linux-kernel - RE: [PATCH v1] hyperv: reduce size of ms_hyperv

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Message-ID: <SA1PR21MB133593AABC414CB427B84FABBFFAA@SA1PR21MB1335.namprd21.prod.outlook.com>
Date:   Tue, 19 Sep 2023 06:18:53 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Olaf Hering <olaf@...fle.de>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Arnd Bergmann <arnd@...db.de>
Subject: RE: [PATCH v1] hyperv: reduce size of ms_hyperv_info

> From: Olaf Hering <olaf@...fle.de>
> Sent: Monday, September 18, 2023 9:02 AM
> [...]
> Use the hole prior shared_gpa_boundary to store the result of get_vtl.
> This reduces the size by 8 bytes.
>  [...]
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -53,8 +53,8 @@ struct ms_hyperv_info {
>  			u32 reserved_b2 : 20;
>  		};
>  	};
> -	u64 shared_gpa_boundary;
>  	u8 vtl;
> +	u64 shared_gpa_boundary;
>  };
>  extern struct ms_hyperv_info ms_hyperv;
>  extern bool hv_nested;

How about moving the 'vtl' field to an even earlier place:

--- a/include/asm-generic/mshyperv.h
+++ b/include/asm-generic/mshyperv.h
@@ -36,6 +36,9 @@ struct ms_hyperv_info {
        u32 nested_features;
        u32 max_vp_index;
        u32 max_lp_index;
+
+       u8 vtl;
+
        union {
                u32 isolation_config_a;
                struct {
@@ -54,7 +57,6 @@ struct ms_hyperv_info {
                };
        };
        u64 shared_gpa_boundary;
-       u8 vtl;
 };
 extern struct ms_hyperv_info ms_hyperv;
 extern bool hv_nested;

This also reduces the size by 8 bytes, and keeps
the isolation_config_a/ isolation_config_b/ shared_gpa_boundary
together, which are related.