Re: [Xen-devel] [PATCH v4 1/4] x86/PVHv2: Add memory map pointer to hvm_start_info struct

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Mar 20, 2018 at 09:48:56AM -0700, Maran Wilson wrote:
> The start info structure that is defined as part of the x86/HVM direct boot
> ABI and used for starting Xen PVH guests would be more versatile if it also
> included a way to pass information about the memory map to the guest. This
> would allow KVM guests to share the same entry point.
> 
> Signed-off-by: Maran Wilson <maran.wilson@xxxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Just a couple of nit suggestions...

> ---
>  xen/include/public/arch-x86/hvm/start_info.h | 65 
> +++++++++++++++++++++++++++-
>  1 file changed, 64 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/include/public/arch-x86/hvm/start_info.h 
> b/xen/include/public/arch-x86/hvm/start_info.h
> index 6484159..d491f2d 100644
> --- a/xen/include/public/arch-x86/hvm/start_info.h
> +++ b/xen/include/public/arch-x86/hvm/start_info.h
> @@ -33,7 +33,7 @@
>   *    | magic          | Contains the magic value XEN_HVM_START_MAGIC_VALUE
>   *    |                | ("xEn3" with the 0x80 bit of the "E" set).
>   *  4 +----------------+
> - *    | version        | Version of this structure. Current version is 0. New
> + *    | version        | Version of this structure. Current version is 1. New
>   *    |                | versions are guaranteed to be backwards-compatible.
>   *  8 +----------------+
>   *    | flags          | SIF_xxx flags.
> @@ -48,6 +48,15 @@
>   * 32 +----------------+
>   *    | rsdp_paddr     | Physical address of the RSDP ACPI data structure.
>   * 40 +----------------+
> + *    | memmap_paddr   | Physical address of the (optional) memory map. Only
> + *    |                | present in version 1 and newer of the structure.
> + * 48 +----------------+
> + *    | memmap_entries | Number of entries in the memory map table. Zero
> + *    |                | if there is no memory map being provided. Only
> + *    |                | present in version 1 and newer of the structure.
> + * 52 +----------------+
> + *    | reserved       | Version 1 and newer only.
> + * 56 +----------------+
>   *
>   * The layout of each entry in the module structure is the following:
>   *
> @@ -62,14 +71,53 @@
>   *    | reserved       |
>   * 32 +----------------+
>   *
> + * The layout of each entry in the memory map table is as follows:
> + *
> + *  0 +----------------+
> + *    | addr           | Base address
> + *  8 +----------------+
> + *    | size           | Size of mapping in bytes
> + * 16 +----------------+
> + *    | type           | Type of mapping as defined between the hypervisor
> + *    |                | and guest it's starting. See XEN_HVM_MEMMAP_TYPE_*

I would remove "it's starting" here.

> + *    |                | values below.
> + * 20 +----------------|
> + *    | reserved       |
> + * 24 +----------------+
> + *
>   * The address and sizes are always a 64bit little endian unsigned integer.
>   *
>   * NB: Xen on x86 will always try to place all the data below the 4GiB
>   * boundary.
> + *
> + * Version numbers of the hvm_start_info structure have evolved like this:
> + *
> + * Version 0:  Initial implementation.
> + *
> + * Version 1:  Added the memmap_paddr/memmap_entries fields (plus 4 bytes of
> + *             padding) to the end of the hvm_start_info struct. These new
> + *             fields can be used to pass a memory map to the guest. The
> + *             memory map is optional and so guests that understand version 1
> + *             of the structure must check that memmap_entries is non-zero
> + *             before trying to read the memory map.
>   */
>  #define XEN_HVM_START_MAGIC_VALUE 0x336ec578
>  
>  /*
> + * The values used in the type field of the memory map table entries are
> + * defined below and match the Address Range Types as defined in the "System
> + * Address Map Interfaces" section of the ACPI Specification. Please refer to
> + * section 15 in version 6.2 of the ACPI spec: http://uefi.org/specifications
> + */
> +#define XEN_HVM_MEMMAP_TYPE_RAM       1
> +#define XEN_HVM_MEMMAP_TYPE_RESERVED  2
> +#define XEN_HVM_MEMMAP_TYPE_ACPI      3
> +#define XEN_HVM_MEMMAP_TYPE_NVS       4
> +#define XEN_HVM_MEMMAP_TYPE_UNUSABLE  5
> +#define XEN_HVM_MEMMAP_TYPE_DISABLED  6
> +#define XEN_HVM_MEMMAP_TYPE_PMEM      7
> +
> +/*
>   * C representation of the x86/HVM start info layout.
>   *
>   * The canonical definition of this layout is above, this is just a way to
> @@ -86,6 +134,14 @@ struct hvm_start_info {
>      uint64_t cmdline_paddr;     /* Physical address of the command line.     
> */
>      uint64_t rsdp_paddr;        /* Physical address of the RSDP ACPI data    
> */
>                                  /* structure.                                
> */
> +    uint64_t memmap_paddr;      /* Physical address of an array of           
> */
> +                                /* hvm_memmap_table_entry. Only present in   
> */
> +                                /* version 1 and newer of the structure      
> */
> +    uint32_t memmap_entries;    /* Number of entries in the memmap table.    
> */
> +                                /* Only present in version 1 and newer of    
> */
> +                                /* the structure. Value will be zero if      
> */
> +                                /* there is no memory map being provided.    
> */
> +    uint32_t reserved;          /* Must be zero for Version 1.               
> */

I would write "Must be zero." only. If at some point we introduce
version 2 we would likely have to fixup this comment to mention
version 1 and version 2.

Thanks, Roger.

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