Re: [PATCH V3 02/12] KVM: X86/MMU: Add using_local_root_page()

[Sean Christopherson]

On Sat, May 21, 2022, Lai Jiangshan wrote:
> +static bool using_local_root_page(struct kvm_mmu *mmu)

Hmm, I agree with David that "local" isn't the most intuitive terminology.  But
I also do want to avoid private vs. shared to avoid confusion with confidential VMs.

Luckily, I don't think we need to come up with new terminology, just be literal
and call 'em "per-vCPU root pages".  E.g.

  static bool kvm_mmu_has_per_vcpu_root_page()

That way readers don't have to understand what "local" means, and that also captures
per-vCPU roots are an exception, i.e. that most roots are NOT per-vCPU.

> +{
> +	return mmu->root_role.level == PT32E_ROOT_LEVEL ||
> +	       (!mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL);
> +}
> +
>  static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu, int direct)
>  {
>  	struct kvm_mmu_page *sp;
> @@ -4252,10 +4285,11 @@ static bool fast_pgd_switch(struct kvm *kvm, struct kvm_mmu *mmu,
>  {
>  	/*
>  	 * For now, limit the caching to 64-bit hosts+VMs in order to avoid
> -	 * having to deal with PDPTEs. We may add support for 32-bit hosts/VMs
> -	 * later if necessary.
> +	 * having to deal with PDPTEs.  Local roots can not be put into
> +	 * mmu->prev_roots[] because mmu->pae_root can not be shared for
> +	 * different roots at the same time.
>  	 */
> -	if (VALID_PAGE(mmu->root.hpa) && !to_shadow_page(mmu->root.hpa))
> +	if (unlikely(using_local_root_page(mmu)))

I don't know that I like using the local/per-vCPU helper.  The problem isn't _just_
that KVM is using a per-vCPU root, KVM is also deliberately punting on dealing with
PDTPRs.  E.g. the per-vCPU aspect doesn't explain why KVM doesn't allow reusing the
current root.  I don't like that the using_local_root_page() obfuscates that check.

My preference for this would be to revert back to a streamlined variation of the
code prior to commit 5499ea73e7db ("KVM: x86/mmu: look for a cached PGD when going
from 32-bit to 64-bit").

KVM switched to the !to_shadow_page() check to _avoid_ consuming (what is now)
mmu->root_role because, at the time of the patch, mmu held the _old_ data, which
was wrong/stale for nested virtualization transitions.

In other words, I would prefer that explicitly do (in a separate patch):

	/*
	 * For now, limit the fast switch to 64-bit VMs in order to avoid having
	 * to deal with PDPTEs.  32-bit VMs can be supported later if necessary.
	 */
	if (new_role.level < PT64_ROOT_LEVEL4)
		kvm_mmu_free_roots(kvm, mmu, KVM_MMU_ROOT_CURRENT);

The "hosts+VMs" can be shortened to just "VMs", because running a 64-bit VM with
a 32-bit host just doesn't work for a variety of reasons, i.e. doesn't need to be
called out here.