linux-kernel - [PATCH v10 062/108] KVM: x86/tdp_mmu: implement MapGPA hypercall for TDX

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Date:   Fri,  9 Dec 2022 00:01:30 +0000
From:   Vishal Annapurve <vannapurve@...gle.com>
To:     binbin.wu@...ux.intel.com
Cc:     dmatlack@...gle.com, erdemaktas@...gle.com,
        isaku.yamahata@...il.com, isaku.yamahata@...el.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, sagis@...gle.com, seanjc@...gle.com
Subject: [PATCH v10 062/108] KVM: x86/tdp_mmu: implement MapGPA hypercall for TDX

> +int __kvm_mmu_map_gpa(struct kvm *kvm, gfn_t *startp, gfn_t end,
> +		      bool map_private)
> +{
> +	gfn_t start = *startp;
> +	int attr;
> +	int ret;
> +
> +	if (!kvm_gfn_shared_mask(kvm))
> +		return -EOPNOTSUPP;
> +
> +	attr = map_private ? KVM_MEM_ATTR_PRIVATE : KVM_MEM_ATTR_SHARED;
> +	start = start & ~kvm_gfn_shared_mask(kvm);
> +	end = end & ~kvm_gfn_shared_mask(kvm);
> +
> +	/*
> +	 * To make the following kvm_vm_set_mem_attr() success within spinlock
> +	 * without memory allocation.
> +	 */
> +	ret = kvm_vm_reserve_mem_attr(kvm, start, end);
> +	if (ret)
> +		return ret;
> +
> +	write_lock(&kvm-> mmu_lock);
> +	if (is_tdp_mmu_enabled(kvm)) {
> +		gfn_t s = start;
> +
> +		ret = kvm_tdp_mmu_map_gpa(kvm, &s, end, map_private);
> +		if (!ret) {
> +			KVM_BUG_ON(kvm_vm_set_mem_attr(kvm, attr, start, end), kvm);

This will result in no exits to userspace during memory conversion requests from
guests. And as a result, userspace will not be able to explicitly back/unback
shared/private memory during conversions leading to double allocation of memory.

Is this an intended behavior for memory conversion with TDX VMs as per earlier
discussion?

> +		} else if (ret == -EAGAIN) {
> +			KVM_BUG_ON(kvm_vm_set_mem_attr(kvm, attr, start, s), kvm);
> +			start = s;
> +		}
> +	} else {
> +		ret = -EOPNOTSUPP;
> +	}
> +	write_unlock(&kvm-> mmu_lock);
> +
> +	if (ret == -EAGAIN) {
> +		if (map_private)