linux-kernel - Re: [PATCH v18 064/121] KVM: TDX: Create initial guest memory

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Message-ID: <Zbrj5WKVgMsUFDtb@google.com>
Date: Wed, 31 Jan 2024 16:20:53 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: isaku.yamahata@...el.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com, 
	Sagi Shahar <sagis@...gle.com>, Kai Huang <kai.huang@...el.com>, chen.bo@...el.com, 
	hang.yuan@...el.com, tina.zhang@...el.com, gkirkpatrick@...gle.com
Subject: Re: [PATCH v18 064/121] KVM: TDX: Create initial guest memory

On Mon, Jan 22, 2024, isaku.yamahata@...el.com wrote:
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 4cbcedff4f16..1a5a91b99de9 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -591,6 +591,69 @@ static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn,
>  	return 0;
>  }
>  
> +static int tdx_mem_page_add(struct kvm *kvm, gfn_t gfn,
> +			    enum pg_level level, kvm_pfn_t pfn)
> +{
> +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> +	hpa_t hpa = pfn_to_hpa(pfn);
> +	gpa_t gpa = gfn_to_gpa(gfn);
> +	struct tdx_module_args out;
> +	hpa_t source_pa;
> +	bool measure;
> +	u64 err;
> +	int i;
> +
> +	/*
> +	 * KVM_INIT_MEM_REGION, tdx_init_mem_region(), supports only 4K page
> +	 * because tdh_mem_page_add() supports only 4K page.
> +	 */
> +	if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm))
> +		return -EINVAL;
> +
> +	/*
> +	 * In case of TDP MMU, fault handler can run concurrently.  Note
> +	 * 'source_pa' is a TD scope variable, meaning if there are multiple
> +	 * threads reaching here with all needing to access 'source_pa', it
> +	 * will break.  However fortunately this won't happen, because below
> +	 * TDH_MEM_PAGE_ADD code path is only used when VM is being created
> +	 * before it is running, using KVM_TDX_INIT_MEM_REGION ioctl (which
> +	 * always uses vcpu 0's page table and protected by vcpu->mutex).
> +	 */

Most of the above is superflous.  tdx_mem_page_add() is called if and only if
the TD is finalized, and the TDX module disallow running vCPUs before the TD is
finalized.  That's it.  And maybe throw in a lockdep to assert that kvm->lock is
held.

> +	if (KVM_BUG_ON(kvm_tdx->source_pa == INVALID_PAGE, kvm)) {
> +		tdx_unpin(kvm, pfn);
> +		return -EINVAL;
> +	}
> +
> +	source_pa = kvm_tdx->source_pa & ~KVM_TDX_MEASURE_MEMORY_REGION;
> +	measure = kvm_tdx->source_pa & KVM_TDX_MEASURE_MEMORY_REGION;
> +	kvm_tdx->source_pa = INVALID_PAGE;
> +
> +	do {
> +		err = tdh_mem_page_add(kvm_tdx->tdr_pa, gpa, hpa, source_pa,
> +				       &out);
> +		/*
> +		 * This path is executed during populating initial guest memory
> +		 * image. i.e. before running any vcpu.  Race is rare.

How are races possible at all?

> +		 */
> +	} while (unlikely(err == TDX_ERROR_SEPT_BUSY));
> +	if (KVM_BUG_ON(err, kvm)) {
> +		pr_tdx_error(TDH_MEM_PAGE_ADD, err, &out);
> +		tdx_unpin(kvm, pfn);
> +		return -EIO;
> +	} else if (measure) {
> +		for (i = 0; i < PAGE_SIZE; i += TDX_EXTENDMR_CHUNKSIZE) {
> +			err = tdh_mr_extend(kvm_tdx->tdr_pa, gpa + i, &out);
> +			if (KVM_BUG_ON(err, &kvm_tdx->kvm)) {
> +				pr_tdx_error(TDH_MR_EXTEND, err, &out);
> +				break;
> +			}
> +		}

Why is measurement done deep within the MMU?  At a glance, I don't see why this
can't be done up in the ioctl, outside of a spinlock.

And IIRC, the order affects the measurement but doesn't truly matter, e.g. KVM
could choose to completely separate tdh_mr_extend() from tdh_mem_page_add(), no?

> +static int tdx_init_mem_region(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
> +{
> +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> +	struct kvm_tdx_init_mem_region region;
> +	struct kvm_vcpu *vcpu;
> +	struct page *page;
> +	int idx, ret = 0;
> +	bool added = false;
> +
> +	/* Once TD is finalized, the initial guest memory is fixed. */
> +	if (is_td_finalized(kvm_tdx))
> +		return -EINVAL;
> +
> +	/* The BSP vCPU must be created before initializing memory regions. */
> +	if (!atomic_read(&kvm->online_vcpus))
> +		return -EINVAL;
> +
> +	if (cmd->flags & ~KVM_TDX_MEASURE_MEMORY_REGION)
> +		return -EINVAL;
> +
> +	if (copy_from_user(&region, (void __user *)cmd->data, sizeof(region)))
> +		return -EFAULT;
> +
> +	/* Sanity check */
> +	if (!IS_ALIGNED(region.source_addr, PAGE_SIZE) ||
> +	    !IS_ALIGNED(region.gpa, PAGE_SIZE) ||
> +	    !region.nr_pages ||
> +	    region.nr_pages & GENMASK_ULL(63, 63 - PAGE_SHIFT) ||
> +	    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)))
> +		return -EINVAL;
> +
> +	vcpu = kvm_get_vcpu(kvm, 0);
> +	if (mutex_lock_killable(&vcpu->mutex))
> +		return -EINTR;

The real reason for this drive-by pseudo-review is that I am hoping/wishing we
can turn this into a generic KVM ioctl() to allow userspace to pre-map guest
memory[*].

If we're going to carry non-trivial code, we might as well squeeze as much use
out of it as we can.

Beyond wanting to shove this into KVM_MEMORY_ENCRYPT_OP, is there any reason why
this is a VM ioctl() and not a vCPU ioctl()?  Very roughly, couldn't we use a
struct like this as input to a vCPU ioctl() that maps memory, and optionally
initializes memory from @source?

	struct kvm_memory_mapping {
		__u64 base_gfn;
		__u64 nr_pages;
		__u64 flags;
		__u64 source;
	}

TDX would need to do special things for copying the source, but beyond that most
of the code in this function is generic.

[*] https://lore.kernel.org/all/65262e67-7885-971a-896d-ad9c0a760907@polito.it