Re: [PATCH v12 08/18] xen/pvh: Load GDT/GS in early PV bootup code for BSP.

[David Vrabel <david.vrabel@citrix.com>]

On 01/01/14 04:35, Konrad Rzeszutek Wilk wrote:
> From: Mukesh Rathor <mukesh.rathor@oracle.com>
> 
> During early bootup we start life using the Xen provided
> GDT, which means that we are running with %cs segment set
> to FLAT_KERNEL_CS (FLAT_RING3_CS64 0xe033, GDT index 261).
> 
> But for PVH we want to be use HVM type mechanism for
> segment operations. As such we need to switch to the HVM
> one and also reload ourselves with the __KERNEL_CS:eip
> to run in the proper GDT and segment.
> 
> For HVM this is usually done in 'secondary_startup_64' in
> (head_64.S) but since we are not taking that bootup
> path (we start in PV - xen_start_kernel) we need to do
> that in the early PV bootup paths.
> 
> For good measure we also zero out the %fs, %ds, and %es
> (not strictly needed as Xen has already cleared them
> for us). The %gs is loaded by 'switch_to_new_gdt'.
[...]
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1414,8 +1414,43 @@ static void __init xen_boot_params_init_edd(void)
>   * we do this, we have to be careful not to call any stack-protected
>   * function, which is most of the kernel.
>   */
> -static void __init xen_setup_stackprotector(void)
> +static void __init xen_setup_gdt(void)
>  {
> +	if (xen_feature(XENFEAT_auto_translated_physmap)) {
> +#ifdef CONFIG_X86_64
> +		unsigned long dummy;
> +
> +		switch_to_new_gdt(0); /* GDT and GS set */
> +
> +		/* We are switching of the Xen provided GDT to our HVM mode
> +		 * GDT. The new GDT has  __KERNEL_CS with CS.L = 1
> +		 * and we are jumping to reload it.
> +		 */
> +		asm volatile ("pushq %0\n"
> +			      "leaq 1f(%%rip),%0\n"
> +			      "pushq %0\n"
> +			      "lretq\n"
> +			      "1:\n"
> +			      : "=&r" (dummy) : "0" (__KERNEL_CS));
> +
> +		/*
> +		 * While not needed, we also set the %es, %ds, and %fs
> +		 * to zero. We don't care about %ss as it is NULL.
> +		 * Strictly speaking this is not needed as Xen zeros those
> +		 * out (and also MSR_FS_BASE, MSR_GS_BASE, MSR_KERNEL_GS_BASE)
> +		 *
> +		 * Linux zeros them in cpu_init() and in secondary_startup_64
> +		 * (for BSP).
> +		 */
> +		loadsegment(es, 0);
> +		loadsegment(ds, 0);
> +		loadsegment(fs, 0);
> +#else
> +		/* PVH: TODO Implement. */
> +		BUG();
> +#endif
> +		return;
> +	}
>  	pv_cpu_ops.write_gdt_entry = xen_write_gdt_entry_boot;
>  	pv_cpu_ops.load_gdt = xen_load_gdt_boot;

If PVH uses native GDT why are these (and possibly other?) GDT ops needed?

David