Re: [PATCH v7 03/14] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR]

Sean Christopherson
2020-03-21T19:22:14Z

On Wed, Mar 18, 2020 at 12:37:09PM -0400, Peter Xu wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e54c6ad628a8..a5123a0aa7d6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9786,7 +9786,34 @@ void kvm_arch_sync_events(struct kvm *kvm)
> kvm_free_pit(kvm);
> }
>
> -int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
> +#define ERR_PTR_USR(e) ((void __user *)ERR_PTR(e))
> +
> +/**
> + * __x86_set_memory_region: Setup KVM internal memory slot
> + *
> + * @kvm: the kvm pointer to the VM.
> + * @id: the slot ID to setup.
> + * @gpa: the GPA to install the slot (unused when @size == 0).
> + * @size: the size of the slot. Set to zero to uninstall a slot.
> + *
> + * This function helps to setup a KVM internal memory slot. Specify
> + * @size > 0 to install a new slot, while @size == 0 to uninstall a
> + * slot. The return code can be one of the following:
> + *
> + * HVA: on success (uninstall will return a bogus HVA)
> + * -errno: on error
> + *
> + * The caller should always use IS_ERR() to check the return value
> + * before use. NOTE: KVM internal memory slots are guaranteed and

"are guaranteed" to ...

> + * won't change until the VM is destroyed. This is also true to the
> + * returned HVA when installing a new memory slot. The HVA can be
> + * invalidated by either an errornous userspace program or a VM under
> + * destruction, however as long as we use __copy_{to|from}_user()
> + * properly upon the HVAs and handle the failure paths always then
> + * we're safe.

Regarding the HVA, it's a bit confusing saying that it's guaranteed to be
valid, and then contradicting that in the second clause. Maybe something
like this to explain the GPA->HVA is guaranteed to be valid, but the
HVA->HPA is not.

/*
* before use. Note, KVM internal memory slots are guaranteed to remain valid
* and unchanged until the VM is destroyed, i.e. the GPA->HVA translation will
* not change. However, the HVA is a user address, i.e. its accessibility is
* not guaranteed, and must be accessed via __copy_{to,from}_user().
*/