Re: [PATCH v9 7/8] KVM: Handle page fault for private memory

[Isaku Yamahata]

On Tue, Oct 25, 2022 at 11:13:43PM +0800,
Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote:

> A memslot with KVM_MEM_PRIVATE being set can include both fd-based
> private memory and hva-based shared memory. Architecture code (like TDX
> code) can tell whether the on-going fault is private or not. This patch
> adds a 'is_private' field to kvm_page_fault to indicate this and
> architecture code is expected to set it.
> 
> To handle page fault for such memslot, the handling logic is different
> depending on whether the fault is private or shared. KVM checks if
> 'is_private' matches the host's view of the page (maintained in
> mem_attr_array).
>   - For a successful match, private pfn is obtained with
>     restrictedmem_get_page () from private fd and shared pfn is obtained
>     with existing get_user_pages().
>   - For a failed match, KVM causes a KVM_EXIT_MEMORY_FAULT exit to
>     userspace. Userspace then can convert memory between private/shared
>     in host's view and retry the fault.
> 
> Co-developed-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c          | 56 +++++++++++++++++++++++++++++++--
>  arch/x86/kvm/mmu/mmu_internal.h | 14 ++++++++-
>  arch/x86/kvm/mmu/mmutrace.h     |  1 +
>  arch/x86/kvm/mmu/spte.h         |  6 ++++
>  arch/x86/kvm/mmu/tdp_mmu.c      |  3 +-
>  include/linux/kvm_host.h        | 28 +++++++++++++++++
>  6 files changed, 103 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 67a9823a8c35..10017a9f26ee 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3030,7 +3030,7 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn,
>  
>  int kvm_mmu_max_mapping_level(struct kvm *kvm,
>  			      const struct kvm_memory_slot *slot, gfn_t gfn,
> -			      int max_level)
> +			      int max_level, bool is_private)
>  {
>  	struct kvm_lpage_info *linfo;
>  	int host_level;
> @@ -3042,6 +3042,9 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm,
>  			break;
>  	}
>  
> +	if (is_private)
> +		return max_level;

Below PG_LEVEL_NUM is passed by zap_collapsible_spte_range().  It doesn't make
sense.

