linux-kernel - Re: [PATCH v2 03/15] mshyperv: Introduce numa_node_to_proximity_domain

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Message-ID: <3b1ca61c-fa3f-a802-6705-a8c1f37ad58f@intel.com>
Date:   Thu, 17 Aug 2023 16:22:18 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-arch@...r.kernel.org
Cc:     patches@...ts.linux.dev, mikelley@...rosoft.com, kys@...rosoft.com,
        wei.liu@...nel.org, haiyangz@...rosoft.com, decui@...rosoft.com,
        apais@...ux.microsoft.com, Tianyu.Lan@...rosoft.com,
        ssengar@...ux.microsoft.com, mukeshrathor@...rosoft.com,
        stanislav.kinsburskiy@...il.com, jinankjain@...ux.microsoft.com,
        vkuznets@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        will@...nel.org, catalin.marinas@....com
Subject: Re: [PATCH v2 03/15] mshyperv: Introduce
 numa_node_to_proximity_domain_info

On 8/17/23 15:01, Nuno Das Neves wrote:
> +static inline union hv_proximity_domain_info
> +numa_node_to_proximity_domain_info(int node)
> +{
> +	union hv_proximity_domain_info proximity_domain_info;
> +
> +	if (node != NUMA_NO_NODE) {
> +		proximity_domain_info.domain_id = node_to_pxm(node);
> +		proximity_domain_info.flags.reserved = 0;
> +		proximity_domain_info.flags.proximity_info_valid = 1;
> +		proximity_domain_info.flags.proximity_preferred = 1;
> +	} else {
> +		proximity_domain_info.as_uint64 = 0;
> +	}
> +
> +	return proximity_domain_info;
> +}

Pop quiz: What are the rules for the 30 bits of uninitialized data of
proximity_domain_info.flags in the (node != NUMA_NO_NODE) case?

I actually don't know off the top of my head.  I generally avoid
bitfields, but if they were normal stack-allocated variable space,
they'd be garbage.

I'd also *much* rather see the "as_uint64 = 0" coded up as a memset() or
even explicitly zeroing all the same variables as the other half of the
if().  As it stands, it's not 100% obvious that proximity_domain_info is
64 bits and that .as_uint64=0 zeroes the whole thing.  It *WOULD* be
totally obvious if it were a memset().