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    SubjectRe: [PATCH v2 04/16] KVM: arm64: Optimize host memory aborts
    On Mon, 26 Jul 2021 10:28:53 +0100,
    Quentin Perret <qperret@google.com> wrote:
    >
    > The kvm_pgtable_stage2_find_range() function is used in the host memory
    > abort path to try and look for the largest block mapping that can be
    > used to map the faulting address. In order to do so, the function
    > currently walks the stage-2 page-table and looks for existing
    > incompatible mappings within the range of the largest possible block.
    > If incompatible mappings are found, it tries the same procedure again,
    > but using a smaller block range, and repeats until a matching range is
    > found (potentially up to page granularity). While this approach has
    > benefits (mostly in the fact that it proactively coalesces host stage-2
    > mappings), it can be slow if the ranges are fragmented, and it isn't
    > optimized to deal with CPUs faulting on the same IPA as all of them will
    > do all the work every time.
    >
    > To avoid these issues, remove kvm_pgtable_stage2_find_range(), and walk
    > the page-table only once in the host_mem_abort() path to find the
    > closest leaf to the input address. With this, use the corresponding
    > range if it is invalid and not owned by another entity. If a valid leaf
    > is found, return -EAGAIN similar to what is done in the
    > kvm_pgtable_stage2_map() path to optimize concurrent faults.
    >
    > Signed-off-by: Quentin Perret <qperret@google.com>
    > ---
    > arch/arm64/include/asm/kvm_pgtable.h | 30 -----------
    > arch/arm64/kvm/hyp/nvhe/mem_protect.c | 39 +++++++++++++-
    > arch/arm64/kvm/hyp/pgtable.c | 74 ---------------------------
    > 3 files changed, 38 insertions(+), 105 deletions(-)
    >
    > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
    > index 5a7a13bbd4a1..cec76a49f521 100644
    > --- a/arch/arm64/include/asm/kvm_pgtable.h
    > +++ b/arch/arm64/include/asm/kvm_pgtable.h
    > @@ -141,16 +141,6 @@ enum kvm_pgtable_prot {
    > #define PAGE_HYP_RO (KVM_PGTABLE_PROT_R)
    > #define PAGE_HYP_DEVICE (PAGE_HYP | KVM_PGTABLE_PROT_DEVICE)
    >
    > -/**
    > - * struct kvm_mem_range - Range of Intermediate Physical Addresses
    > - * @start: Start of the range.
    > - * @end: End of the range.
    > - */
    > -struct kvm_mem_range {
    > - u64 start;
    > - u64 end;
    > -};
    > -
    > /**
    > * enum kvm_pgtable_walk_flags - Flags to control a depth-first page-table walk.
    > * @KVM_PGTABLE_WALK_LEAF: Visit leaf entries, including invalid
    > @@ -477,24 +467,4 @@ int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
    > */
    > int kvm_pgtable_get_leaf(struct kvm_pgtable *pgt, u64 addr,
    > kvm_pte_t *ptep, u32 *level);
    > -
    > -/**
    > - * kvm_pgtable_stage2_find_range() - Find a range of Intermediate Physical
    > - * Addresses with compatible permission
    > - * attributes.
    > - * @pgt: Page-table structure initialised by kvm_pgtable_stage2_init*().
    > - * @addr: Address that must be covered by the range.
    > - * @prot: Protection attributes that the range must be compatible with.
    > - * @range: Range structure used to limit the search space at call time and
    > - * that will hold the result.
    > - *
    > - * The offset of @addr within a page is ignored. An IPA is compatible with @prot
    > - * iff its corresponding stage-2 page-table entry has default ownership and, if
    > - * valid, is mapped with protection attributes identical to @prot.
    > - *
    > - * Return: 0 on success, negative error code on failure.
    > - */
    > -int kvm_pgtable_stage2_find_range(struct kvm_pgtable *pgt, u64 addr,
    > - enum kvm_pgtable_prot prot,
    > - struct kvm_mem_range *range);
    > #endif /* __ARM64_KVM_PGTABLE_H__ */
    > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
    > index 871149246f5f..01700a908bb7 100644
    > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
    > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
    > @@ -159,6 +159,11 @@ static int host_stage2_unmap_dev_all(void)
    > return kvm_pgtable_stage2_unmap(pgt, addr, BIT(pgt->ia_bits) - addr);
    > }
    >
    > +struct kvm_mem_range {
    > + u64 start;
    > + u64 end;
    > +};
    > +
    > static bool find_mem_range(phys_addr_t addr, struct kvm_mem_range *range)
    > {
    > int cur, left = 0, right = hyp_memblock_nr;
    > @@ -227,6 +232,38 @@ static inline int __host_stage2_idmap(u64 start, u64 end,
    > __ret; \
    > })
    >
    > +static int host_stage2_find_range(u64 addr, struct kvm_mem_range *range)

    nit: I find 'find_range' a bit odd. We already have found a
    range. We're just trying to narrow it down to something that fits in a
    single block mapping. How about 'host_stage2_adjust_range'?

    > +{
    > + u64 granule, start, end;
    > + kvm_pte_t pte;
    > + u32 level;
    > + int ret;
    > +
    > + ret = kvm_pgtable_get_leaf(&host_kvm.pgt, addr, &pte, &level);
    > + if (ret)
    > + return ret;
    > +
    > + if (kvm_pte_valid(pte))
    > + return -EAGAIN;
    > +
    > + if (pte)
    > + return -EPERM;
    > +
    > + do {
    > + granule = kvm_granule_size(level);
    > + start = ALIGN_DOWN(addr, granule);
    > + end = start + granule;
    > + level++;
    > + } while ((level < KVM_PGTABLE_MAX_LEVELS) &&
    > + (!kvm_level_supports_block_mapping(level) ||
    > + start < range->start || range->end < end));
    > +

    This expression does my head in. You are trying to find the largest
    block mapping that entirely fits in range, right? Can we just express
    that directly (with a global negation for the purpose of the loop)?

    do {
    [...]
    } while (level < KVM_PGTABLE_MAX_LEVELS &&
    !(kvm_level_supports_block_mapping(level) &&
    start >= range->start &&
    end <= range->end));

    I personally find this much more readable, because it expresses the
    condition we are looking for rather than a lot of conditions forcing
    us to continue.

    You could also use a kvm_mem_range for the iteration, and add a helper
    that checks for the inclusion.

    > + range->start = start;
    > + range->end = end;
    > +
    > + return 0;
    > +}
    > +
    > static int host_stage2_idmap(u64 addr)
    > {
    > enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W;
    > @@ -238,7 +275,7 @@ static int host_stage2_idmap(u64 addr)
    > prot |= KVM_PGTABLE_PROT_X;
    >
    > hyp_spin_lock(&host_kvm.lock);
    > - ret = kvm_pgtable_stage2_find_range(&host_kvm.pgt, addr, prot, &range);
    > + ret = host_stage2_find_range(addr, &range);
    > if (ret)
    > goto unlock;

    Thanks,

    M.

    --
    Without deviation from the norm, progress is not possible.

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