On 12/08/2016 20:30, Jim Mattson wrote:
> kvm_arch_vcpu_ioctl_set_sregs() has the following code to read the
> PDPTEs from CR3:
>
> if (!is_long_mode(vcpu) && is_pae(vcpu)) {
> load_pdptrs(vcpu, vcpu->arch.walk_mmu, kvm_read_cr3(vcpu));
> mmu_reset_needed = 1;
> }
>
> This is incorrect if kvm_arch_vcpu_ioctl_set_sregs() is used to
> restore checkpointed CPU state. The guest may have changed the
> in-memory PDPTEs after they were cached by the CPU at CR3-load. There
> should be a mechanism to save/restore the Guest PDPTE[0-3] fields from
> the VMCS.
>
> Are we cutting corners here because SVM does? Or because no guest OS
> is known to behave this way?
A mix of both. There was no reason _not_ to pass PDPTRs around before
SVM NPT was introduced (the first versions of KVM of course used shadow
paging only), but no guest OS treats PDPTRs in a different way than
other non-leaf entries of the page tables. Nowadays, the lack of PDPTR
fields in the SVM VMCB is not a great incentive for implementing this...
Paolo
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