Re: [Xen-devel] [PATCH] x86/PVH/libxl: Check whether Linux guest can handle RSDP at 4G boundary

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

On Sat, Apr 07, 2018 at 07:03:32PM -0400, Boris Ostrovsky wrote:
> Commit 4a5733771e6f ("libxl: put RSDP for PVH guest near 4GB") breaks
> pre-4.17 Linux guests since they do not use start_info's rsdp_paddr
> pointer and instread scan BIOS memory for RSDP signature.
> 
> Introduce XENFEAT_rsdp_unrestricted feature flag that indicates whether
> the guest can handle RSDP at locations pointed to by rsdp_paddr.
> 
> Since only Linux PVH guests suffer from this problem (BSD has always
> relied on rsdp_paddr) we check this flag just for those guests. If the
> flag is not set we place RSDP in BIOS, as before.

LGTM if we agree to do this based on the contents of
XEN_ELFNOTE_GUEST_OS. I think this is the first time we have a feature
flag that depends on the OS type.

> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> ---
>  tools/libxl/libxl_x86_acpi.c  | 13 +++++++++++--
>  xen/include/public/features.h |  8 ++++++++
>  2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/libxl/libxl_x86_acpi.c b/tools/libxl/libxl_x86_acpi.c
> index fe87418bc1..046d4866c7 100644
> --- a/tools/libxl/libxl_x86_acpi.c
> +++ b/tools/libxl/libxl_x86_acpi.c
> @@ -218,8 +218,17 @@ int libxl__dom_load_acpi(libxl__gc *gc,
>  
>      dom->acpi_modules[0].data = (void *)config.rsdp;
>      dom->acpi_modules[0].length = 64;
> -    dom->acpi_modules[0].guest_addr_out = ACPI_INFO_PHYSICAL_ADDRESS +
> -        (1 + acpi_pages_num) * libxl_ctxt.page_size;
> +    /*
> +     * Some Linux versions cannot properly process hvm_start_info.rsdp_paddr
> +     * and so we need to put RSDP in location that can be discovered by 
> ACPI's
> +     * standard search method, in R-O BIOS memory (we chose last 64 bytes)
> +     */
> +    if (strcmp(dom->parms.guest_os, "linux") ||
> +        elf_xen_feature_get(XENFEAT_rsdp_unrestricted, 
> dom->parms.f_supported))
> +        dom->acpi_modules[0].guest_addr_out = ACPI_INFO_PHYSICAL_ADDRESS +
> +            (1 + acpi_pages_num) * libxl_ctxt.page_size;
> +    else
> +        dom->acpi_modules[0].guest_addr_out = 0x100000 - 64;
>  
>      dom->acpi_modules[1].data = (void *)config.infop;
>      dom->acpi_modules[1].length = 4096;
> diff --git a/xen/include/public/features.h b/xen/include/public/features.h
> index 1a989b8bf9..c5319fbfbf 100644
> --- a/xen/include/public/features.h
> +++ b/xen/include/public/features.h
> @@ -105,6 +105,14 @@
>  /* arm: Hypervisor supports ARM SMC calling convention. */
>  #define XENFEAT_ARM_SMCCC_supported       14
>  
> +/*
> + * x86/PVH: If set, ACPI RSDP can be placed at any address. Otherwise RSDP
> + * must be located in lower 1MB, as required by ACPI Specification for IA-PC
> + * systems.
> + * This feature flag is only consulted for Linux guests.

I would slightly change the above to be more explicit:

This feature flag is only consulted if XEN_ELFNOTE_GUEST_OS contains
the "linux" string.

Thanks, Roger.

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