diff mbox series

[RFC,v6,036/104] KVM: x86/mmu: Explicitly check for MMIO spte in fast page fault

Message ID d1a1da631b44f425d929767fda74c90de2d87a8d.1651774250.git.isaku.yamahata@intel.com (mailing list archive)
State New, archived
Headers show
Series KVM TDX basic feature support | expand

Commit Message

Isaku Yamahata May 5, 2022, 6:14 p.m. UTC 
From: Sean Christopherson <sean.j.christopherson@intel.com>

Explicitly check for an MMIO spte in the fast page fault flow.  TDX will
use a not-present entry for MMIO sptes, which can be mistaken for an
access-tracked spte since both have SPTE_SPECIAL_MASK set.

MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so this
patch does not affect them.  TDX will handle MMIO emulation through a
hypercall instead.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
---
 arch/x86/kvm/mmu/mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

David Matlack Aug. 1, 2022, 10:27 p.m. UTC | #1 
On Thu, May 05, 2022 at 11:14:30AM -0700, isaku.yamahata@intel.com wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
> 
> Explicitly check for an MMIO spte in the fast page fault flow.  TDX will
> use a not-present entry for MMIO sptes, which can be mistaken for an
> access-tracked spte since both have SPTE_SPECIAL_MASK set.
> 
> MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so this
> patch does not affect them.  TDX will handle MMIO emulation through a
> hypercall instead.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index d1c37295bb6e..4a12d862bbb6 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3184,7 +3184,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  		else
>  			sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
>  
> -		if (!is_shadow_present_pte(spte))
> +		if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))

I wonder if this patch is really necessary. is_shadow_present_pte()
checks if SPTE_MMU_PRESENT_MASK is set (which is bit 11, not
shadow_present_mask). Do TDX VMs set bit 11 in MMIO SPTEs?

>  			break;
>  
>  		sp = sptep_to_sp(sptep);
> -- 
> 2.25.1
>
Sean Christopherson Aug. 1, 2022, 11:27 p.m. UTC | #2 
On Mon, Aug 01, 2022, David Matlack wrote:
> On Thu, May 05, 2022 at 11:14:30AM -0700, isaku.yamahata@intel.com wrote:
> > From: Sean Christopherson <sean.j.christopherson@intel.com>
> > 
> > Explicitly check for an MMIO spte in the fast page fault flow.  TDX will
> > use a not-present entry for MMIO sptes, which can be mistaken for an
> > access-tracked spte since both have SPTE_SPECIAL_MASK set.
> > 
> > MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so this
> > patch does not affect them.  TDX will handle MMIO emulation through a
> > hypercall instead.
> > 
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index d1c37295bb6e..4a12d862bbb6 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3184,7 +3184,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> >  		else
> >  			sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
> >  
> > -		if (!is_shadow_present_pte(spte))
> > +		if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
> 
> I wonder if this patch is really necessary. is_shadow_present_pte()
> checks if SPTE_MMU_PRESENT_MASK is set (which is bit 11, not
> shadow_present_mask). Do TDX VMs set bit 11 in MMIO SPTEs?

This patch should be unnecessary, TDX's not-present SPTEs was one of my motivations
for adding MMU_PRESENT.   Bit 11 most definitely must not be set for MMIO SPTEs.
Kai Huang Aug. 2, 2022, 1:46 a.m. UTC | #3 
> On Mon, Aug 01, 2022, David Matlack wrote:
> > On Thu, May 05, 2022 at 11:14:30AM -0700, isaku.yamahata@intel.com wrote:
> > > From: Sean Christopherson <sean.j.christopherson@intel.com>
> > >
> > > Explicitly check for an MMIO spte in the fast page fault flow.  TDX
> > > will use a not-present entry for MMIO sptes, which can be mistaken
> > > for an access-tracked spte since both have SPTE_SPECIAL_MASK set.
> > >
> > > MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so
> > > this patch does not affect them.  TDX will handle MMIO emulation
> > > through a hypercall instead.
> > >
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > > ---
> > >  arch/x86/kvm/mmu/mmu.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index
> > > d1c37295bb6e..4a12d862bbb6 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -3184,7 +3184,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu,
> struct kvm_page_fault *fault)
> > >  		else
> > >  			sptep = fast_pf_get_last_sptep(vcpu, fault->addr,
> &spte);
> > >
> > > -		if (!is_shadow_present_pte(spte))
> > > +		if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
> >
> > I wonder if this patch is really necessary. is_shadow_present_pte()
> > checks if SPTE_MMU_PRESENT_MASK is set (which is bit 11, not
> > shadow_present_mask). Do TDX VMs set bit 11 in MMIO SPTEs?
> 
> This patch should be unnecessary, TDX's not-present SPTEs was one of my
> motivations
> for adding MMU_PRESENT.   Bit 11 most definitely must not be set for MMIO
> SPTEs.

