linux-kernel - Re: [PATCH v18 040/121] KVM: x86/mmu: Disallow fast page fault on private GPA

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Date: Mon, 26 Feb 2024 10:02:43 -0800
From: Isaku Yamahata <isaku.yamahata@...ux.intel.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: isaku.yamahata@...el.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
	erdemaktas@...gle.com, Sean Christopherson <seanjc@...gle.com>,
	Sagi Shahar <sagis@...gle.com>, Kai Huang <kai.huang@...el.com>,
	chen.bo@...el.com, hang.yuan@...el.com, tina.zhang@...el.com,
	isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v18 040/121] KVM: x86/mmu: Disallow fast page fault on
 private GPA

On Mon, Feb 26, 2024 at 09:55:10AM -0800,
Isaku Yamahata <isaku.yamahata@...ux.intel.com> wrote:

> On Mon, Feb 12, 2024 at 06:01:54PM +0100,
> Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> > On Tue, Jan 23, 2024 at 12:55 AM <isaku.yamahata@...el.com> wrote:
> > >
> > > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > >
> > > TDX requires TDX SEAMCALL to operate Secure EPT instead of direct memory
> > > access and TDX SEAMCALL is heavy operation.  Fast page fault on private GPA
> > > doesn't make sense.  Disallow fast page fault on private GPA.
> > >
> > > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> > > Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
> > > ---
> > >  arch/x86/kvm/mmu/mmu.c | 12 ++++++++++--
> > >  1 file changed, 10 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index b2924bd9b668..54d4c8f1ba68 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -3339,8 +3339,16 @@ static int kvm_handle_noslot_fault(struct kvm_vcpu *vcpu,
> > >         return RET_PF_CONTINUE;
> > >  }
> > >
> > > -static bool page_fault_can_be_fast(struct kvm_page_fault *fault)
> > > +static bool page_fault_can_be_fast(struct kvm *kvm, struct kvm_page_fault *fault)
> > >  {
> > > +       /*
> > > +        * TDX private mapping doesn't support fast page fault because the EPT
> > > +        * entry is read/written with TDX SEAMCALLs instead of direct memory
> > > +        * access.
> > > +        */
> > > +       if (kvm_is_private_gpa(kvm, fault->addr))
> > > +               return false;
> > 
> > I think this does not apply to SNP? If so, it would be better to check
> > the SPTE against the shared-page mask inside the do...while loop.
> 
> No, this won't apply to SNP. Let me update the patch corresponding in v19.

shared-page mask is against GPA or faulting address. Not SPTE unlike SNP.
So it doesn't make sense to check inside the do..while loop.
-- 
Isaku Yamahata <isaku.yamahata@...ux.intel.com>