Re: [PATCH 19/21] KVM: TDX: Add an ioctl to create initial guest memory

[Yan Zhao]

On Tue, Sep 03, 2024 at 08:07:49PM -0700, Rick Edgecombe wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> 
> Add a new ioctl for the user space VMM to initialize guest memory with the
> specified memory contents.
> 
> Because TDX protects the guest's memory, the creation of the initial guest
> memory requires a dedicated TDX module API, TDH.MEM.PAGE.ADD(), instead of
> directly copying the memory contents into the guest's memory in the case of
> the default VM type.
> 
> Define a new subcommand, KVM_TDX_INIT_MEM_REGION, of vCPU-scoped
> KVM_MEMORY_ENCRYPT_OP.  Check if the GFN is already pre-allocated, assign
> the guest page in Secure-EPT, copy the initial memory contents into the
> guest memory, and encrypt the guest memory.  Optionally, extend the memory
> measurement of the TDX guest.
> 
> Discussion history:
> - Originally, KVM_TDX_INIT_MEM_REGION used the callback of the TDP MMU of
>   the KVM page fault handler.  It issues TDX SEAMCALL deep in the call
>   stack, and the ioctl passes down the necessary parameters.  [2] rejected
>   it.  [3] suggests that the call to the TDX module should be invoked in a
>   shallow call stack.
> 
> - Instead, introduce guest memory pre-population [1] that doesn't update
>   vendor-specific part (Secure-EPT in TDX case) and the vendor-specific
>   code (KVM_TDX_INIT_MEM_REGION) updates only vendor-specific parts without
>   modifying the KVM TDP MMU suggested at [4]
> 
>     Crazy idea.  For TDX S-EPT, what if KVM_MAP_MEMORY does all of the
>     SEPT.ADD stuff, which doesn't affect the measurement, and even fills in
>     KVM's copy of the leaf EPTE, but tdx_sept_set_private_spte() doesn't do
>     anything if the TD isn't finalized?
> 
>     Then KVM provides a dedicated TDX ioctl(), i.e. what is/was
>     KVM_TDX_INIT_MEM_REGION, to do PAGE.ADD.  KVM_TDX_INIT_MEM_REGION
>     wouldn't need to map anything, it would simply need to verify that the
>     pfn from guest_memfd() is the same as what's in the TDP MMU.
> 
> - Use the common guest_memfd population function, kvm_gmem_populate()
>   instead of a custom function.  It should check whether the PFN
>   from TDP MMU is the same as the one from guest_memfd. [1]
> 
> - Instead of forcing userspace to do two passes, pre-map the guest
>   initial memory in tdx_gmem_post_populate. [5]
> 
> Link: https://lore.kernel.org/kvm/20240419085927.3648704-1-pbonzini@xxxxxxxxxx/ [1]
> Link: https://lore.kernel.org/kvm/Zbrj5WKVgMsUFDtb@xxxxxxxxxx/ [2]
> Link: https://lore.kernel.org/kvm/Zh8DHbb8FzoVErgX@xxxxxxxxxx/ [3]
> Link: https://lore.kernel.org/kvm/Ze-TJh0BBOWm9spT@xxxxxxxxxx/ [4]
> Link: https://lore.kernel.org/kvm/CABgObfa=a3cKcKJHQRrCs-3Ty8ppSRou=dhi6Q+KdZnom0Zegw@xxxxxxxxxxxxxx/ [5]
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> Co-developed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
> TDX MMU part 2 v1:
>  - Update the code according to latest gmem update.
>    https://lore.kernel.org/kvm/CABgObfa=a3cKcKJHQRrCs-3Ty8ppSRou=dhi6Q+KdZnom0Zegw@xxxxxxxxxxxxxx/
>  - Fixup a aligment bug reported by Binbin.
>  - Rename KVM_MEMORY_MAPPING => KVM_MAP_MEMORY (Sean)
>  - Drop issueing TDH.MEM.PAGE.ADD() on KVM_MAP_MEMORY(), defer it to
>    KVM_TDX_INIT_MEM_REGION. (Sean)
>  - Added nr_premapped to track the number of premapped pages
>  - Drop tdx_post_mmu_map_page().
