Re: [PATCH 4/6] KVM MMU: optimize for writing cr4

Avi Kivity
2010-04-12T08:34:47Z

On 04/12/2010 11:03 AM, Xiao Guangrong wrote:
> Usually, OS changes CR4.PGE bit to flush all global page, under this
> case, no need reset mmu and just flush tlb
>
> Signed-off-by: Xiao Guangrong<xiaoguangrong@cn.fujitsu.com>
> ---
> arch/x86/kvm/x86.c | 9 +++++++++
> 1 files changed, 9 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fd5c3d3..2aaa6fb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -463,6 +463,15 @@ void kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> unsigned long old_cr4 = kvm_read_cr4(vcpu);
> unsigned long pdptr_bits = X86_CR4_PGE | X86_CR4_PSE | X86_CR4_PAE;
>
> + if (cr4 == old_cr4)
> + return;
> +
> + if ((cr4 ^ old_cr4) == X86_CR4_PGE) {
> + kvm_mmu_sync_roots(vcpu);
> + kvm_mmu_flush_tlb(vcpu);
> + return;
> + }
> +
> if (cr4& CR4_RESERVED_BITS) {
> kvm_inject_gp(vcpu, 0);
> return;
>

Later we have:

> kvm_x86_ops->set_cr4(vcpu, cr4);
> vcpu->arch.cr4 = cr4;
> vcpu->arch.mmu.base_role.cr4_pge = (cr4 & X86_CR4_PGE) &&
> !tdp_enabled;

All of which depend on cr4.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.