On Mon, Feb 21, 2022, Paolo Bonzini wrote: > The ept_ad field is used during page walk to determine if the guest PTEs > have accessed and dirty bits. In the MMU role, the ad_disabled > bit represents whether the *shadow* PTEs have the bits, so it > would be incorrect to replace PT_HAVE_ACCESSED_DIRTY with just > !mmu->mmu_role.base.ad_disabled. > > However, the similar field in the CPU mode, ad_disabled, is initialized > correctly: to the opposite value of ept_ad for shadow EPT, and zero > for non-EPT guest paging modes (which always have A/D bits). It is > therefore possible to compute PT_HAVE_ACCESSED_DIRTY from the CPU mode, > like other page-format fields; it just has to be inverted to account > for the different polarity. > > Having a CPU mode that is distinct from the MMU roles in fact would even > allow to remove PT_HAVE_ACCESSED_DIRTY macro altogether, and always use > !mmu->cpu_mode.base.ad_disabled. I am not doing this because the macro > has a small effect in terms of dead code elimination: > > text data bss dec hex > 103544 16665 112 120321 1d601 # as of this patch > 103746 16665 112 120523 1d6cb # without PT_HAVE_ACCESSED_DIRTY > > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > --- Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
