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

[Sean Christopherson]

On Thu, Mar 07, 2019 at 05:41:39PM +0100, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:
> 
> > 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?
> 
> Oh, thank you for taking a look! You've probably missed my original v1
> patch where I tried to explaing the issue:
> 
> "I noticed that fast_cr3_switch() always fails when we switch back from
> L2 to L1 as it is not able to find a cached root. This is odd: host's
> CR3 usually stays the same, we expect to always follow the fast
> path. Turns out the problem is that page role is always mismatched
> because kvm_mmu_get_page() filters out cr4_pae when direct, the value is
> stored in page header and later compared with new_role in
> cached_root_available(). As cr4_pae is always set in long mode
> prev_roots cache is dysfunctional.
> 
> The problem appeared after we introduced kvm_calc_mmu_role_common():
> previously, we were only setting this bit for shadow MMU root but now we
> set it for everything. Restore the original behavior.
> 
> Fixes: 7dcd57552008 ("x86/kvm/mmu: check if tdp/shadow MMU reconfiguration is needed")
> "
> 
> and the patch was just removing role.cr4_pae setting and moving it to
> kvm_calc_shadow_mmu_root_page_role(). This is an alternative approach -
> always set cr4_pae but count on kvm_mmu_get_page() filtering out cr4_pae
> when direct is set.

Ah, but neither approach solves the underlying problem that KVM can't
identify a shadow page that is associated with a nested EPT entry.
For example, kvm_calc_shadow_ept_root_page_role() should set cr4_pae=1,
i.e. always has 8-byte guest PTEs regardless of PAE.  But that would
break e.g. __kvm_sync_page() as it would always mismatch if L2 is a
non-PAE guest (I think I got that right...).

I think what we could do is repurpose role's nxe, cr0_wp, and
sm{a,e}p_andnot_wp bits to uniquely identify a nested EPT/NPT entry.
E.g. cr0_wp=1 and sm{a,e}p_andnot_wp=1 are an impossible combination.
I'll throw together a patch to see what breaks.  In fact, I think we
could revamp kvm_calc_shadow_ept_root_page_role() to completely ignore
all legacy paging bits, i.e. handling changes in L2's configuration is
L1's problem.