Re: [PATCH v5 2/5] KVM: arm64: New function to determine hardware cache management support

[Donald Dutile <ddutile@redhat.com>]

On 5/23/25 11:44 AM, ankita@nvidia.com wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
> 
> The hardware supports safely mapping PFNMAP as cacheable if it
> is capable of managing cache. This can be determined by the presence
> of FWB (Force Write Back) and CACHE_DIC feature.
> 
> When FWB is not enabled, the kernel expects to trivially do cache
> management by flushing the memory by linearly converting a kvm_pte to
> phys_addr to a KVA. The cache management thus relies on memory being
> mapped. Since the GPU device memory is not kernel mapped, exit when
> the FWB is not supported. Similarly, ARM64_HAS_CACHE_DIC allows KVM
> to avoid flushing the icache and turns icache_inval_pou() into a NOP.
> So the cacheable PFNMAP is contingent on these two hardware features.
> 
> Introduce a new function to make the check for presence of those
> features.
> 
> CC: David Hildenbrand <david@redhat.com>
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> ---
>   arch/arm64/kvm/mmu.c     | 12 ++++++++++++
>   include/linux/kvm_host.h |  2 ++
>   2 files changed, 14 insertions(+)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 305a0e054f81..124655da02ca 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1287,6 +1287,18 @@ void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
>   	kvm_nested_s2_wp(kvm);
>   }
>   
> +/**
> + * kvm_arch_supports_cacheable_pfnmap() - Determine whether hardware
> + *      supports cache management.
> + *
> + * Return: True if FWB and DIC is supported.
> + */
> +bool kvm_arch_supports_cacheable_pfnmap(void)
> +{
> +	return cpus_have_final_cap(ARM64_HAS_STAGE2_FWB) &&
> +	       cpus_have_final_cap(ARM64_HAS_CACHE_DIC);
> +}
> +
>   static void kvm_send_hwpoison_signal(unsigned long address, short lsb)
>   {
>   	send_sig_mceerr(BUS_MCEERR_AR, (void __user *)address, lsb, current);
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 291d49b9bf05..3750d216d456 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1231,6 +1231,8 @@ void kvm_arch_flush_shadow_all(struct kvm *kvm);
>   /* flush memory translations pointing to 'slot' */
>   void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
>   				   struct kvm_memory_slot *slot);
> +/* hardware support cache management */
> +bool kvm_arch_supports_cacheable_pfnmap(void);
>   
Won't this cause a build warning on non-ARM builds, b/c there is no
resolution of this function for the other arch's?
Need #ifdef or default-rtn-0 function for arch's that don't have this function?

>   int kvm_prefetch_pages(struct kvm_memory_slot *slot, gfn_t gfn,
>   		       struct page **pages, int nr_pages);