Subject: Re: [PATCH 01/18] powerpc/64: Don't try to use radix MMU under a hypervisor

From: Balbir Singh <[email-masked]>
Date: Thu, 12 Jan 2017 22:18:59 +0530

On Thu, Jan 12, 2017 at 08:07:09PM +1100, Paul Mackerras wrote:
> 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

						H_REGISTER_PROCESS_TABLE
for consistency

> hcall).  The result is that the guest kernel will crash when it tries
> to turn on the MMU.
> 
> This fixes it by looking for the /chosen/ibm,architecture-vec-5
> property, and if it exists, clears the radix MMU feature bit,
> 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 will indicate whether the hypervisor
> agreed to us using radix.
> 
> Systems without a hypervisor may have this property also (for
> example, skiboot creates it), so we check the HV bit in the MSR
> to see whether we are running as a guest or not.  If we are in
> hypervisor mode, then we can do whatever we like including
> using the radix MMU.
> 
> 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.
> 
> Cc: [email-masked] # v4.8+
> Signed-off-by: Paul Mackerras <[email-masked]>
> ---
>  arch/powerpc/mm/init_64.c | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
> index 93abf8a..4d9481e 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>
> @@ -344,12 +346,43 @@ 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.
						       _PROCESS_TABLE
> + * 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)
> +{

The function is called early_check, but it also disables MMU_FTR_TYPE_RADIX,
should the disabling be moved out to the caller, since the check has
nothing to do with disabling the feature?

> +	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. */
>  	if (disable_radix)
>  		cur_cpu_spec->mmu_features &= ~MMU_FTR_TYPE_RADIX;
>  
> +	/*
> +	 * Check /chosen/ibm,architecture-vec-5 if running as a guest.
> +	 * When running bare-metal, we can use radix if we like
> +	 * even though the ibm,architecture-vec-5 property created by
> +	 * skiboot doesn't have the necessary bits set.
> +	 */
> +	if (early_radix_enabled() && !(mfmsr() & MSR_HV))
> +		early_check_vec5();

Balbir Singh.