Re: [RFC PATCH] KVM: TDX: Decouple TDX init mem region from kvm_gmem_populate()

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

On Thu, Jul 10, 2025 at 09:24:13AM -0700, Sean Christopherson wrote:
> On Wed, Jul 09, 2025, Michael Roth wrote:
> > On Thu, Jul 03, 2025 at 02:26:41PM +0800, Yan Zhao wrote:
> > > Rather than invoking kvm_gmem_populate(), allow tdx_vcpu_init_mem_region()
> > > to use open code to populate the initial memory region into the mirror page
> > > table, and add the region to S-EPT.
> > > 
> > > Background
> > > ===
> > > Sean initially suggested TDX to populate initial memory region in a 4-step
> > > way [1]. Paolo refactored guest_memfd and introduced kvm_gmem_populate()
> > > interface [2] to help TDX populate init memory region.
> 
> I wouldn't give my suggestion too much weight; I did qualify it with "Crazy idea."
> after all :-)
> 
> > > tdx_vcpu_init_mem_region
> > >     guard(mutex)(&kvm->slots_lock)
> > >     kvm_gmem_populate
> > >         filemap_invalidate_lock(file->f_mapping)
> > >             __kvm_gmem_get_pfn      //1. get private PFN
> > >             post_populate           //tdx_gmem_post_populate
> > >                 get_user_pages_fast //2. get source page
> > >                 kvm_tdp_map_page    //3. map private PFN to mirror root
> > >                 tdh_mem_page_add    //4. add private PFN to S-EPT and copy
> > >                                          source page to it.
> > > 
> > > kvm_gmem_populate() helps TDX to "get private PFN" in step 1. Its file
> > > invalidate lock also helps ensure the private PFN remains valid when
> > > tdh_mem_page_add() is invoked in TDX's post_populate hook.
> > > 
> > > Though TDX does not need the folio prepration code, kvm_gmem_populate()
> > > helps on sharing common code between SEV-SNP and TDX.
> > > 
> > > Problem
> > > ===
> > > (1)
> > > In Michael's series "KVM: gmem: 2MB THP support and preparedness tracking
> > > changes" [4], kvm_gmem_get_pfn() was modified to rely on the filemap
> > > invalidation lock for protecting its preparedness tracking. Similarly, the
> > > in-place conversion version of guest_memfd series by Ackerly also requires
> > > kvm_gmem_get_pfn() to acquire filemap invalidation lock [5].
> > > 
> > > kvm_gmem_get_pfn
> > >     filemap_invalidate_lock_shared(file_inode(file)->i_mapping);
> > > 
> > > However, since kvm_gmem_get_pfn() is called by kvm_tdp_map_page(), which is
> > > in turn invoked within kvm_gmem_populate() in TDX, a deadlock occurs on the
> > > filemap invalidation lock.
> > 
> > Bringing the prior discussion over to here: it seems wrong that
> > kvm_gmem_get_pfn() is getting called within the kvm_gmem_populate()
> > chain, because:
> > 
> > 1) kvm_gmem_populate() is specifically passing the gmem PFN down to
> >    tdx_gmem_post_populate(), but we are throwing it away to grab it
> >    again kvm_gmem_get_pfn(), which is then creating these locking issues
> >    that we are trying to work around. If we could simply pass that PFN down
> >    to kvm_tdp_map_page() (or some variant), then we would not trigger any
> >    deadlocks in the first place.
> 
> Yes, doing kvm_mmu_faultin_pfn() in tdx_gmem_post_populate() is a major flaw.
> 
> > 2) kvm_gmem_populate() is intended for pre-boot population of guest
> >    memory, and allows the post_populate callback to handle setting
> >    up the architecture-specific preparation, whereas kvm_gmem_get_pfn()
> >    calls kvm_arch_gmem_prepare(), which is intended to handle post-boot
> >    setup of private memory. Having kvm_gmem_get_pfn() called as part of
> >    kvm_gmem_populate() chain brings things 2 things in conflict with
> >    each other, and TDX seems to be relying on that fact that it doesn't
> >    implement a handler for kvm_arch_gmem_prepare(). 
> > 
> > I don't think this hurts anything in the current code, and I don't
> > personally see any issue with open-coding the population path if it doesn't
> > fit TDX very well, but there was some effort put into making
> > kvm_gmem_populate() usable for both TDX/SNP, and if the real issue isn't the
> > design of the interface itself, but instead just some inflexibility on the
> > KVM MMU mapping side, then it seems more robust to address the latter if
> > possible.
> > 
> > Would something like the below be reasonable? 
> 
> No, polluting the page fault paths is a non-starter for me.  TDX really shouldn't
> be synthesizing a page fault when it has the PFN in hand.  And some of the behavior
> that's desirable for pre-faults looks flat out wrong for TDX.  E.g. returning '0'
> on RET_PF_WRITE_PROTECTED and RET_PF_SPURIOUS (though maybe spurious is fine?).
> 
> I would much rather special case this path, because it absolutely is a special
> snowflake.  This even eliminates several exports of low level helpers that frankly
> have no business being used by TDX, e.g. kvm_mmu_reload().
> 
> ---
>  arch/x86/kvm/mmu.h         |  2 +-
>  arch/x86/kvm/mmu/mmu.c     | 78 ++++++++++++++++++++++++++++++++++++--
>  arch/x86/kvm/mmu/tdp_mmu.c |  1 -
>  arch/x86/kvm/vmx/tdx.c     | 24 ++----------
>  4 files changed, 78 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index b4b6860ab971..9cd7a34333af 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -258,7 +258,7 @@ extern bool tdp_mmu_enabled;
>  #endif
>  
>  bool kvm_tdp_mmu_gpa_is_mapped(struct kvm_vcpu *vcpu, u64 gpa);
> -int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, u8 *level);
> +int kvm_tdp_mmu_map_private_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn);
>  
>  static inline bool kvm_memslots_have_rmaps(struct kvm *kvm)
>  {
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6e838cb6c9e1..bc937f8ed5a0 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4900,7 +4900,8 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  	return direct_page_fault(vcpu, fault);
>  }
>  
> -int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, u8 *level)
> +static int kvm_tdp_prefault_page(struct kvm_vcpu *vcpu, gpa_t gpa,
> +				 u64 error_code, u8 *level)
>  {
>  	int r;
>  
> @@ -4942,7 +4943,6 @@ int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, u8 *level
>  		return -EIO;
>  	}
>  }
> -EXPORT_SYMBOL_GPL(kvm_tdp_map_page);
>  
>  long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
>  				    struct kvm_pre_fault_memory *range)
> @@ -4978,7 +4978,7 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
>  	 * Shadow paging uses GVA for kvm page fault, so restrict to
>  	 * two-dimensional paging.
>  	 */
> -	r = kvm_tdp_map_page(vcpu, range->gpa | direct_bits, error_code, &level);
> +	r = kvm_tdp_prefault_page(vcpu, range->gpa | direct_bits, error_code, &level);
>  	if (r < 0)
>  		return r;
>  
> @@ -4990,6 +4990,77 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
>  	return min(range->size, end - range->gpa);
>  }
>  
> +int kvm_tdp_mmu_map_private_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
> +{
> +	struct kvm_page_fault fault = {
> +		.addr = gfn_to_gpa(gfn),
> +		.error_code = PFERR_GUEST_FINAL_MASK | PFERR_PRIVATE_ACCESS,
> +		.prefetch = true,
> +		.is_tdp = true,
> +		.nx_huge_page_workaround_enabled = is_nx_huge_page_enabled(vcpu->kvm),
> +
> +		.max_level = KVM_MAX_HUGEPAGE_LEVEL,
> +		.req_level = PG_LEVEL_4K,
kvm_mmu_hugepage_adjust() will replace the PG_LEVEL_4K here to PG_LEVEL_2M,
because the private_max_mapping_level hook is only invoked in
kvm_mmu_faultin_pfn_gmem().

