Re: [PATCH RFC v7 04/64] KVM: x86: Add 'fault_is_private' x86 op

[Michael Roth]

On Thu, Dec 29, 2022 at 05:14:03PM +0100, Borislav Petkov wrote:
> On Wed, Dec 14, 2022 at 01:39:56PM -0600, Michael Roth wrote:
> > This callback is used by the KVM MMU to check whether a #NPF was
> > or a private GPA or not.
> 
> s/or //
> 
> > 
> > Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> > ---
> >  arch/x86/include/asm/kvm-x86-ops.h |  1 +
> >  arch/x86/include/asm/kvm_host.h    |  1 +
> >  arch/x86/kvm/mmu/mmu.c             |  3 +--
> >  arch/x86/kvm/mmu/mmu_internal.h    | 40 +++++++++++++++++++++++++++---
> >  4 files changed, 39 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> > index f530a550c092..efae987cdce0 100644
> > --- a/arch/x86/include/asm/kvm-x86-ops.h
> > +++ b/arch/x86/include/asm/kvm-x86-ops.h
> > @@ -132,6 +132,7 @@ KVM_X86_OP(complete_emulated_msr)
> >  KVM_X86_OP(vcpu_deliver_sipi_vector)
> >  KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
> >  KVM_X86_OP_OPTIONAL_RET0(private_mem_enabled);
> > +KVM_X86_OP_OPTIONAL_RET0(fault_is_private);
> >  
> >  #undef KVM_X86_OP
> >  #undef KVM_X86_OP_OPTIONAL
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 9317abffbf68..92539708f062 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1636,6 +1636,7 @@ struct kvm_x86_ops {
> >  	void (*load_mmu_pgd)(struct kvm_vcpu *vcpu, hpa_t root_hpa,
> >  			     int root_level);
> >  	int (*private_mem_enabled)(struct kvm *kvm);
> > +	int (*fault_is_private)(struct kvm *kvm, gpa_t gpa, u64 error_code, bool *private_fault);
> 
> bool
> 
> and then you don't need the silly "== 1" at the call site.

Obviously I need to add some proper documentation for this, but a 1
return basically means 'private_fault' pass-by-ref arg has been set
with the appropriate value, whereas 0 means "there's no platform-specific
handling for this, so if you have some generic way to determine this
then use that instead".

This is mainly to handle CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING, which
just parrots whatever kvm_mem_is_private() returns to support running
KVM selftests without needed hardware/platform support. If we don't
take care to skip this check where the above fault_is_private() hook
returns 1, then it ends up breaking SNP in cases where the kernel has
been compiled with CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING, since SNP
relies on the page fault flags to make this determination, not
kvm_mem_is_private(), which normally only tracks the memory attributes
set by userspace via KVM_SET_MEMORY_ATTRIBUTES ioctl.

> 
> >  
> >  	bool (*has_wbinvd_exit)(void);
> 
> ...
> 
> > @@ -261,13 +293,13 @@ enum {
> >  };
> >  
> >  static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> > -					u32 err, bool prefetch)
> > +					u64 err, bool prefetch)
> 
> The u32 -> u64 change of err could use a sentence or two of
> clarification in the commit message...

Will do.

-Mike

> 
> >  {
> >  	bool is_tdp = likely(vcpu->arch.mmu->page_fault == kvm_tdp_page_fault);
> >  
> >  	struct kvm_page_fault fault = {
> >  		.addr = cr2_or_gpa,
> > -		.error_code = err,
> > +		.error_code = lower_32_bits(err),
> >  		.exec = err & PFERR_FETCH_MASK,
> >  		.write = err & PFERR_WRITE_MASK,
> >  		.present = err & PFERR_PRESENT_MASK,
> > @@ -281,8 +313,8 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> >  		.max_level = KVM_MAX_HUGEPAGE_LEVEL,
> >  		.req_level = PG_LEVEL_4K,
> >  		.goal_level = PG_LEVEL_4K,
> > -		.is_private = IS_ENABLED(CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING) && is_tdp &&
> > -				kvm_mem_is_private(vcpu->kvm, cr2_or_gpa >> PAGE_SHIFT),
> > +		.is_private = is_tdp && kvm_mmu_fault_is_private(vcpu->kvm,
> > +								 cr2_or_gpa, err),
> >  	};
> >  	int r;
> >  
> > -- 
> > 2.25.1
> > 
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette