linux-kernel - Re: [PATCH v4 2/7] KVM: x86/mmu: Factor out allocating memslot rmap

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Date:   Tue, 11 May 2021 20:56:44 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Kai Huang <kai.huang@...el.com>,
        Keqian Zhu <zhukeqian1@...wei.com>
Subject: Re: [PATCH v4 2/7] KVM: x86/mmu: Factor out allocating memslot rmap

On 11.05.21 20:17, Ben Gardon wrote:
> On Tue, May 11, 2021 at 10:56 AM Sean Christopherson <seanjc@...gle.com> wrote:
>>
>> On Tue, May 11, 2021, Ben Gardon wrote:
>>> Small refactor to facilitate allocating rmaps for all memslots at once.
>>>
>>> No functional change expected.
>>>
>>> Signed-off-by: Ben Gardon <bgardon@...gle.com>
>>> ---
>>>   arch/x86/kvm/x86.c | 39 ++++++++++++++++++++++++++++++---------
>>>   1 file changed, 30 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 1e1f4f31e586..cc0440b5b35d 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -10911,10 +10911,35 @@ void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot)
>>>        kvm_page_track_free_memslot(slot);
>>>   }
>>>
>>> +static int memslot_rmap_alloc(struct kvm_memory_slot *slot,
>>> +                           unsigned long npages)
>>> +{
>>> +     int i;
>>> +
>>> +     for (i = 0; i < KVM_NR_PAGE_SIZES; ++i) {
>>> +             int lpages;
>>> +             int level = i + 1;
>>> +
>>> +             lpages = gfn_to_index(slot->base_gfn + npages - 1,
>>> +                                   slot->base_gfn, level) + 1;
>>
>> Might as well assign lpages at its declaration, i.e.
>>
>>                  int lpages = gfn_to_index(slot->base_gfn + npages - 1,
>>                                            slot->base_gfn, level) + 1;
> 
> I'll do this if I end up sending out a v5.
> 
>>> +
>>> +             slot->arch.rmap[i] =
>>> +                     kvcalloc(lpages, sizeof(*slot->arch.rmap[i]),
>>> +                              GFP_KERNEL_ACCOUNT);
>>
>> Eh, I don't think avoiding a 3 char overrun is worth splitting across three lines.
>> E.g. this is perfectly readable
>>
>>                  slot->arch.rmap[i] = kvcalloc(lpages, sizeof(*slot->arch.rmap[i]),
>>                                                GFP_KERNEL_ACCOUNT);
>>
>> Alternatively, the rmap size could be captured in a local var, e.g.
>>
>>          const int sz = sizeof(*slot->arch.rmap[0]);
>>
>>          ...
>>
>>                  slot->arch.rmap[i] = kvcalloc(lpages, sz, GFP_KERNEL_ACCOUNT);
> 
> I like this suggestion. Much nicer. Will incorporate if I send a v5.
> 
>>                  if (!slot->arch.rmap[i]) {
>>                          memslot_rmap_free(slot);
>>                          return -ENOMEM;
>>                  }
>>
>>> +             if (!slot->arch.rmap[i]) {
>>> +                     memslot_rmap_free(slot);
>>> +                     return -ENOMEM;
>>
>> Reaaaally getting into nitpicks, what do you think about changing this to a goto
>> with the error handling at the bottom?  Obviously not necessary by any means,
>> but for me it makes it easier to see that all rmaps are freed on failure.  My
>> eyes skipped over that on the first read through.  E.g.
>>
>>                  if (!slot_arch.rmap[i])
>>                          goto err;
>>          }
>>
>>          return 0;
>>
>> err:
>>          memslot_rmap_free(slot);
>>          return -ENOMEM;
>>
> 
> Lol, I had a goto in v3, but David Hildenbrand suggested removing it
> and putting the free in the loop. I think I like it more this way too.

No strong opinion, I tend to stick to 
Documentation/process/coding-style.rst which states

"The goto statement comes in handy when a function exits from multiple 
locations and some common work such as cleanup has to be done."

As we only have a single error exit and no complicated locking, at least 
for me the "goto" makes it unnecessary hard to read.


-- 
Thanks,

David / dhildenb