Re: [PATCH 04/24] kvm: x86/mmu: change TDP MMU yield function returns to match cond_resched

Paolo Bonzini
2021-01-26T14:15:36Z

On 20/01/21 19:38, Sean Christopherson wrote:
> On Tue, Jan 12, 2021, Ben Gardon wrote:
>> Currently the TDP MMU yield / cond_resched functions either return
>> nothing or return true if the TLBs were not flushed. These are confusing
>> semantics, especially when making control flow decisions in calling
>> functions.
>>
>> To clean things up, change both functions to have the same
>> return value semantics as cond_resched: true if the thread yielded,
>> false if it did not. If the function yielded in the _flush_ version,
>> then the TLBs will have been flushed.
>>
>> Reviewed-by: Peter Feiner <pfeiner@google.com>
>> Signed-off-by: Ben Gardon <bgardon@google.com>
>> ---
>> arch/x86/kvm/mmu/tdp_mmu.c | 38 +++++++++++++++++++++++++++++---------
>> 1 file changed, 29 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
>> index 2ef8615f9dba..b2784514ca2d 100644
>> --- a/arch/x86/kvm/mmu/tdp_mmu.c
>> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
>> @@ -413,8 +413,15 @@ static inline void tdp_mmu_set_spte_no_dirty_log(struct kvm *kvm,
>> _mmu->shadow_root_level, _start, _end)
>>
>> /*
>> - * Flush the TLB if the process should drop kvm->mmu_lock.
>> - * Return whether the caller still needs to flush the tlb.
>> + * Flush the TLB and yield if the MMU lock is contended or this thread needs to
>> + * return control to the scheduler.
>> + *
>> + * If this function yields, it will also reset the tdp_iter's walk over the
>> + * paging structure and the calling function should allow the iterator to
>> + * continue its traversal from the paging structure root.
>> + *
>> + * Return true if this function yielded, the TLBs were flushed, and the
>> + * iterator's traversal was reset. Return false if a yield was not needed.
>> */
>> static bool tdp_mmu_iter_flush_cond_resched(struct kvm *kvm, struct tdp_iter *iter)
>> {
>> @@ -422,18 +429,30 @@ static bool tdp_mmu_iter_flush_cond_resched(struct kvm *kvm, struct tdp_iter *it
>> kvm_flush_remote_tlbs(kvm);
>> cond_resched_lock(&kvm->mmu_lock);
>> tdp_iter_refresh_walk(iter);
>> - return false;
>> - } else {
>> return true;
>> - }
>> + } else
>> + return false;
>
> Kernel style is to have curly braces on all branches if any branch has 'em. Or,
> omit the else since the taken branch always returns. I think I prefer the latter?
>
>> }
>>
>> -static void tdp_mmu_iter_cond_resched(struct kvm *kvm, struct tdp_iter *iter)
>> +/*
>> + * Yield if the MMU lock is contended or this thread needs to return control
>> + * to the scheduler.
>> + *
>> + * If this function yields, it will also reset the tdp_iter's walk over the
>> + * paging structure and the calling function should allow the iterator to
>> + * continue its traversal from the paging structure root.
>> + *
>> + * Return true if this function yielded and the iterator's traversal was reset.
>> + * Return false if a yield was not needed.
>> + */
>> +static bool tdp_mmu_iter_cond_resched(struct kvm *kvm, struct tdp_iter *iter)
>> {
>> if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
>> cond_resched_lock(&kvm->mmu_lock);
>> tdp_iter_refresh_walk(iter);
>> - }
>> + return true;
>> + } else
>> + return false;
>
> Same here.
>
>> }
>>
>> /*
>> @@ -470,7 +489,8 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>> tdp_mmu_set_spte(kvm, &iter, 0);
>>
>> if (can_yield)
>> - flush_needed = tdp_mmu_iter_flush_cond_resched(kvm, &iter);
>> + flush_needed = !tdp_mmu_iter_flush_cond_resched(kvm,
>> + &iter);
>
> As with the existing code, I'd let this poke out. Alternatively, this could be
> written as:
>
> flush_needed = !can_yield ||
> !tdp_mmu_iter_flush_cond_resched(kvm, &iter);
>
>> else
>> flush_needed = true;
>> }
>> @@ -1072,7 +1092,7 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
>>
>> tdp_mmu_set_spte(kvm, &iter, 0);
>>
>> - spte_set = tdp_mmu_iter_flush_cond_resched(kvm, &iter);
>> + spte_set = !tdp_mmu_iter_flush_cond_resched(kvm, &iter);
>> }
>>
>> if (spte_set)
>> --
>> 2.30.0.284.gd98b1dd5eaa7-goog
>>
>

Tweaked and queued, thanks.

Paolo