Re: [PATCH 1/2] KVM: arm64: Introduce KVM_PGTABLE_S2_NOFWB Stage-2 flag

[Will Deacon]

On Wed, Mar 17, 2021 at 02:17:13PM +0000, Quentin Perret wrote:
> In order to further configure stage-2 page-tables, pass flags to the
> init function using a new enum.
> 
> The first of these flags allows to disable FWB even if the hardware
> supports it as we will need to do so for the host stage-2.
> 
> Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
> 
> ---
> 
> One question is, do we want to use stage2_has_fwb() everywhere, including
> guest-specific paths (e.g. kvm_arch_prepare_memory_region(), ...) ?
> 
> That'd make this patch more intrusive, but would make the whole codebase
> work with FWB enabled on a guest by guest basis. I don't see us use that
> anytime soon (other than maybe debug of some sort?) but it'd be good to
> have an agreement.

I don't see the value in spreading this everywhere for now.

>  arch/arm64/include/asm/kvm_pgtable.h  | 19 +++++++++--
>  arch/arm64/include/asm/pgtable-prot.h |  4 +--
>  arch/arm64/kvm/hyp/pgtable.c          | 49 +++++++++++++++++----------
>  3 files changed, 50 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index b93a2a3526ab..7382bdfb6284 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -56,6 +56,15 @@ struct kvm_pgtable_mm_ops {
>  	phys_addr_t	(*virt_to_phys)(void *addr);
>  };
>  
> +/**
> + * enum kvm_pgtable_stage2_flags - Stage-2 page-table flags.
> + * @KVM_PGTABLE_S2_NOFWB:	Don't enforce Normal-WB even if the CPUs have
> + *				ARM64_HAS_STAGE2_FWB.
> + */
> +enum kvm_pgtable_stage2_flags {
> +	KVM_PGTABLE_S2_NOFWB			= BIT(0),
> +};
> +
>  /**
>   * struct kvm_pgtable - KVM page-table.
>   * @ia_bits:		Maximum input address size, in bits.
> @@ -72,6 +81,7 @@ struct kvm_pgtable {
>  
>  	/* Stage-2 only */
>  	struct kvm_s2_mmu			*mmu;
> +	enum kvm_pgtable_stage2_flags		flags;
>  };
>  
>  /**
> @@ -201,11 +211,16 @@ u64 kvm_get_vtcr(u64 mmfr0, u64 mmfr1, u32 phys_shift);
>   * @arch:	Arch-specific KVM structure representing the guest virtual
>   *		machine.
>   * @mm_ops:	Memory management callbacks.
> + * @flags:	Stage-2 configuration flags.
>   *
>   * Return: 0 on success, negative error code on failure.
>   */
> -int kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm_arch *arch,
> -			    struct kvm_pgtable_mm_ops *mm_ops);
> +int kvm_pgtable_stage2_init_flags(struct kvm_pgtable *pgt, struct kvm_arch *arch,
> +				  struct kvm_pgtable_mm_ops *mm_ops,
> +				  enum kvm_pgtable_stage2_flags flags);
> +
> +#define kvm_pgtable_stage2_init(pgt, arch, mm_ops) \
> +	kvm_pgtable_stage2_init_flags(pgt, arch, mm_ops, 0)

nit: I think some of the kerneldoc refers to "kvm_pgtable_stage_init()"
so that needs a trivial update to e.g. "kvm_pgtable_stage_init*()".

> diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
> index 046be789fbb4..beeb722a82d3 100644
> --- a/arch/arm64/include/asm/pgtable-prot.h
> +++ b/arch/arm64/include/asm/pgtable-prot.h
> @@ -72,10 +72,10 @@ extern bool arm64_use_ng_mappings;
>  #define PAGE_KERNEL_EXEC	__pgprot(PROT_NORMAL & ~PTE_PXN)
>  #define PAGE_KERNEL_EXEC_CONT	__pgprot((PROT_NORMAL & ~PTE_PXN) | PTE_CONT)
>  
> -#define PAGE_S2_MEMATTR(attr)						\
> +#define PAGE_S2_MEMATTR(attr, has_fwb)					\
>  	({								\
>  		u64 __val;						\
> -		if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))		\
> +		if (has_fwb)						\
>  			__val = PTE_S2_MEMATTR(MT_S2_FWB_ ## attr);	\
>  		else							\
>  			__val = PTE_S2_MEMATTR(MT_S2_ ## attr);		\

Can you take the pgt structure instead of a bool here, or does it end up
being really ugly?

Will