Re: [PATCH v2 RFC] x86/kvm/mmu: make mmu->prev_roots cache work for NPT case

[Sean Christopherson]

On Thu, Mar 07, 2019 at 03:07:05PM +0100, Vitaly Kuznetsov wrote:
> Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> writes:
> 
> > Alternative patch: remove the filtering from kvm_mmu_get_page() and check
> > for direct on call sites. cr4_pae setting in kvm_calc_mmu_role_common()
> > can be preserved for consistency.
> >
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index f2d1d230d5b8..7fb8118f2af6 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -2420,8 +2420,6 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> >  	role = vcpu->arch.mmu->mmu_role.base;
> >  	role.level = level;
> >  	role.direct = direct;
> > -	if (role.direct)
> > -		role.cr4_pae = 0;
> >  	role.access = access;
> >  	if (!vcpu->arch.mmu->direct_map
> >  	    && vcpu->arch.mmu->root_level <= PT32_ROOT_LEVEL) {
> > @@ -5176,7 +5174,7 @@ static bool detect_write_misaligned(struct kvm_mmu_page *sp, gpa_t gpa,
> >  		 gpa, bytes, sp->role.word);
> >  
> >  	offset = offset_in_page(gpa);
> > -	pte_size = sp->role.cr4_pae ? 8 : 4;
> > +	pte_size = (sp->role.cr4_pae && !sp->role.direct) ? 8 : 4;
> >  
> >  	/*
> >  	 * Sometimes, the OS only writes the last one bytes to update status
> > @@ -5200,7 +5198,7 @@ static u64 *get_written_sptes(struct kvm_mmu_page *sp, gpa_t gpa, int *nspte)
> >  	page_offset = offset_in_page(gpa);
> >  	level = sp->role.level;
> >  	*nspte = 1;
> > -	if (!sp->role.cr4_pae) {
> > +	if (!sp->role.cr4_pae || sp->role.direct) {
> >  		page_offset <<= 1;	/* 32->64 */
> >  		/*
> >  		 * A 32-bit pde maps 4MB while the shadow pdes map
> 
> While I personally prefer this approach to not setting role.cr4_pae in
> kvm_calc_mmu_role_common() I'd like to get maintainers opinion (I did
> test the patch with ept=off but I have to admit I don't know much about
> shadow page tables which actually use detect_write_misaligned()/
> get_written_sptes())
> 
> Paolo, Radim, (anyone else) - any thoughts?

The changes to detect_write_misaligned() and get_written_sptes() are
wrong/unnecessary as those functions should only be called for indirect
shadow PTEs, e.g.:

	for_each_gfn_indirect_valid_sp(vcpu->kvm, sp, gfn) {
		if (detect_write_misaligned(sp, gpa, bytes) ||
		      detect_write_flooding(sp)) {
			kvm_mmu_prepare_zap_page(vcpu->kvm, sp, &invalid_list);
			++vcpu->kvm->stat.mmu_flooded;
			continue;
		}

		spte = get_written_sptes(sp, gpa, &npte);
		if (!spte)
			continue;

		...
	}

If anything, they could WARN_ON(sp->role.direct), but IMO that's overkill
since they're static helpers with a single call site (above).

I'm missing the background for this patch, why does clearing role.cr4_pae
for direct SPTEs cause problems with the prev_roots cache?