Re: [PATCH] powerpc: mm: Limit rma_size to 1TB when running without HV mode

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Jul 10, 2019 at 03:20:18PM +1000, Suraj Jitindar Singh wrote:
> The virtual real mode addressing (VRMA) mechanism is used when a
> partition is using HPT (Hash Page Table) translation and performs
> real mode accesses (MSR[IR|DR] = 0) in non-hypervisor mode. In this
> mode effective address bits 0:23 are treated as zero (i.e. the access
> is aliased to 0) and the access is performed using an implicit 1TB SLB
> entry.
> 
> The size of the RMA (Real Memory Area) is communicated to the guest as
> the size of the first memory region in the device tree. And because of
> the mechanism described above can be expected to not exceed 1TB. In the
> event that the host erroneously represents the RMA as being larger than
> 1TB, guest accesses in real mode to memory addresses above 1TB will be
> aliased down to below 1TB. This means that a memory access performed in
> real mode may differ to one performed in virtual mode for the same memory
> address, which would likely have unintended consequences.
> 
> To avoid this outcome have the guest explicitly limit the size of the
> RMA to the current maximum, which is 1TB. This means that even if the
> first memory block is larger than 1TB, only the first 1TB should be
> accessed in real mode.
> 
> Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Although I'd really like to also see some comments added in
allocate_paca_ptrs() explaining the constraints there.

Oh, also, basing this on the non-compat PVR is bogus, but it's still
better than what we had.

> ---
>  arch/powerpc/mm/book3s64/hash_utils.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
> index 28ced26f2a00..4d0e2cce9cd5 100644
> --- a/arch/powerpc/mm/book3s64/hash_utils.c
> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> @@ -1901,11 +1901,19 @@ void hash__setup_initial_memory_limit(phys_addr_t first_memblock_base,
>  	 *
>  	 * For guests on platforms before POWER9, we clamp the it limit to 1G
>  	 * to avoid some funky things such as RTAS bugs etc...
> +	 * On POWER9 we limit to 1TB in case the host erroneously told us that
> +	 * the RMA was >1TB. Effective address bits 0:23 are treated as zero
> +	 * (meaning the access is aliased to zero i.e. addr = addr % 1TB)
> +	 * for virtual real mode addressing and so it doesn't make sense to
> +	 * have an area larger than 1TB as it can't be addressed.
>  	 */
>  	if (!early_cpu_has_feature(CPU_FTR_HVMODE)) {
>  		ppc64_rma_size = first_memblock_size;
>  		if (!early_cpu_has_feature(CPU_FTR_ARCH_300))
>  			ppc64_rma_size = min_t(u64, ppc64_rma_size, 0x40000000);
> +		else
> +			ppc64_rma_size = min_t(u64, ppc64_rma_size,
> +					       1UL << SID_SHIFT_1T);
>  
>  		/* Finally limit subsequent allocations */
>  		memblock_set_current_limit(ppc64_rma_size);

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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