On Wed, Apr 10, 2019 at 12:55:53PM +0000, David Laight wrote: > From: Paolo Bonzini > > Sent: 10 April 2019 10:55 > > > > This check will soon be done on every nested vmentry and vmexit, > > "parallelize" it using bitwise operations. > > > > Signed-off-by: Paolo Bonzini <[email protected]> > > --- > ... > > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h > > index 28406aa1136d..7bc7ac9d2a44 100644 > > --- a/arch/x86/kvm/x86.h > > +++ b/arch/x86/kvm/x86.h > > @@ -347,4 +347,12 @@ static inline void kvm_after_interrupt(struct kvm_vcpu > > *vcpu) > > __this_cpu_write(current_vcpu, NULL); > > } > > > > +static inline bool kvm_pat_valid(u64 data) > > +{ > > + if (data & 0xF8F8F8F8F8F8F8F8) > > + return false; > > + /* 0, 1, 4, 5, 6, 7 are valid values. */ > > + return (data | ((data & 0x0202020202020202) << 1)) == data; > > +} > > + > > How about: > /* > * Each byte must be 0, 1, 4, 5, 6 or 7. > * Convert 001x to 011x then 100x so 2 and 3 fail the test. > */ > data |= (data ^ 0x0404040404040404ULL)) + 0x0202020202020202ULL; > if (data & 0xF8F8F8F8F8F8F8F8ULL) > return false;
Woah. My vote is for Paolo's version as the separate checks allow the reader to walk through step-by-step. The generated assembly isn't much different from a performance perspective since the TEST+JNE will be not taken in the fast path. Fancy: 0x000000000004844f <+255>: movabs $0xf8f8f8f8f8f8f8f8,%rcx 0x0000000000048459 <+265>: xor %eax,%eax 0x000000000004845b <+267>: test %rcx,%rdx 0x000000000004845e <+270>: jne 0x4848b <kvm_mtrr_valid+315> 0x0000000000048460 <+272>: movabs $0x202020202020202,%rax 0x000000000004846a <+282>: and %rdx,%rax 0x000000000004846d <+285>: add %rax,%rax 0x0000000000048470 <+288>: or %rdx,%rax 0x0000000000048473 <+291>: cmp %rdx,%rax 0x0000000000048476 <+294>: sete %al 0x0000000000048479 <+297>: retq Really fancy: 0x0000000000048447 <+247>: movabs $0x404040404040404,%rcx 0x0000000000048451 <+257>: movabs $0x202020202020202,%rax 0x000000000004845b <+267>: xor %rdx,%rcx 0x000000000004845e <+270>: add %rax,%rcx 0x0000000000048461 <+273>: movabs $0xf8f8f8f8f8f8f8f8,%rax 0x000000000004846b <+283>: or %rcx,%rdx 0x000000000004846e <+286>: test %rax,%rdx 0x0000000000048471 <+289>: sete %al 0x0000000000048474 <+292>: retq
