Re: [PATCH 5/8] KVM: x86/mmu: Also record spteps in shadow_page_walk

[Sean Christopherson <seanjc@google.com>]

On Fri, Jun 11, 2021, David Matlack wrote:
> In order to use walk_shadow_page_lockless() in fast_page_fault() we need
> to also record the spteps.
> 
> No functional change intended.
> 
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  arch/x86/kvm/mmu/mmu.c          | 1 +
>  arch/x86/kvm/mmu/mmu_internal.h | 3 +++
>  arch/x86/kvm/mmu/tdp_mmu.c      | 1 +
>  3 files changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 8140c262f4d3..765f5b01768d 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3538,6 +3538,7 @@ static bool walk_shadow_page_lockless(struct kvm_vcpu *vcpu, u64 addr,
>  		spte = mmu_spte_get_lockless(it.sptep);
>  		walk->last_level = it.level;
>  		walk->sptes[it.level] = spte;
> +		walk->spteps[it.level] = it.sptep;
>  
>  		if (!is_shadow_present_pte(spte))
>  			break;
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index 26da6ca30fbf..0fefbd5d6c95 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -178,6 +178,9 @@ struct shadow_page_walk {
>  
>  	/* The spte value at each level. */
>  	u64 sptes[PT64_ROOT_MAX_LEVEL + 1];
> +
> +	/* The spte pointers at each level. */
> +	u64 *spteps[PT64_ROOT_MAX_LEVEL + 1];

Hrm.  I'm not sure how I feel about this patch, or about shadow_page_walk in
general.  On the one hand, I like having a common API.  On the other hand, I
really don't like mixing two different protection schemes, e.g. this really
should be

        tdp_ptep_t spteps;

in order to gain the RCU annotations for TDP, but those RCU annotations are
problematic because the legacy MMU doesn't use RCU to protect its page tables.

I also don't like forcing the caller to hold the "lock" for longer than is
necessary, e.g. get_mmio_spte() doesn't require access to the page tables after
the initial walk, but the common spteps[] kinda sorta forces its hand.

The two use cases (and the only common use cases I can see) have fairly different
requirements.  The MMIO check wants the SPTEs at _all_ levels, whereas the fast
page fault handler wants the SPTE _and_ its pointer at a single level.  So I
wonder if by providing a super generic API we'd actually increase complexity.

I.e. rather than provide a completely generic API, maybe it would be better to
have two distinct API.  That wouldn't fix the tdp_ptep_t issue, but it would at
least bound it to some degree and make the code more obvious.  I suspect it would
also reduce the code churn, though that's not necessarily a goal in and of itself.

E.g. for fast_page_fault():

        walk_shadow_page_lockless_begin(vcpu);

        do {
                sptep = get_spte_lockless(..., &spte);
                if (!is_shadow_present_pte(spte))
                        break;

                sp = sptep_to_sp(sptep);
                if (!is_last_spte(spte, sp->role.level))
                        break;

                ...
        } while(true);

        walk_shadow_page_lockless_end(vcpu);


and for get_mmio_spte():
        walk_shadow_page_lockless_begin(vcpu);
        leaf = get_sptes_lockless(vcpu, addr, sptes, &root);
        if (unlikely(leaf < 0)) {
                *sptep = 0ull;
                return reserved;
        }

        walk_shadow_page_lockless_end(vcpu);


And if we look at the TDP MMU implementations, they aren't sharing _that_ much
code, and the code that is shared isn't all that interesting, e.g. if we really
wanted to we could macro-magic away the boilerplate, but I think even I would
balk at the result :-)

int kvm_tdp_mmu_get_sptes_lockless(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
				   int *root_level)
{
	struct tdp_iter iter;
	struct kvm_mmu *mmu = vcpu->arch.mmu;
	gfn_t gfn = addr >> PAGE_SHIFT;
	int leaf = -1;

	*root_level = vcpu->arch.mmu->shadow_root_level;

	tdp_mmu_for_each_pte(iter, mmu, gfn, gfn + 1) {
		leaf = iter.level;
		sptes[leaf] = iter.old_spte;
	}

	return leaf;
}

u64 *kvm_tdp_mmu_get_spte_lockless(struct kvm_vcpu *vcpu, u64 addr, u64 *spte)
{
	struct kvm_mmu *mmu = vcpu->arch.mmu;
	gfn_t gfn = addr >> PAGE_SHIFT;
	struct tdp_iter iter;
	u64 *sptep = NULL;

	*spte = 0ull;

	tdp_mmu_for_each_pte(iter, mmu, gfn, gfn + 1) {
		/*
		 * Here be a comment about the unfortunate differences between
		 * the TDP MMU and the legacy MMU.
		 */
		sptep = (u64 * __force)iter.sptep;
		*spte = iter.old_spte;
	}
	return sptep;
}