Re: [RFC PATCH v2 14/23] KVM: TDX: Split and inhibit huge mappings if a VMExit carries level info

[Yan Zhao <yan.y.zhao@intel.com>]

On Wed, Nov 19, 2025 at 01:51:26PM +0800, Binbin Wu wrote:
> 
> 
> On 8/7/2025 5:44 PM, Yan Zhao wrote:
> > TDX requires guests to accept S-EPT mappings created by the host KVM. Due
> > to the current implementation of the TDX module, if a guest accepts a GFN
> > at a lower level after KVM maps it at a higher level, the TDX module will
> > emulate an EPT violation VMExit to KVM instead of returning a size mismatch
> > error to the guest. If KVM fails to perform page splitting in the VMExit
> > handler, the guest's accept operation will be triggered again upon
> > re-entering the guest, causing a repeated EPT violation VMExit.
> > 
> > The TDX module thus enables the EPT violation VMExit to carry the guest's
> > accept level when the VMExit is caused by the guest's accept operation.
> > 
> > Therefore, in TDX's EPT violation handler
> > (1) Set the guest inhibit bit in the lpage info to prevent KVM MMU core
> >      from mapping at a higher a level than the guest's accept level.
>                              ^
>                             an extra 'a'
Thanks.

> > 
> > (2) Split any existing huge mapping at the fault GFN to avoid unsupported
> >      splitting under the shared mmu_lock by TDX.
> > 
> > Use write mmu_lock to pretect (1) and (2) for now. If future KVM TDX can
> 
> pretect -> protect
Thanks.

> > perform the actual splitting under shared mmu_lock with enhanced TDX
> > modules, (1) is possible to be called under shared mmu_lock, and (2) would
> > become unnecessary.
> > 
> > As an optimization, this patch calls hugepage_test_guest_inhibit() without
> > holding the mmu_lock to reduce the frequency of acquiring the write
> > mmu_lock. The write mmu_lock is thus only acquired if the guest inhibit bit
> > is not already set. This is safe because the guest inhibit bit is set in a
> > one-way manner while the splitting under the write mmu_lock is performed
> > before setting the guest inhibit bit.
> > 
> > Link: https://lore.kernel.org/all/a6ffe23fb97e64109f512fa43e9f6405236ed40a.camel@intel.com
> > Suggested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> > Suggested-by: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> > ---
> > RFC v2
> > - Change tdx_get_accept_level() to tdx_check_accept_level().
> > - Invoke kvm_split_cross_boundary_leafs() and hugepage_set_guest_inhibit()
> >    to change KVM mapping level in a global way according to guest accept
> >    level. (Rick, Sean).
> > 
> > RFC v1:
> > - Introduce tdx_get_accept_level() to get guest accept level.
> > - Use tdx->violation_request_level and tdx->violation_gfn* to pass guest
> >    accept level to tdx_gmem_private_max_mapping_level() to detemine KVM
> >    mapping level.
> > ---
> >   arch/x86/kvm/vmx/tdx.c      | 50 +++++++++++++++++++++++++++++++++++++
> >   arch/x86/kvm/vmx/tdx_arch.h |  3 +++
> >   2 files changed, 53 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index 035d81275be4..71115058e5e6 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -2019,6 +2019,53 @@ static inline bool tdx_is_sept_violation_unexpected_pending(struct kvm_vcpu *vcp
> >   	return !(eq & EPT_VIOLATION_PROT_MASK) && !(eq & EPT_VIOLATION_EXEC_FOR_RING3_LIN);
> >   }
> > +static inline int tdx_check_accept_level(struct kvm_vcpu *vcpu, gfn_t gfn)
> 
> The function name sounds like it is just doing check, but it may split a
> hugepage on mismatch.
> 
> How about tdx_enforce_accept_level_mapping() or something else to reflect
> the change could be make?
What about tdx_honor_guest_accept_level()?


> > +{
> > +	struct kvm_memory_slot *slot = gfn_to_memslot(vcpu->kvm, gfn);
> > +	struct vcpu_tdx *tdx = to_tdx(vcpu);
> > +	struct kvm *kvm = vcpu->kvm;
> > +	u64 eeq_type, eeq_info;
> > +	int level = -1;
> > +
> > +	if (!slot)
> > +		return 0;
> > +
> > +	eeq_type = tdx->ext_exit_qualification & TDX_EXT_EXIT_QUAL_TYPE_MASK;
> > +	if (eeq_type != TDX_EXT_EXIT_QUAL_TYPE_ACCEPT)
> > +		return 0;
> > +
> > +	eeq_info = (tdx->ext_exit_qualification & TDX_EXT_EXIT_QUAL_INFO_MASK) >>
> > +		   TDX_EXT_EXIT_QUAL_INFO_SHIFT;
> > +
> > +	level = (eeq_info & GENMASK(2, 0)) + 1;
> > +
> > +	if (level == PG_LEVEL_4K || level == PG_LEVEL_2M) {
> > +		if (!hugepage_test_guest_inhibit(slot, gfn, level + 1)) {
> > +			gfn_t base_gfn = gfn_round_for_level(gfn, level);
> > +			struct kvm_gfn_range gfn_range = {
> > +				.start = base_gfn,
> > +				.end = base_gfn + KVM_PAGES_PER_HPAGE(level),
> > +				.slot = slot,
> > +				.may_block = true,
> > +				.attr_filter = KVM_FILTER_PRIVATE,
> > +			};
> > +
> > +			scoped_guard(write_lock, &kvm->mmu_lock) {
> > +				int ret;
> > +
> > +				ret = kvm_split_cross_boundary_leafs(kvm, &gfn_range, false);
> > +				if (ret)
> > +					return ret;
> > +
> > +				hugepage_set_guest_inhibit(slot, gfn, level + 1);
> > +				if (level == PG_LEVEL_4K)
> > +					hugepage_set_guest_inhibit(slot, gfn, level + 2);
> > +			}
> > +		}
> > +	}
> > +	return 0;
> > +}
> > +
> >   static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
> >   {
> >   	unsigned long exit_qual;
> > @@ -2044,6 +2091,9 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
> >   		 */
> >   		exit_qual = EPT_VIOLATION_ACC_WRITE;
> > +		if (tdx_check_accept_level(vcpu, gpa_to_gfn(gpa)))
> > +			return RET_PF_RETRY;
> > +
> >   		/* Only private GPA triggers zero-step mitigation */
> >   		local_retry = true;
> >   	} else {
> > diff --git a/arch/x86/kvm/vmx/tdx_arch.h b/arch/x86/kvm/vmx/tdx_arch.h
> > index a30e880849e3..af006a73ee05 100644
> > --- a/arch/x86/kvm/vmx/tdx_arch.h
> > +++ b/arch/x86/kvm/vmx/tdx_arch.h
> > @@ -82,7 +82,10 @@ struct tdx_cpuid_value {
> >   #define TDX_TD_ATTR_PERFMON		BIT_ULL(63)
> >   #define TDX_EXT_EXIT_QUAL_TYPE_MASK	GENMASK(3, 0)
> > +#define TDX_EXT_EXIT_QUAL_TYPE_ACCEPT  1
> >   #define TDX_EXT_EXIT_QUAL_TYPE_PENDING_EPT_VIOLATION  6
> > +#define TDX_EXT_EXIT_QUAL_INFO_MASK	GENMASK(63, 32)
> > +#define TDX_EXT_EXIT_QUAL_INFO_SHIFT	32
> >   /*
> >    * TD_PARAMS is provided as an input to TDH_MNG_INIT, the size of which is 1024B.
> >    */
>