> +
>  	if (max_level == PG_LEVEL_4K)
>  		return PG_LEVEL_4K;
>  
> @@ -3070,7 +3073,8 @@ void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>  	 * level, which will be used to do precise, accurate accounting.
>  	 */
>  	fault->req_level = kvm_mmu_max_mapping_level(vcpu->kvm, slot,
> -						     fault->gfn, fault->max_level);
> +						     fault->gfn, fault->max_level,
> +						     fault->is_private);
>  	if (fault->req_level == PG_LEVEL_4K || fault->huge_page_disallowed)
>  		return;
>  
> @@ -4141,6 +4145,32 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
>  	kvm_mmu_do_page_fault(vcpu, work->cr2_or_gpa, 0, true);
>  }
>  
> +static inline u8 order_to_level(int order)
> +{
> +	BUILD_BUG_ON(KVM_MAX_HUGEPAGE_LEVEL > PG_LEVEL_1G);
> +
> +	if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_1G))
> +		return PG_LEVEL_1G;
> +
> +	if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M))
> +		return PG_LEVEL_2M;
> +
> +	return PG_LEVEL_4K;
> +}
> +
> +static int kvm_faultin_pfn_private(struct kvm_page_fault *fault)
> +{
> +	int order;
> +	struct kvm_memory_slot *slot = fault->slot;
> +
> +	if (kvm_restricted_mem_get_pfn(slot, fault->gfn, &fault->pfn, &order))
> +		return RET_PF_RETRY;
> +
> +	fault->max_level = min(order_to_level(order), fault->max_level);
> +	fault->map_writable = !(slot->flags & KVM_MEM_READONLY);
> +	return RET_PF_CONTINUE;
> +}
> +
>  static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  {
>  	struct kvm_memory_slot *slot = fault->slot;
> @@ -4173,6 +4203,22 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  			return RET_PF_EMULATE;
>  	}
>  
> +	if (kvm_slot_can_be_private(slot) &&
> +	    fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> +		vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
> +		if (fault->is_private)
> +			vcpu->run->memory.flags = KVM_MEMORY_EXIT_FLAG_PRIVATE;
> +		else
> +			vcpu->run->memory.flags = 0;
> +		vcpu->run->memory.padding = 0;
> +		vcpu->run->memory.gpa = fault->gfn << PAGE_SHIFT;
> +		vcpu->run->memory.size = PAGE_SIZE;
> +		return RET_PF_USER;
> +	}
> +
> +	if (fault->is_private)
> +		return kvm_faultin_pfn_private(fault);
> +
>  	async = false;
>  	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, &async,
>  					  fault->write, &fault->map_writable,
> @@ -5557,6 +5603,9 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
>  			return -EIO;
>  	}
>  
> +	if (r == RET_PF_USER)
> +		return 0;
> +
>  	if (r < 0)
>  		return r;
>  	if (r != RET_PF_EMULATE)
> @@ -6408,7 +6457,8 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
>  		 */
>  		if (sp->role.direct &&
>  		    sp->role.level < kvm_mmu_max_mapping_level(kvm, slot, sp->gfn,
> -							       PG_LEVEL_NUM)) {
> +							       PG_LEVEL_NUM,
> +							       false)) {
>  			kvm_zap_one_rmap_spte(kvm, rmap_head, sptep);
>  
>  			if (kvm_available_flush_tlb_with_range())
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index 582def531d4d..5cdff5ca546c 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -188,6 +188,7 @@ struct kvm_page_fault {
>  
>  	/* Derived from mmu and global state.  */
>  	const bool is_tdp;
> +	const bool is_private;
>  	const bool nx_huge_page_workaround_enabled;
>  
>  	/*
> @@ -236,6 +237,7 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
>   * RET_PF_RETRY: let CPU fault again on the address.
>   * RET_PF_EMULATE: mmio page fault, emulate the instruction directly.
>   * RET_PF_INVALID: the spte is invalid, let the real page fault path update it.
> + * RET_PF_USER: need to exit to userspace to handle this fault.
>   * RET_PF_FIXED: The faulting entry has been fixed.
>   * RET_PF_SPURIOUS: The faulting entry was already fixed, e.g. by another vCPU.
>   *
> @@ -252,6 +254,7 @@ enum {
>  	RET_PF_RETRY,
>  	RET_PF_EMULATE,
>  	RET_PF_INVALID,
> +	RET_PF_USER,
>  	RET_PF_FIXED,
>  	RET_PF_SPURIOUS,
>  };
> @@ -309,7 +312,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  
>  int kvm_mmu_max_mapping_level(struct kvm *kvm,
>  			      const struct kvm_memory_slot *slot, gfn_t gfn,
> -			      int max_level);
> +			      int max_level, bool is_private);
>  void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
>  void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_level);
>  
> @@ -318,4 +321,13 @@ void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
>  void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp);
>  void unaccount_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp);
>  
> +#ifndef CONFIG_HAVE_KVM_RESTRICTED_MEM
> +static inline int kvm_restricted_mem_get_pfn(struct kvm_memory_slot *slot,
> +					gfn_t gfn, kvm_pfn_t *pfn, int *order)
> +{
> +	WARN_ON_ONCE(1);
> +	return -EOPNOTSUPP;
> +}
> +#endif /* CONFIG_HAVE_KVM_RESTRICTED_MEM */
> +
>  #endif /* __KVM_X86_MMU_INTERNAL_H */
> diff --git a/arch/x86/kvm/mmu/mmutrace.h b/arch/x86/kvm/mmu/mmutrace.h
> index ae86820cef69..2d7555381955 100644
> --- a/arch/x86/kvm/mmu/mmutrace.h
> +++ b/arch/x86/kvm/mmu/mmutrace.h
> @@ -58,6 +58,7 @@ TRACE_DEFINE_ENUM(RET_PF_CONTINUE);
>  TRACE_DEFINE_ENUM(RET_PF_RETRY);
>  TRACE_DEFINE_ENUM(RET_PF_EMULATE);
>  TRACE_DEFINE_ENUM(RET_PF_INVALID);
> +TRACE_DEFINE_ENUM(RET_PF_USER);
>  TRACE_DEFINE_ENUM(RET_PF_FIXED);
>  TRACE_DEFINE_ENUM(RET_PF_SPURIOUS);
>  
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 7670c13ce251..9acdf72537ce 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -315,6 +315,12 @@ static inline bool is_dirty_spte(u64 spte)
>  	return dirty_mask ? spte & dirty_mask : spte & PT_WRITABLE_MASK;
>  }
>  
> +static inline bool is_private_spte(u64 spte)
> +{
> +	/* FIXME: Query C-bit/S-bit for SEV/TDX. */
> +	return false;
> +}
> +

PFN encoded in spte doesn't make sense.  In VMM for TDX, private-vs-shared is
determined by S-bit of GFN.


>  static inline u64 get_rsvd_bits(struct rsvd_bits_validate *rsvd_check, u64 pte,
>  				int level)
>  {
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 672f0432d777..9f97aac90606 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1768,7 +1768,8 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
>  			continue;
>  
>  		max_mapping_level = kvm_mmu_max_mapping_level(kvm, slot,
> -							      iter.gfn, PG_LEVEL_NUM);
> +						iter.gfn, PG_LEVEL_NUM,
> +						is_private_spte(iter.old_spte));
>  		if (max_mapping_level < iter.level)
>  			continue;

This is to merge pages into a large page on the next kvm page fault.  large page
support is not yet supported.  Let's skip the private slot until large page
support is done.
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>