Re: [PATCH] x86/platform: make XENPF_get_dom0_console actually usable

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.02.2023 15:51, Jan Beulich wrote:
> struct dom0_vga_console_info has been extended in the past, and it may
> be extended again. The use in PV Dom0's start info already covers for
> that by supplying the size of the provided data. For the recently
> introduced platform-op size needs providing similarly. Go the easiest
> available route and simply supply size via the hypercall return value.
> 
> While there also add a build-time check that possibly future growth of
> the struct won't affect xen_platform_op_t's size.
> 
> Fixes: 4dd160583c79 ("x86/platform: introduce hypercall to get initial video 
> console settings")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Noticed while trying to actually use the new hypercall in Linux.
> 
> --- a/xen/arch/x86/platform_hypercall.c
> +++ b/xen/arch/x86/platform_hypercall.c
> @@ -841,6 +841,8 @@ ret_t do_platform_op(
>  
>  #ifdef CONFIG_VIDEO
>      case XENPF_get_dom0_console:
> +        BUILD_BUG_ON(sizeof(op->u.dom0_console) > sizeof(op->u.pad));
> +        ret = sizeof(op->u.dom0_console);
>          if ( !fill_console_start_info(&op->u.dom0_console) )
>          {
>              ret = -ENODEV;
> 

Thinking about it, I've added the hunk below.

Jan

--- a/xen/include/public/platform.h
+++ b/xen/include/public/platform.h
@@ -605,7 +605,11 @@ struct xenpf_symdata {
 typedef struct xenpf_symdata xenpf_symdata_t;
 DEFINE_XEN_GUEST_HANDLE(xenpf_symdata_t);
 
-/* Fetch the video console information and mode setup by Xen. */
+/*
+ * Fetch the video console information and mode setup by Xen.  A non-
+ * negative return value indicates the size of the (part of the) structure
+ * which was filled.
+ */
 #define XENPF_get_dom0_console 64
 typedef struct dom0_vga_console_info xenpf_dom0_console_t;
 DEFINE_XEN_GUEST_HANDLE(xenpf_dom0_console_t);