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SubjectRe: [PATCH] kvm: don't lose the higher 32 bits of tlbs_dirty
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On 14/12/20 18:20, Sean Christopherson wrote:
> On Sun, Dec 13, 2020, Lai Jiangshan wrote:
>> From: Lai Jiangshan <laijs@linux.alibaba.com>
>>
>> In kvm_mmu_notifier_invalidate_range_start(), tlbs_dirty is used as:
>> need_tlb_flush |= kvm->tlbs_dirty;
>> with need_tlb_flush's type being int and tlbs_dirty's type being long.
>>
>> It means that tlbs_dirty is always used as int and the higher 32 bits
>> is useless.
>
> It's probably worth noting in the changelog that it's _extremely_ unlikely this
> bug can cause problems in practice. It would require encountering tlbs_dirty
> on a 4 billion count boundary, and KVM would need to be using shadow paging or
> be running a nested guest.
>
>> We can just change need_tlb_flush's type to long to
>> make full use of tlbs_dirty.
>
> Hrm, this does solve the problem, but I'm not a fan of continuing to use an
> integer variable as a boolean. Rather than propagate tlbs_dirty to
> need_tlb_flush, what if this bug fix patch checks tlbs_dirty directly, and then
> a follow up patch converts need_tlb_flush to a bool and removes the unnecessary
> initialization (see below).

Indeed, the compiler should be able to convert || to | if useful and
valid (it may or may not do it depending on the sizes of types involved,
but that's Someone Else's Problem and this is not really a path where
every instruction matter).

Paolo

> E.g. the net result of both patches would be:
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 3abcb2ce5b7d..93b6986d3dfc 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -473,7 +473,8 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> const struct mmu_notifier_range *range)
> {
> struct kvm *kvm = mmu_notifier_to_kvm(mn);
> - int need_tlb_flush = 0, idx;
> + bool need_tlb_flush;
> + int idx;
>
> idx = srcu_read_lock(&kvm->srcu);
> spin_lock(&kvm->mmu_lock);
> @@ -483,11 +484,10 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> * count is also read inside the mmu_lock critical section.
> */
> kvm->mmu_notifier_count++;
> - need_tlb_flush = kvm_unmap_hva_range(kvm, range->start, range->end,
> - range->flags);
> - need_tlb_flush |= kvm->tlbs_dirty;
> + need_tlb_flush = !!kvm_unmap_hva_range(kvm, range->start, range->end,
> + range->flags);
> /* we've to flush the tlb before the pages can be freed */
> - if (need_tlb_flush)
> + if (need_tlb_flush || kvm->tlbs_dirty)
> kvm_flush_remote_tlbs(kvm);
>
> spin_unlock(&kvm->mmu_lock);
>
> Cc: stable@vger.kernel.org
> Fixes: a4ee1ca4a36e ("KVM: MMU: delay flush all tlbs on sync_page path")
>
>> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
>> ---
>> virt/kvm/kvm_main.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 2541a17ff1c4..4e519a517e9f 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -470,7 +470,8 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>> const struct mmu_notifier_range *range)
>> {
>> struct kvm *kvm = mmu_notifier_to_kvm(mn);
>> - int need_tlb_flush = 0, idx;
>> + long need_tlb_flush = 0;
>
> need_tlb_flush doesn't need to be initialized here, it's explicitly set via the
> call to kvm_unmap_hva_range().
>
>> + int idx;
>>
>> idx = srcu_read_lock(&kvm->srcu);
>> spin_lock(&kvm->mmu_lock);
>> --
>> 2.19.1.6.gb485710b
>>
>

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