>  - Drop kvm_slot_can_be_private() check (Paolo)
>  - Use kvm_tdp_mmu_gpa_is_mapped() (Paolo)
> 
> v19:
>  - Switched to use KVM_MEMORY_MAPPING
>  - Dropped measurement extension
>  - updated commit message. private_page_add() => set_private_spte()
> ---
>  arch/x86/include/uapi/asm/kvm.h |   9 ++
>  arch/x86/kvm/vmx/tdx.c          | 150 ++++++++++++++++++++++++++++++++
>  virt/kvm/kvm_main.c             |   1 +
>  3 files changed, 160 insertions(+)
> 
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index 39636be5c891..789d1d821b4f 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -931,6 +931,7 @@ enum kvm_tdx_cmd_id {
>  	KVM_TDX_CAPABILITIES = 0,
>  	KVM_TDX_INIT_VM,
>  	KVM_TDX_INIT_VCPU,
> +	KVM_TDX_INIT_MEM_REGION,
>  	KVM_TDX_GET_CPUID,
>  
>  	KVM_TDX_CMD_NR_MAX,
> @@ -996,4 +997,12 @@ struct kvm_tdx_init_vm {
>  	struct kvm_cpuid2 cpuid;
>  };
>  
> +#define KVM_TDX_MEASURE_MEMORY_REGION   _BITULL(0)
> +
> +struct kvm_tdx_init_mem_region {
> +	__u64 source_addr;
> +	__u64 gpa;
> +	__u64 nr_pages;
> +};
> +
>  #endif /* _ASM_X86_KVM_H */
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 50ce24905062..796d1a495a66 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -8,6 +8,7 @@
>  #include "tdx_ops.h"
>  #include "vmx.h"
>  #include "mmu/spte.h"
> +#include "common.h"
>  
>  #undef pr_fmt
>  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> @@ -1586,6 +1587,152 @@ static int tdx_vcpu_init(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
>  	return 0;
>  }
>  
> +struct tdx_gmem_post_populate_arg {
> +	struct kvm_vcpu *vcpu;
> +	__u32 flags;
> +};
> +
> +static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> +				  void __user *src, int order, void *_arg)
> +{
> +	u64 error_code = PFERR_GUEST_FINAL_MASK | PFERR_PRIVATE_ACCESS;
> +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> +	struct tdx_gmem_post_populate_arg *arg = _arg;
> +	struct kvm_vcpu *vcpu = arg->vcpu;
> +	gpa_t gpa = gfn_to_gpa(gfn);
> +	u8 level = PG_LEVEL_4K;
> +	struct page *page;
> +	int ret, i;
> +	u64 err, entry, level_state;
> +
> +	/*
> +	 * Get the source page if it has been faulted in. Return failure if the
> +	 * source page has been swapped out or unmapped in primary memory.
> +	 */
> +	ret = get_user_pages_fast((unsigned long)src, 1, 0, &page);
> +	if (ret < 0)
> +		return ret;
> +	if (ret != 1)
> +		return -ENOMEM;
> +
> +	if (!kvm_mem_is_private(kvm, gfn)) {
> +		ret = -EFAULT;
> +		goto out_put_page;
> +	}
> +
> +	ret = kvm_tdp_map_page(vcpu, gpa, error_code, &level);
> +	if (ret < 0)
> +		goto out_put_page;
> +
> +	read_lock(&kvm->mmu_lock);
Although mirrored root can't be zapped with shared lock currently, is it
better to hold write_lock() here?

It should bring no extra overhead in a normal condition when the
tdx_gmem_post_populate() is called.