Updating lpage_info can fix it though.

> +		.goal_level = PG_LEVEL_4K,
> +		.is_private = true,
> +
> +		.gfn = gfn,
> +		.slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn),
> +		.pfn = pfn,
> +		.map_writable = true,
> +	};
> +	struct kvm *kvm = vcpu->kvm;
> +	int r;
> +
> +	lockdep_assert_held(&kvm->slots_lock);
> +
> +	if (KVM_BUG_ON(!tdp_mmu_enabled, kvm))
> +		return -EIO;
> +
> +	if (kvm_gfn_is_write_tracked(kvm, fault.slot, fault.gfn))
> +		return -EPERM;
> +
> +	r = kvm_mmu_reload(vcpu);
> +	if (r)
> +		return r;
> +
> +	r = mmu_topup_memory_caches(vcpu, false);
> +	if (r)
> +		return r;
> +
> +	do {
> +		if (signal_pending(current))
> +			return -EINTR;
> +
> +		if (kvm_test_request(KVM_REQ_VM_DEAD, vcpu))
> +			return -EIO;
> +
> +		cond_resched();
> +
> +		guard(read_lock)(&kvm->mmu_lock);
> +
> +		r = kvm_tdp_mmu_map(vcpu, &fault);
> +	} while (r == RET_PF_RETRY);
> +
> +	if (r != RET_PF_FIXED)
> +		return -EIO;
> +
> +	/*
> +	 * The caller is responsible for ensuring that no MMU invalidations can
> +	 * occur.  Sanity check that the mapping hasn't been zapped.
> +	 */
> +	if (IS_ENABLED(CONFIG_KVM_PROVE_MMU)) {
> +		cond_resched();
> +
> +		scoped_guard(read_lock, &kvm->mmu_lock) {
> +			if (KVM_BUG_ON(!kvm_tdp_mmu_gpa_is_mapped(vcpu, fault.addr), kvm))
> +				return -EIO;
> +		}
> +	}
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(kvm_tdp_mmu_map_private_pfn);

Besides, it can't address the 2nd AB-BA lock issue as mentioned in the patch
log:

Problem
===
...
(2)
Moreover, in step 2, get_user_pages_fast() may acquire mm->mmap_lock,
resulting in the following lock sequence in tdx_vcpu_init_mem_region():
- filemap invalidation lock --> mm->mmap_lock

However, in future code, the shared filemap invalidation lock will be held
in kvm_gmem_fault_shared() (see [6]), leading to the lock sequence:
- mm->mmap_lock --> filemap invalidation lock