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Message-ID: <aLgMd6X1qINCjozy@yzhao56-desk.sh.intel.com>
Date: Wed, 3 Sep 2025 17:37:59 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
CC: <pbonzini@...hat.com>, <seanjc@...gle.com>,
<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>, <x86@...nel.org>,
<rick.p.edgecombe@...el.com>, <dave.hansen@...el.com>, <kas@...nel.org>,
<tabba@...gle.com>, <ackerleytng@...gle.com>, <quic_eberman@...cinc.com>,
<michael.roth@....com>, <david@...hat.com>, <vannapurve@...gle.com>,
<vbabka@...e.cz>, <thomas.lendacky@....com>, <pgonda@...gle.com>,
<zhiquan1.li@...el.com>, <fan.du@...el.com>, <jun.miao@...el.com>,
<ira.weiny@...el.com>, <isaku.yamahata@...el.com>, <xiaoyao.li@...el.com>,
<chao.p.peng@...el.com>
Subject: Re: [RFC PATCH v2 14/23] KVM: TDX: Split and inhibit huge mappings
if a VMExit carries level info
On Wed, Sep 03, 2025 at 03:36:49PM +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.
> >
> > (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
> > 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.
>
> The description for (1) and (2) reversed?
No.
After supporting splitting under shared mmu_lock,
- setting guest inhibit bit can be performed under shared mmu_lock. (*)
- splitting existing huge mapping under write mmu_lock here would be unnecessary.
(*) is still required to convey the info of which max level the guest requires.
(as explained in "Open 1: How to pass guest's ACCEPT level info" in the
cover letter).
> > 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@...el.com>
> > Suggested-by: Sean Christopherson <seanjc@...gle.com>
> > Signed-off-by: Yan Zhao <yan.y.zhao@...el.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)
> > +{
> > + 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;
>
> kvm_split_cross_boundary_leafs() calls kvm_tdp_mmu_gfn_range_split_cross_boundary_leafs(), which could return flush as 1 if any of the huge page crossing boundary is split, return directly when ret is non-zero seems not right. Also, the TLB flush should also be taken care because in kvm_tdp_mmu_gfn_range_split_cross_boundary_leafs(), TLB flush is only done for negative return value.
Oh, good catch!
I forgot about the 2 facts. Will fix them.
> > +
> > + 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.
> > */
>
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