Re: [PATCH v7 5/8] KVM: MMU: Add helper function to get pkr bits

["Wang, Lei" <lei4.wang@intel.com>]

On 5/25/2022 7:21 AM, Sean Christopherson wrote:
> On Sun, Apr 24, 2022, Lei Wang wrote:
>> Extra the PKR stuff to a separate, non-inline helper, which is a
> s/Extra/Extract

My mistake, will fix it.

>> preparation to introduce pks support.
> Please provide more justification.  The change is justified, by random readers of
> this patch/commit will be clueless.
>
>    Extract getting the effective PKR bits to a helper that lives in mmu.c
>    in order to keep the is_cr4_*() helpers contained to mmu.c.  Support for
>    PKRS (versus just PKRU) will require querying MMU state to see if the
>    relevant CR4 bit is enabled because pkr_mask will be non-zero if _either_
>    bit is enabled).
>
>    PKR{U,S} are exposed to the guest if and only if TDP is enabled, and
>    while permission_fault() is performance critical for ia32 shadow paging,
>    it's a rarely used path with TDP is enabled.  I.e. moving the PKR code
>    out-of-line is not a performance concern.

Will add more justification to make it clearer to random readers.

>> Signed-off-by: Lei Wang <lei4.wang@intel.com>
>> ---
>>   arch/x86/kvm/mmu.h     | 20 +++++---------------
>>   arch/x86/kvm/mmu/mmu.c | 21 +++++++++++++++++++++
>>   2 files changed, 26 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
>> index cb3f07e63778..cea03053a153 100644
>> --- a/arch/x86/kvm/mmu.h
>> +++ b/arch/x86/kvm/mmu.h
>> @@ -204,6 +204,9 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>>   	return vcpu->arch.mmu->page_fault(vcpu, &fault);
>>   }
>>   
>> +u32 kvm_mmu_pkr_bits(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> kvm_mmu_get_pkr_bits() so that there's a verb in there.

Will fix it.

>> +		     unsigned pte_access, unsigned pte_pkey, unsigned int pfec);
>> +
>>   /*
>>    * Check if a given access (described through the I/D, W/R and U/S bits of a
>>    * page fault error code pfec) causes a permission fault with the given PTE
>> @@ -240,21 +243,8 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>>   
>>   	WARN_ON(pfec & (PFERR_PK_MASK | PFERR_RSVD_MASK));
>>   	if (unlikely(mmu->pkr_mask)) {
>> -		u32 pkr_bits, offset;
>> -
>> -		/*
>> -		* PKRU defines 32 bits, there are 16 domains and 2
>> -		* attribute bits per domain in pkru.  pte_pkey is the
>> -		* index of the protection domain, so pte_pkey * 2 is
>> -		* is the index of the first bit for the domain.
>> -		*/
>> -		pkr_bits = (vcpu->arch.pkru >> (pte_pkey * 2)) & 3;
>> -
>> -		/* clear present bit, replace PFEC.RSVD with ACC_USER_MASK. */
>> -		offset = (pfec & ~1) +
>> -			((pte_access & PT_USER_MASK) << (PFERR_RSVD_BIT - PT_USER_SHIFT));
>> -
>> -		pkr_bits &= mmu->pkr_mask >> offset;
>> +		u32 pkr_bits =
>> +			kvm_mmu_pkr_bits(vcpu, mmu, pte_access, pte_pkey, pfec);
> Nit, I prefer wrapping in the params, that way the first line shows the most
> important information, e.g. what variable is being set and how (by a function call).
> And then there won't be overflow with the longer helper name:
>
> 		u32 pkr_bits = kvm_mmu_get_pkr_bits(vcpu, mmu, pte_access,
> 						    pte_pkey, pfec);

Make sense, will make the wrapping more reasonable.

> Comment needs to be aligned, and it can be adjust to wrap at 80 chars (its
> indentation has changed).
>
> 	/*
> 	 * PKRU and PKRS both define 32 bits. There are 16 domains and 2
> 	 * attribute bits per domain in them. pte_key is the index of the
> 	 * protection domain, so pte_pkey * 2 is the index of the first bit for
> 	 * the domain. The use of PKRU versus PKRS is selected by the address
> 	 * type, as determined by the U/S bit in the paging-structure entries.
> 	 */
Will align and adjust it.