Re: [PATCH v3 09/21] KVM: arm64: Convert unmap_stage2_range() to generic page-table API

Alexandru Elisei
2020-09-02T16:23:39Z

Hello,

On 8/25/20 10:39 AM, Will Deacon wrote:
> Convert unmap_stage2_range() to use kvm_pgtable_stage2_unmap() instead
> of walking the page-table directly.
>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Quentin Perret <qperret@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> arch/arm64/kvm/mmu.c | 57 +++++++++++++++++++++++++-------------------
> 1 file changed, 32 insertions(+), 25 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 704b471a48ce..751ce2462765 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -39,6 +39,33 @@ static bool is_iomap(unsigned long flags)
> return flags & KVM_S2PTE_FLAG_IS_IOMAP;
> }
>
> +/*
> + * Release kvm_mmu_lock periodically if the memory region is large. Otherwise,
> + * we may see kernel panics with CONFIG_DETECT_HUNG_TASK,
> + * CONFIG_LOCKUP_DETECTOR, CONFIG_LOCKDEP. Additionally, holding the lock too
> + * long will also starve other vCPUs. We have to also make sure that the page
> + * tables are not freed while we released the lock.
> + */
> +#define stage2_apply_range(kvm, addr, end, fn, resched) \
> +({ \
> + int ret; \
> + struct kvm *__kvm = (kvm); \
> + bool __resched = (resched); \
> + u64 next, __addr = (addr), __end = (end); \
> + do { \
> + struct kvm_pgtable *pgt = __kvm->arch.mmu.pgt; \
> + if (!pgt) \
> + break; \

I'm 100% sure there's a reason why we've dropped the READ_ONCE, but it still looks
to me like the compiler might decide to optimize by reading pgt once at the start
of the loop and stashing it in a register. Would you mind explaining what I am
missing?

> + next = stage2_pgd_addr_end(__kvm, __addr, __end); \
> + ret = fn(pgt, __addr, next - __addr); \
> + if (ret) \
> + break; \
> + if (__resched && next != __end) \
> + cond_resched_lock(&__kvm->mmu_lock); \
> + } while (__addr = next, __addr != __end); \
> + ret; \
> +})

This seems unusual to me. We have a non-trivial, multiline macro which calls
cond_resched(), has 6 local variables, and is called from exactly one place.I am
curious why we are not open coding the loop in __unmap_stage2_range() or using a
function.

> +
> static bool memslot_is_logging(struct kvm_memory_slot *memslot)
> {
> return memslot->dirty_bitmap && !(memslot->flags & KVM_MEM_READONLY);
> @@ -220,8 +247,8 @@ static inline void kvm_pgd_populate(pgd_t *pgdp, p4d_t *p4dp)
> * end up writing old data to disk.
> *
> * This is why right after unmapping a page/section and invalidating
> - * the corresponding TLBs, we call kvm_flush_dcache_p*() to make sure
> - * the IO subsystem will never hit in the cache.
> + * the corresponding TLBs, we flush to make sure the IO subsystem will
> + * never hit in the cache.
> *
> * This is all avoided on systems that have ARM64_HAS_STAGE2_FWB, as
> * we then fully enforce cacheability of RAM, no matter what the guest
> @@ -344,32 +371,12 @@ static void __unmap_stage2_range(struct kvm_s2_mmu *mmu, phys_addr_t start, u64
> bool may_block)
> {
> struct kvm *kvm = mmu->kvm;
> - pgd_t *pgd;
> - phys_addr_t addr = start, end = start + size;
> - phys_addr_t next;
> + phys_addr_t end = start + size;
>
> assert_spin_locked(&kvm->mmu_lock);
> WARN_ON(size & ~PAGE_MASK);
> -
> - pgd = mmu->pgd + stage2_pgd_index(kvm, addr);
> - do {
> - /*
> - * Make sure the page table is still active, as another thread
> - * could have possibly freed the page table, while we released
> - * the lock.
> - */
> - if (!READ_ONCE(mmu->pgd))
> - break;
> - next = stage2_pgd_addr_end(kvm, addr, end);
> - if (!stage2_pgd_none(kvm, *pgd))
> - unmap_stage2_p4ds(mmu, pgd, addr, next);
> - /*
> - * If the range is too large, release the kvm->mmu_lock
> - * to prevent starvation and lockup detector warnings.
> - */
> - if (may_block && next != end)
> - cond_resched_lock(&kvm->mmu_lock);
> - } while (pgd++, addr = next, addr != end);
> + WARN_ON(stage2_apply_range(kvm, start, end, kvm_pgtable_stage2_unmap,
> + may_block));
> }
>
> static void unmap_stage2_range(struct kvm_s2_mmu *mmu, phys_addr_t start, u64 size)