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SubjectRe: [PATCH 02/22] kvm: mmu: Introduce tdp_iter
On Fri, Sep 25, 2020 at 02:22:42PM -0700, Ben Gardon wrote:
> diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
> new file mode 100644
> index 0000000000000..ee90d62d2a9b1
> --- /dev/null
> +++ b/arch/x86/kvm/mmu/tdp_iter.c
> @@ -0,0 +1,163 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#include "mmu_internal.h"
> +#include "tdp_iter.h"
> +
> +/*
> + * Recalculates the pointer to the SPTE for the current GFN and level and
> + * reread the SPTE.
> + */
> +static void tdp_iter_refresh_sptep(struct tdp_iter *iter)
> +{
> + iter->sptep = iter->pt_path[iter->level - 1] +
> + SHADOW_PT_INDEX(iter->gfn << PAGE_SHIFT, iter->level);
> + iter->old_spte = READ_ONCE(*iter->sptep);
> +}
> +
> +/*
> + * Sets a TDP iterator to walk a pre-order traversal of the paging structure
> + * rooted at root_pt, starting with the walk to translate goal_gfn.
> + */
> +void tdp_iter_start(struct tdp_iter *iter, u64 *root_pt, int root_level,
> + gfn_t goal_gfn)
> +{
> + WARN_ON(root_level < 1);
> + WARN_ON(root_level > PT64_ROOT_MAX_LEVEL);
> +
> + iter->goal_gfn = goal_gfn;
> + iter->root_level = root_level;
> + iter->level = root_level;
> + iter->pt_path[iter->level - 1] = root_pt;
> +
> + iter->gfn = iter->goal_gfn -
> + (iter->goal_gfn % KVM_PAGES_PER_HPAGE(iter->level));

Maybe use the params, if only to avoid the line wrap?

iter->gfn = goal_gfn - (goal_gfn % KVM_PAGES_PER_HPAGE(root_level));

Actually, peeking further into the file, this calculation is repeated in both
try_step_up and try_step_down, probably worth adding a helper of some form.

> + tdp_iter_refresh_sptep(iter);
> +
> + iter->valid = true;
> +}
> +
> +/*
> + * Given an SPTE and its level, returns a pointer containing the host virtual
> + * address of the child page table referenced by the SPTE. Returns null if
> + * there is no such entry.
> + */
> +u64 *spte_to_child_pt(u64 spte, int level)
> +{
> + u64 *pt;
> + /* There's no child entry if this entry isn't present */
> + if (!is_shadow_present_pte(spte))
> + return NULL;
> +
> + /* There is no child page table if this is a leaf entry. */
> + if (is_last_spte(spte, level))
> + return NULL;

I'd collapse the checks and their comments.

> +
> + pt = (u64 *)__va(spte_to_pfn(spte) << PAGE_SHIFT);
> + return pt;

No need for the local variable or the explicit cast.

/* There's no child if this entry is non-present or a leaf entry. */
if (!is_shadow_present_pte(spte) || is_last_spte(spte, level))
return NULL;

return __va(spte_to_pfn(spte) << PAGE_SHIFT);

> +}

...

> +void tdp_iter_next(struct tdp_iter *iter)
> +{
> + bool done;
> +
> + done = try_step_down(iter);
> + if (done)
> + return;
> +
> + done = try_step_side(iter);
> + while (!done) {
> + if (!try_step_up(iter)) {
> + iter->valid = false;
> + break;
> + }
> + done = try_step_side(iter);
> + }

At the risk of being too clever:

bool done;

if (try_step_down(iter))
return;

do {
done = try_step_side(iter);
} while (!done && try_step_up(iter));

iter->valid = done;

> +}

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