deactivate fpu when cr0.PE ON?

deactivate fpu when cr0.PE ON?

[Dong, Eddie]

	Following code deactivate fpu when CR0.PE is on, any  explaination? Rest of code active/deactive fpu based on cr0.TS bit.
	

thx, eddie




static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
{
        unsigned long guest_cr3;
        u64 eptp;

        guest_cr3 = cr3;
        if (enable_ept) {
                eptp = construct_eptp(cr3);
                vmcs_write64(EPT_POINTER, eptp);
                ept_sync_context(eptp);
                ept_load_pdptrs(vcpu);
                guest_cr3 = is_paging(vcpu) ? vcpu->arch.cr3 :
                        VMX_EPT_IDENTITY_PAGETABLE_ADDR;
        }

        vmx_flush_tlb(vcpu);
        vmcs_writel(GUEST_CR3, guest_cr3);
        if (vcpu->arch.cr0 & X86_CR0_PE)
                vmx_fpu_deactivate(vcpu);
}

Re: deactivate fpu when cr0.PE ON?

[Avi Kivity]

Dong, Eddie wrote:
> 	Following code deactivate fpu when CR0.PE is on, any  explaination? Rest of code active/deactive fpu based on cr0.TS bit.
> 	
>
>   

The idea is, if the guest is doing a context switch, then it will likely 
set its own lazy fpu code, so we anticipate it and deactivate the cpu.

When cr0.pe is cleared, lazy fpu usually doesn't work so we leave it 
enabled.

However the mechanism of deactivating the fpu on cr3 write doesn't work 
with npt or ept, I'd like to replace it with something better.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.