Subject: Re: [PATCH] powerpc/64: Don't try to use radix MMU under a hypervisor

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 20 Dec 2016 20:19:02 +0530

Paul Mackerras <[email-masked]> writes:

> Currently, if the kernel is running on a POWER9 processor under a
> hypervisor, it will try to use the radix MMU even though it doesn't
> have the necessary code to use radix under a hypervisor (it doesn't
> negotiate use of radix, and it doesn't do the H_REGISTER_PROC_TBL
> hcall).  The result is that the guest kernel will crash when it tries
> to turn on the MMU, because it will still actually be using the HPT
> MMU, but it won't have set up any SLB or HPT entries.  It does this
> because the only thing that the kernel looks at in deciding to use
> radix, on any platform, is the ibm,pa-features property on the cpu
> device nodes.
>
> This fixes it by looking for the /chosen/ibm,architecture-vec-5
> property, and if it exists, clearing the radix MMU feature bit.
> We do this before we decide whether to initialize for radix or HPT.
> This property is created by the hypervisor as a result of the guest
> calling the ibm,client-architecture-support method to indicate
> its capabilities, so it only exists on systems with a hypervisor.
> The reason for using this property is that in future, when we
> have support for using radix under a hypervisor, we will need
> to check this property to see whether the hypervisor agreed to
> us using radix.

Hypervisor that doesn't support radix should clear the ibm,pa-features
radix feature bit right ? We look at that before setting
MMU_FTR_TYPE_RADIX. So how did we end up enabling radix in the above
case ?

>
> Fixes: 17a3dd2f5fc7 ("powerpc/mm/radix: Use firmware feature to enable Radix MMU")
> Cc: [email-masked] # v4.7+
> Signed-off-by: Paul Mackerras <[email-masked]>
> ---
>  arch/powerpc/mm/init_64.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
>
> diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
> index a000c35..098531d 100644
> --- a/arch/powerpc/mm/init_64.c
> +++ b/arch/powerpc/mm/init_64.c
> @@ -42,6 +42,8 @@
>  #include <linux/memblock.h>
>  #include <linux/hugetlb.h>
>  #include <linux/slab.h>
> +#include <linux/of_fdt.h>
> +#include <linux/libfdt.h>
>
>  #include <asm/pgalloc.h>
>  #include <asm/page.h>
> @@ -344,6 +346,28 @@ static int __init parse_disable_radix(char *p)
>  }
>  early_param("disable_radix", parse_disable_radix);
>
> +/*
> + * If we're running under a hypervisor, we currently can't do radix
> + * since we don't have the code to do the H_REGISTER_PROC_TBL hcall.
> + * We tell that we're running under a hypervisor by looking for the
> + * /chosen/ibm,architecture-vec-5 property.
> + */
> +static void early_check_vec5(void)
> +{
> +	unsigned long root, chosen;
> +	int size;
> +	const u8 *vec5;
> +
> +	root = of_get_flat_dt_root();
> +	chosen = of_get_flat_dt_subnode_by_name(root, "chosen");
> +	if (chosen == -FDT_ERR_NOTFOUND)
> +		return;
> +	vec5 = of_get_flat_dt_prop(chosen, "ibm,architecture-vec-5", &size);
> +	if (!vec5)
> +		return;
> +	cur_cpu_spec->mmu_features &= ~MMU_FTR_TYPE_RADIX;
> +}
> +
>  void __init mmu_early_init_devtree(void)
>  {
>  	/* Disable radix mode based on kernel command line. */
> @@ -351,6 +375,9 @@ void __init mmu_early_init_devtree(void)
>  		cur_cpu_spec->mmu_features &= ~MMU_FTR_TYPE_RADIX;
>
>  	if (early_radix_enabled())
> +		early_check_vec5();
> +
> +	if (early_radix_enabled())
>  		radix__early_init_devtree();
>  	else
>  		hash__early_init_devtree();
> -- 
> 2.7.4