[PATCH v3 6/7] KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE enabled

[Steven Price]

On 02/09/2022 14:45, Catalin Marinas wrote:
> On Wed, Aug 10, 2022 at 12:30:32PM -0700, Peter Collingbourne wrote:
>> Certain VMMs such as crosvm have features (e.g. sandboxing) that depend
>> on being able to map guest memory as MAP_SHARED. The current restriction
>> on sharing MAP_SHARED pages with the guest is preventing the use of
>> those features with MTE. Now that the races between tasks concurrently
>> clearing tags on the same page have been fixed, remove this restriction.
>>
>> Signed-off-by: Peter Collingbourne <pcc at google.com>
>> ---
>>  arch/arm64/kvm/mmu.c | 8 --------
>>  1 file changed, 8 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index d54be80e31dd..fc65dc20655d 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1075,14 +1075,6 @@ static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
>>  
>>  static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
>>  {
>> -	/*
>> -	 * VM_SHARED mappings are not allowed with MTE to avoid races
>> -	 * when updating the PG_mte_tagged page flag, see
>> -	 * sanitise_mte_tags for more details.
>> -	 */
>> -	if (vma->vm_flags & VM_SHARED)
>> -		return false;
> 
> I think this is fine with the locking in place (BTW, it may be worth
> mentioning in the commit message that it's a relaxation of the ABI). I'd
> like Steven to have a look as well when he gets the time, in case we
> missed anything on the KVM+MTE side.
> 
> Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>

Looks fine to me, and thanks for doing the work: I was never very
pleased with the !VM_SHARED restriction, but I couldn't figure a good
way of getting the locking to work.

Reviewed-by: Steven Price <steven.price at arm.com>