[PATCH kernel 3/9] KVM: PPC: Use preregistered memory API to access TCE list
Alexey Kardashevskiy
aik at ozlabs.ru
Wed Mar 9 19:55:53 AEDT 2016
On 03/08/2016 05:30 PM, David Gibson wrote:
> On Tue, Mar 08, 2016 at 04:47:20PM +1100, Alexey Kardashevskiy wrote:
>> On 03/07/2016 05:00 PM, David Gibson wrote:
>>> On Mon, Mar 07, 2016 at 02:41:11PM +1100, Alexey Kardashevskiy wrote:
>>>> VFIO on sPAPR already implements guest memory pre-registration
>>>> when the entire guest RAM gets pinned. This can be used to translate
>>>> the physical address of a guest page containing the TCE list
>>> >from H_PUT_TCE_INDIRECT.
>>>>
>>>> This makes use of the pre-registrered memory API to access TCE list
>>>> pages in order to avoid unnecessary locking on the KVM memory
>>>> reverse map.
>>>>
>>>> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
>>>
>>> Ok.. so, what's the benefit of not having to lock the rmap?
>>
>> Less locking -> less racing == good, no?
>
> Well.. maybe. The increased difficulty in verifying that the code is
> correct isn't always a good price to pay.
>
>>>> ---
>>>> arch/powerpc/kvm/book3s_64_vio_hv.c | 86 ++++++++++++++++++++++++++++++-------
>>>> 1 file changed, 70 insertions(+), 16 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/kvm/book3s_64_vio_hv.c b/arch/powerpc/kvm/book3s_64_vio_hv.c
>>>> index 44be73e..af155f6 100644
>>>> --- a/arch/powerpc/kvm/book3s_64_vio_hv.c
>>>> +++ b/arch/powerpc/kvm/book3s_64_vio_hv.c
>>>> @@ -180,6 +180,38 @@ long kvmppc_gpa_to_ua(struct kvm *kvm, unsigned long gpa,
>>>> EXPORT_SYMBOL_GPL(kvmppc_gpa_to_ua);
>>>>
>>>> #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
>>>> +static mm_context_t *kvmppc_mm_context(struct kvm_vcpu *vcpu)
>>>> +{
>>>> + struct task_struct *task;
>>>> +
>>>> + task = vcpu->arch.run_task;
>>>> + if (unlikely(!task || !task->mm))
>>>> + return NULL;
>>>> +
>>>> + return &task->mm->context;
>>>> +}
>>>> +
>>>> +static inline bool kvmppc_preregistered(struct kvm_vcpu *vcpu)
>>>> +{
>>>> + mm_context_t *mm = kvmppc_mm_context(vcpu);
>>>> +
>>>> + if (unlikely(!mm))
>>>> + return false;
>>>> +
>>>> + return mm_iommu_preregistered(mm);
>>>> +}
>>>> +
>>>> +static struct mm_iommu_table_group_mem_t *kvmppc_rm_iommu_lookup(
>>>> + struct kvm_vcpu *vcpu, unsigned long ua, unsigned long size)
>>>> +{
>>>> + mm_context_t *mm = kvmppc_mm_context(vcpu);
>>>> +
>>>> + if (unlikely(!mm))
>>>> + return NULL;
>>>> +
>>>> + return mm_iommu_lookup_rm(mm, ua, size);
>>>> +}
>>>> +
>>>> long kvmppc_h_put_tce(struct kvm_vcpu *vcpu, unsigned long liobn,
>>>> unsigned long ioba, unsigned long tce)
>>>> {
>>>> @@ -261,23 +293,44 @@ long kvmppc_rm_h_put_tce_indirect(struct kvm_vcpu *vcpu,
>>>> if (ret != H_SUCCESS)
>>>> return ret;
>>>>
>>>> - if (kvmppc_gpa_to_ua(vcpu->kvm, tce_list, &ua, &rmap))
>>>> - return H_TOO_HARD;
>>>> + if (kvmppc_preregistered(vcpu)) {
>>>> + /*
>>>> + * We get here if guest memory was pre-registered which
>>>> + * is normally VFIO case and gpa->hpa translation does not
>>>> + * depend on hpt.