> +
> +	if (!kvm_tdp_mmu_gpa_is_mapped(vcpu, gpa)) {
> +		ret = -ENOENT;
> +		goto out;
> +	}
> +
> +	ret = 0;
> +	do {
> +		err = tdh_mem_page_add(kvm_tdx, gpa, pfn_to_hpa(pfn),
> +				       pfn_to_hpa(page_to_pfn(page)),
> +				       &entry, &level_state);
> +	} while (err == TDX_ERROR_SEPT_BUSY);
> +	if (err) {
> +		ret = -EIO;
> +		goto out;
> +	}
> +
> +	WARN_ON_ONCE(!atomic64_read(&kvm_tdx->nr_premapped));
> +	atomic64_dec(&kvm_tdx->nr_premapped);
> +
> +	if (arg->flags & KVM_TDX_MEASURE_MEMORY_REGION) {
> +		for (i = 0; i < PAGE_SIZE; i += TDX_EXTENDMR_CHUNKSIZE) {
> +			err = tdh_mr_extend(kvm_tdx, gpa + i, &entry,
> +					&level_state);
> +			if (err) {
> +				ret = -EIO;
> +				break;
> +			}
> +		}
> +	}
> +
> +out:
> +	read_unlock(&kvm->mmu_lock);
> +out_put_page:
> +	put_page(page);
> +	return ret;
> +}
> +
> +static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
> +{
> +	struct kvm *kvm = vcpu->kvm;
> +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> +	struct kvm_tdx_init_mem_region region;
> +	struct tdx_gmem_post_populate_arg arg;
> +	long gmem_ret;
> +	int ret;
> +
> +	if (!to_tdx(vcpu)->initialized)
> +		return -EINVAL;
> +
> +	/* Once TD is finalized, the initial guest memory is fixed. */
> +	if (is_td_finalized(kvm_tdx))
> +		return -EINVAL;
> +
> +	if (cmd->flags & ~KVM_TDX_MEASURE_MEMORY_REGION)
> +		return -EINVAL;
> +
> +	if (copy_from_user(&region, u64_to_user_ptr(cmd->data), sizeof(region)))
> +		return -EFAULT;
> +
> +	if (!PAGE_ALIGNED(region.source_addr) || !PAGE_ALIGNED(region.gpa) ||
> +	    !region.nr_pages ||
> +	    region.gpa + (region.nr_pages << PAGE_SHIFT) <= region.gpa ||
> +	    !kvm_is_private_gpa(kvm, region.gpa) ||
> +	    !kvm_is_private_gpa(kvm, region.gpa + (region.nr_pages << PAGE_SHIFT) - 1))
> +		return -EINVAL;
> +
> +	mutex_lock(&kvm->slots_lock);
> +
> +	kvm_mmu_reload(vcpu);
> +	ret = 0;
> +	while (region.nr_pages) {
> +		if (signal_pending(current)) {
> +			ret = -EINTR;
> +			break;
> +		}
> +
> +		arg = (struct tdx_gmem_post_populate_arg) {
> +			.vcpu = vcpu,
> +			.flags = cmd->flags,
> +		};
> +		gmem_ret = kvm_gmem_populate(kvm, gpa_to_gfn(region.gpa),
> +					     u64_to_user_ptr(region.source_addr),
> +					     1, tdx_gmem_post_populate, &arg);
> +		if (gmem_ret < 0) {
> +			ret = gmem_ret;
> +			break;
> +		}
> +
> +		if (gmem_ret != 1) {
> +			ret = -EIO;
> +			break;
> +		}
> +
> +		region.source_addr += PAGE_SIZE;
> +		region.gpa += PAGE_SIZE;
> +		region.nr_pages--;
> +
> +		cond_resched();
> +	}
> +
> +	mutex_unlock(&kvm->slots_lock);
> +
> +	if (copy_to_user(u64_to_user_ptr(cmd->data), &region, sizeof(region)))
> +		ret = -EFAULT;
> +	return ret;
> +}
> +
>  int tdx_vcpu_ioctl(struct kvm_vcpu *vcpu, void __user *argp)
>  {
>  	struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> @@ -1605,6 +1752,9 @@ int tdx_vcpu_ioctl(struct kvm_vcpu *vcpu, void __user *argp)
>  	case KVM_TDX_INIT_VCPU:
>  		ret = tdx_vcpu_init(vcpu, &cmd);
>  		break;
> +	case KVM_TDX_INIT_MEM_REGION:
> +		ret = tdx_vcpu_init_mem_region(vcpu, &cmd);
> +		break;
>  	case KVM_TDX_GET_CPUID:
>  		ret = tdx_vcpu_get_cpuid(vcpu, &cmd);
>  		break;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 73fc3334721d..0822db480719 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2639,6 +2639,7 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
>  
>  	return NULL;
>  }
> +EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_memslot);
>  
>  bool kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
>  {
> -- 
> 2.34.1
>