On 3/8/22 19:11, Sean Christopherson wrote:
On Mon, Feb 21, 2022, Paolo Bonzini wrote:
Extended bits are unnecessary because page walking uses the CPU mode,
and EFER.NX/CR0.WP can be set to one unconditionally---matching the
format of shadow pages rather than the format of guest pages.
But they don't match the format of shadow pages. EPT has an equivalent to NX in
that KVM can always clear X, but KVM explicitly supports running with EPT and
EFER.NX=0 in the host (32-bit non-PAE kernels).
In which case bit 2 of EPTs doesn't change meaning, does it?
CR0.WP equally confusing. Yes, both EPT and NPT enforce write protection at all
times, but EPT has no concept of user vs. supervisor in the EPT tables themselves,
at least with respect to writes (thanks mode-based execution for the qualifier...).
NPT is even worse as the APM explicitly states:
The host hCR0.WP bit is ignored under nested paging.
Unless there's some hidden dependency I'm missing, I'd prefer we arbitrarily leave
them zero.
Setting EFER.NX=0 might be okay for EPT/NPT, but I'd prefer to set it
respectively to 1 (X bit always present) and host EFER.NX (NX bit
present depending on host EFER).
For CR0.WP it should really be 1 in my opinion, because CR0.WP=0 implies
having a concept of user vs. supervisor access: CR0.WP=1 is the
"default", while CR0.WP=0 is "always allow *supervisor* writes".
even if only barely so, due to SMM and guest mode; for consistency,
pass it down to kvm_calc_tdp_mmu_root_page_role instead of querying
the vcpu with is_smm or is_guest_mode.
The changelog should call out this is a _significant_ change in behavior for KVM,
as it allows reusing shadow pages with different guest MMU "role bits".
Good point! It's safe and arguably clea{n,r}er, but it's still a pretty
large change.
E.g. if this lands after the changes to not unload MMUs on cr0/cr4
emulation, it will be quite the functional change.
I expect this to land first, so that the part where we don't really
agree on the implementation comes last and benefits from a more
understandable core.
Paolo
