| Message ID | 1583376535-27255-1-git-send-email-linmiaohe@huawei.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86: small optimization for is_mtrr_mask calculation | expand |
On 05/03/20 03:48, linmiaohe wrote: > From: Miaohe Lin <linmiaohe@huawei.com> > > We can get is_mtrr_mask by calculating (msr - 0x200) % 2 directly. > > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> > --- > arch/x86/kvm/mtrr.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c > index 7f0059aa30e1..a98701d9f2bf 100644 > --- a/arch/x86/kvm/mtrr.c > +++ b/arch/x86/kvm/mtrr.c > @@ -348,7 +348,7 @@ static void set_var_mtrr_msr(struct kvm_vcpu *vcpu, u32 msr, u64 data) > int index, is_mtrr_mask; > > index = (msr - 0x200) / 2; > - is_mtrr_mask = msr - 0x200 - 2 * index; > + is_mtrr_mask = (msr - 0x200) % 2; > cur = &mtrr_state->var_ranges[index]; > > /* remove the entry if it's in the list. */ > @@ -424,7 +424,7 @@ int kvm_mtrr_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata) > int is_mtrr_mask; > > index = (msr - 0x200) / 2; > - is_mtrr_mask = msr - 0x200 - 2 * index; > + is_mtrr_mask = (msr - 0x200) % 2; > if (!is_mtrr_mask) > *pdata = vcpu->arch.mtrr_state.var_ranges[index].base; > else > If you're going to do that, might as well use ">> 1" for index instead of "/ 2", and "msr & 1" for is_mtrr_mask. Paolo
From: Paolo Bonzini > Sent: 05 March 2020 14:36 > > On 05/03/20 03:48, linmiaohe wrote: > > From: Miaohe Lin <linmiaohe@huawei.com> > > > > We can get is_mtrr_mask by calculating (msr - 0x200) % 2 directly. > > > > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> > > --- > > arch/x86/kvm/mtrr.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c > > index 7f0059aa30e1..a98701d9f2bf 100644 > > --- a/arch/x86/kvm/mtrr.c > > +++ b/arch/x86/kvm/mtrr.c > > @@ -348,7 +348,7 @@ static void set_var_mtrr_msr(struct kvm_vcpu *vcpu, u32 msr, u64 data) > > int index, is_mtrr_mask; > > > > index = (msr - 0x200) / 2; > > - is_mtrr_mask = msr - 0x200 - 2 * index; > > + is_mtrr_mask = (msr - 0x200) % 2; > > cur = &mtrr_state->var_ranges[index]; > > > > /* remove the entry if it's in the list. */ > > @@ -424,7 +424,7 @@ int kvm_mtrr_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata) > > int is_mtrr_mask; > > > > index = (msr - 0x200) / 2; > > - is_mtrr_mask = msr - 0x200 - 2 * index; > > + is_mtrr_mask = (msr - 0x200) % 2; > > if (!is_mtrr_mask) > > *pdata = vcpu->arch.mtrr_state.var_ranges[index].base; > > else > > > > If you're going to do that, might as well use ">> 1" for index instead > of "/ 2", and "msr & 1" for is_mtrr_mask. Provided the variables are unsigned it makes little difference whether you use / % or >> &. At least with / % the two values are the same. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On 05/03/20 16:10, David Laight wrote: >>> index = (msr - 0x200) / 2; >>> - is_mtrr_mask = msr - 0x200 - 2 * index; >>> + is_mtrr_mask = (msr - 0x200) % 2; >>> cur = &mtrr_state->var_ranges[index]; >>> >>> /* remove the entry if it's in the list. */ >>> @@ -424,7 +424,7 @@ int kvm_mtrr_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata) >>> int is_mtrr_mask; >>> >>> index = (msr - 0x200) / 2; >>> - is_mtrr_mask = msr - 0x200 - 2 * index; >>> + is_mtrr_mask = (msr - 0x200) % 2; >>> if (!is_mtrr_mask) >>> *pdata = vcpu->arch.mtrr_state.var_ranges[index].base; >>> else >>> >> If you're going to do that, might as well use ">> 1" for index instead >> of "/ 2", and "msr & 1" for is_mtrr_mask. > Provided the variables are unsigned it makes little difference > whether you use / % or >> &. > At least with / % the two values are the same. Yes, I'm old-fashioned, but also I prefer ">>" and "&" for both signed and unsigned, because if ever I need to switch from unsigned to signed I will get floor-division instead of round-to-zero division (most likely the code doesn't expect negative remainders if it was using unsigned). (That perhaps also reflects on me working a lot with Smalltalk long before switching to the kernel...). Paolo
diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c index 7f0059aa30e1..a98701d9f2bf 100644 --- a/arch/x86/kvm/mtrr.c +++ b/arch/x86/kvm/mtrr.c @@ -348,7 +348,7 @@ static void set_var_mtrr_msr(struct kvm_vcpu *vcpu, u32 msr, u64 data) int index, is_mtrr_mask; index = (msr - 0x200) / 2; - is_mtrr_mask = msr - 0x200 - 2 * index; + is_mtrr_mask = (msr - 0x200) % 2; cur = &mtrr_state->var_ranges[index]; /* remove the entry if it's in the list. */ @@ -424,7 +424,7 @@ int kvm_mtrr_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata) int is_mtrr_mask; index = (msr - 0x200) / 2; - is_mtrr_mask = msr - 0x200 - 2 * index; + is_mtrr_mask = (msr - 0x200) % 2; if (!is_mtrr_mask) *pdata = vcpu->arch.mtrr_state.var_ranges[index].base; else