>>>> + */
>>>> + struct mm_iommu_table_group_mem_t *mem;
>>>>
>>>> - rmap = (void *) vmalloc_to_phys(rmap);
>>>> + if (kvmppc_gpa_to_ua(vcpu->kvm, tce_list, &ua, NULL))
>>>> + return H_TOO_HARD;
>>>>
>>>> - /*
>>>> - * Synchronize with the MMU notifier callbacks in
>>>> - * book3s_64_mmu_hv.c (kvm_unmap_hva_hv etc.).
>>>> - * While we have the rmap lock, code running on other CPUs
>>>> - * cannot finish unmapping the host real page that backs
>>>> - * this guest real page, so we are OK to access the host
>>>> - * real page.
>>>> - */
>>>> - lock_rmap(rmap);
>>>> - if (kvmppc_rm_ua_to_hpa(vcpu, ua, &tces)) {
>>>> - ret = H_TOO_HARD;
>>>> - goto unlock_exit;
>>>> + mem = kvmppc_rm_iommu_lookup(vcpu, ua, IOMMU_PAGE_SIZE_4K);
>>>> + if (!mem || mm_iommu_rm_ua_to_hpa(mem, ua, &tces))
>>>> + return H_TOO_HARD;
>>>> + } else {
>>>> + /*
>>>> + * This is emulated devices case.
>>>> + * We do not require memory to be preregistered in this case
>>>> + * so lock rmap and do __find_linux_pte_or_hugepte().
>>>> + */
>>>> + if (kvmppc_gpa_to_ua(vcpu->kvm, tce_list, &ua, &rmap))
>>>> + return H_TOO_HARD;
>>>> +
>>>> + rmap = (void *) vmalloc_to_phys(rmap);
>>>> +
>>>> + /*
>>>> + * Synchronize with the MMU notifier callbacks in
>>>> + * book3s_64_mmu_hv.c (kvm_unmap_hva_hv etc.).
>>>> + * While we have the rmap lock, code running on other CPUs
>>>> + * cannot finish unmapping the host real page that backs
>>>> + * this guest real page, so we are OK to access the host
>>>> + * real page.
>>>> + */
>>>> + lock_rmap(rmap);
>>>> + if (kvmppc_rm_ua_to_hpa(vcpu, ua, &tces)) {
>>>> + ret = H_TOO_HARD;
>>>> + goto unlock_exit;
>>>> + }
>>>> }
>>>>
>>>> for (i = 0; i < npages; ++i) {
>>>> @@ -291,7 +344,8 @@ long kvmppc_rm_h_put_tce_indirect(struct kvm_vcpu *vcpu,
>>>> }
>>>>
>>>> unlock_exit:
>>>> - unlock_rmap(rmap);
>>>> + if (rmap)
>>>
>>> I don't see where rmap is initialized to NULL in the case where it's
>>> not being used.
>>
>> @rmap is not new to this function, and it has always been initialized to
>> NULL as it was returned via a pointer from kvmppc_gpa_to_ua().
>
> This comment confuses me. Looking closer at the code I see you're
> right, and it's initialized to NULL where defined, which I missed.
>
> But that has nothing to do with being returned by pointer from
> kvmppc_gpa_to_ua(), since one of your branches in the new code no
> longer passes &rmap to that function.
So? The code is still correct - the "preregistered branch" does not touch
NULL pointer, it remains NULL and unlock_rmap() is not called. I agree the
patch is not the easiest to read but how can I improve it to get your "rb"?
Replace "if(rmap)" with "if (kvmppc_preregistered(vcpu))"? Move that loop
between lock_rmap/unlock_rmap to a helper? kvmppc_rm_h_put_tce_indirect()
is not big enough to justify splitting, the comments inside it are though...
--
Alexey
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