As we already discussed, Isaku will drop this patch.
David Matlack Aug. 2, 2022, 4:34 p.m. UTC | #4 
On Mon, Aug 1, 2022 at 6:46 PM Huang, Kai <kai.huang@intel.com> wrote:
>
> > On Mon, Aug 01, 2022, David Matlack wrote:
> > > On Thu, May 05, 2022 at 11:14:30AM -0700, isaku.yamahata@intel.com wrote:
> > > > From: Sean Christopherson <sean.j.christopherson@intel.com>
> > > >
> > > > Explicitly check for an MMIO spte in the fast page fault flow.  TDX
> > > > will use a not-present entry for MMIO sptes, which can be mistaken
> > > > for an access-tracked spte since both have SPTE_SPECIAL_MASK set.
> > > >
> > > > MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so
> > > > this patch does not affect them.  TDX will handle MMIO emulation
> > > > through a hypercall instead.
> > > >
> > > > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > > > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > > > ---
> > > >  arch/x86/kvm/mmu/mmu.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index
> > > > d1c37295bb6e..4a12d862bbb6 100644
> > > > --- a/arch/x86/kvm/mmu/mmu.c
> > > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > > @@ -3184,7 +3184,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu,
> > struct kvm_page_fault *fault)
> > > >           else
> > > >                   sptep = fast_pf_get_last_sptep(vcpu, fault->addr,
> > &spte);
> > > >
> > > > -         if (!is_shadow_present_pte(spte))
> > > > +         if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
> > >
> > > I wonder if this patch is really necessary. is_shadow_present_pte()
> > > checks if SPTE_MMU_PRESENT_MASK is set (which is bit 11, not
> > > shadow_present_mask). Do TDX VMs set bit 11 in MMIO SPTEs?
> >
> > This patch should be unnecessary, TDX's not-present SPTEs was one of my
> > motivations
> > for adding MMU_PRESENT.   Bit 11 most definitely must not be set for MMIO
> > SPTEs.
>
> As we already discussed, Isaku will drop this patch.

Ah, I missed that discussion. Can you share a link so I can catch up?
Kai Huang Aug. 3, 2022, 12:28 a.m. UTC | #5 
On Tue, 2022-08-02 at 09:34 -0700, David Matlack wrote:
> On Mon, Aug 1, 2022 at 6:46 PM Huang, Kai <kai.huang@intel.com> wrote:
> > 
> > > On Mon, Aug 01, 2022, David Matlack wrote:
> > > > On Thu, May 05, 2022 at 11:14:30AM -0700, isaku.yamahata@intel.com wrote:
> > > > > From: Sean Christopherson <sean.j.christopherson@intel.com>
> > > > > 
> > > > > Explicitly check for an MMIO spte in the fast page fault flow.  TDX
> > > > > will use a not-present entry for MMIO sptes, which can be mistaken
> > > > > for an access-tracked spte since both have SPTE_SPECIAL_MASK set.
> > > > > 
> > > > > MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so
> > > > > this patch does not affect them.  TDX will handle MMIO emulation
> > > > > through a hypercall instead.
> > > > > 
> > > > > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > > > > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > > > > ---
> > > > >  arch/x86/kvm/mmu/mmu.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index
> > > > > d1c37295bb6e..4a12d862bbb6 100644
> > > > > --- a/arch/x86/kvm/mmu/mmu.c
> > > > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > > > @@ -3184,7 +3184,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu,
> > > struct kvm_page_fault *fault)
> > > > >           else
> > > > >                   sptep = fast_pf_get_last_sptep(vcpu, fault->addr,
> > > &spte);
> > > > > 
> > > > > -         if (!is_shadow_present_pte(spte))
> > > > > +         if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
> > > > 
> > > > I wonder if this patch is really necessary. is_shadow_present_pte()
> > > > checks if SPTE_MMU_PRESENT_MASK is set (which is bit 11, not
> > > > shadow_present_mask). Do TDX VMs set bit 11 in MMIO SPTEs?
> > > 
> > > This patch should be unnecessary, TDX's not-present SPTEs was one of my
> > > motivations
> > > for adding MMU_PRESENT.   Bit 11 most definitely must not be set for MMIO
> > > SPTEs.
> > 
> > As we already discussed, Isaku will drop this patch.
> 
> Ah, I missed that discussion. Can you share a link so I can catch up?

Sure.  Isaku has sent out v7 of this series:

https://lore.kernel.org/lkml/20220727220456.GA3669189@ls.amr.corp.intel.com/T/#m8d2229ce31b9bcd084cc43e3478154f5f24d7506

For this particular patch, see here:

https://lore.kernel.org/lkml/20220727220456.GA3669189@ls.amr.corp.intel.com/T/#mcdb118103460c55f8b850e784d1ed57724c0fe2f
diff mbox series

Patch

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d1c37295bb6e..4a12d862bbb6 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3184,7 +3184,7 @@  static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		else
 			sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
 
-		if (!is_shadow_present_pte(spte))
+		if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
 			break;
 
 		sp = sptep_to_sp(sptep);