Re: [PATCH 4/5] KVM: MMU: Optimize pte permission checks

[Avi Kivity <avi@redhat.com>]

On 09/13/2012 03:09 PM, Xiao Guangrong wrote:
>> 
>> The result is short, branch-free code.
>> 
>> Signed-off-by: Avi Kivity <avi@redhat.com>
> 
>> +static void update_permission_bitmask(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
>> +{
>> +	unsigned bit, byte, pfec;
>> +	u8 map;
>> +	bool fault, x, w, u, wf, uf, ff, smep;
>> +
>> +	smep = kvm_read_cr4_bits(vcpu, X86_CR4_SMEP);
>> +	for (byte = 0; byte < ARRAY_SIZE(mmu->permissions); ++byte) {
>> +		pfec = byte << 1;
>> +		map = 0;
>> +		wf = pfec & PFERR_WRITE_MASK;
>> +		uf = pfec & PFERR_USER_MASK;
>> +		ff = pfec & PFERR_FETCH_MASK;
>> +		for (bit = 0; bit < 8; ++bit) {
>> +			x = bit & ACC_EXEC_MASK;
>> +			w = bit & ACC_WRITE_MASK;
>> +			u = bit & ACC_USER_MASK;
>> +
>> +			/* Not really needed: !nx will cause pte.nx to fault */
>> +			x |= !mmu->nx;
>> +			/* Allow supervisor writes if !cr0.wp */
>> +			w |= !is_write_protection(vcpu) && !uf;
>> +			/* Disallow supervisor fetches if cr4.smep */
>> +			x &= !(smep && !uf);
> 
> In the case of smep, supervisor mode can fetch the memory if pte.u == 0,
> so, it should be x &= !(smep && !uf && u)?

Good catch, will fix.

> 
>> @@ -3672,20 +3672,18 @@ static int vcpu_mmio_gva_to_gpa(struct kvm_vcpu *vcpu, unsigned long gva,
>>  				gpa_t *gpa, struct x86_exception *exception,
>>  				bool write)
>>  {
>> -	u32 access = (kvm_x86_ops->get_cpl(vcpu) == 3) ? PFERR_USER_MASK : 0;
>> +	u32 access = ((kvm_x86_ops->get_cpl(vcpu) == 3) ? PFERR_USER_MASK : 0)
>> +		| (write ? PFERR_WRITE_MASK : 0);
>> +	u8 bit = vcpu->arch.access;
>> 
>> -	if (vcpu_match_mmio_gva(vcpu, gva) &&
>> -		  check_write_user_access(vcpu, write, access,
>> -		  vcpu->arch.access)) {
>> +	if (vcpu_match_mmio_gva(vcpu, gva)
>> +	    && ((vcpu->arch.walk_mmu->permissions[access >> 1] >> bit) & 1)) {
> 
> !((vcpu->arch.walk_mmu->permissions[access >> 1] >> bit) & 1) ?
> 
> It is better introducing a function to do the permission check?
> 

Probably, I'll rethink it.

-- 
error compiling committee.c: too many arguments to function