPUS_PER_SPARSE_BANK (64)

I think replacing the magic number with a macro is a good thing.

Where do you get these names? Did you make them up yourself?

I'm trying to dig into internal code to find the most appropriate names,
but I couldn't find any so far. Michael, do you have insight here?

Thanks,
Wei.