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

[Donald Dutile <ddutile@redhat.com>]

On 5/23/25 3:30 PM, Donald Dutile wrote:
> 
> 
> 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?
> 
ah, I see you have the weak function in patch 5/5.
But I think you have to move that hunk to this patch, so a bisect won't cause
a build warning (or failure, depending on how a distro sets -W in its builds).

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