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SubjectRe: [RFC PATCH v2 2/2] KVM: x86: Not wr-protect huge page with init_all_set dirty log
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Hi Ben,

Sorry for the delay reply!

On 2021/4/21 0:30, Ben Gardon wrote:
> On Tue, Apr 20, 2021 at 12:49 AM Keqian Zhu <zhukeqian1@huawei.com> wrote:
>>
>> Hi Ben,
>>
>> On 2021/4/20 3:20, Ben Gardon wrote:
>>> On Fri, Apr 16, 2021 at 1:25 AM Keqian Zhu <zhukeqian1@huawei.com> wrote:
>>>>
>>>> Currently during start dirty logging, if we're with init-all-set,
>>>> we write protect huge pages and leave normal pages untouched, for
>>>> that we can enable dirty logging for these pages lazily.
>>>>
>>>> Actually enable dirty logging lazily for huge pages is feasible
>>>> too, which not only reduces the time of start dirty logging, also
>>>> greatly reduces side-effect on guest when there is high dirty rate.
>>>>
>>>> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
>>>> ---
>>>> arch/x86/kvm/mmu/mmu.c | 48 ++++++++++++++++++++++++++++++++++++++----
>>>> arch/x86/kvm/x86.c | 37 +++++++++-----------------------
>>>> 2 files changed, 54 insertions(+), 31 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>>> index 2ce5bc2ea46d..98fa25172b9a 100644
>>>> --- a/arch/x86/kvm/mmu/mmu.c
>>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>>> @@ -1188,8 +1188,7 @@ static bool __rmap_clear_dirty(struct kvm *kvm, struct kvm_rmap_head *rmap_head,
>>>> * @gfn_offset: start of the BITS_PER_LONG pages we care about
>>>> * @mask: indicates which pages we should protect
>>>> *
>>>> - * Used when we do not need to care about huge page mappings: e.g. during dirty
>>>> - * logging we do not have any such mappings.
>>>> + * Used when we do not need to care about huge page mappings.
>>>> */
>>>> static void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
>>>> struct kvm_memory_slot *slot,
>>>> @@ -1246,13 +1245,54 @@ static void kvm_mmu_clear_dirty_pt_masked(struct kvm *kvm,
>>>> * It calls kvm_mmu_write_protect_pt_masked to write protect selected pages to
>>>> * enable dirty logging for them.
>>>> *
>>>> - * Used when we do not need to care about huge page mappings: e.g. during dirty
>>>> - * logging we do not have any such mappings.
>>>> + * We need to care about huge page mappings: e.g. during dirty logging we may
>>>> + * have any such mappings.
>>>> */
>>>> void kvm_arch_mmu_enable_log_dirty_pt_masked(struct kvm *kvm,
>>>> struct kvm_memory_slot *slot,
>>>> gfn_t gfn_offset, unsigned long mask)
>>>> {
>>>> + gfn_t start, end;
>>>> +
>>>> + /*
>>>> + * Huge pages are NOT write protected when we start dirty log with
>>>> + * init-all-set, so we must write protect them at here.
>>>> + *
>>>> + * The gfn_offset is guaranteed to be aligned to 64, but the base_gfn
>>>> + * of memslot has no such restriction, so the range can cross two large
>>>> + * pages.
>>>> + */
>>>> + if (kvm_dirty_log_manual_protect_and_init_set(kvm)) {
>>>> + start = slot->base_gfn + gfn_offset + __ffs(mask);
>>>> + end = slot->base_gfn + gfn_offset + __fls(mask);
>>>> + kvm_mmu_slot_gfn_write_protect(kvm, slot, start, PG_LEVEL_2M);
>>>> +
>>>> + /* Cross two large pages? */
>>>> + if (ALIGN(start << PAGE_SHIFT, PMD_SIZE) !=
>>>> + ALIGN(end << PAGE_SHIFT, PMD_SIZE))
>>>> + kvm_mmu_slot_gfn_write_protect(kvm, slot, end,
>>>> + PG_LEVEL_2M);
>>>> + }
>>>> +
>>>> + /*
>>>> + * RFC:
>>>> + *
>>>> + * 1. I don't return early when kvm_mmu_slot_gfn_write_protect() returns
>>>> + * true, because I am not very clear about the relationship between
>>>> + * legacy mmu and tdp mmu. AFAICS, the code logic is NOT an if/else
>>>> + * manner.
>>>> + *
>>>> + * The kvm_mmu_slot_gfn_write_protect() returns true when we hit a
>>>> + * writable large page mapping in legacy mmu mapping or tdp mmu mapping.
>>>> + * Do we still have normal mapping in that case? (e.g. We have large
>>>> + * mapping in legacy mmu and normal mapping in tdp mmu).
>>>
>>> Right, we can't return early because the two MMUs could map the page
>>> in different ways, but each MMU could also map the page in multiple
>>> ways independently.
>>> For example, if the legacy MMU was being used and we were running a
>>> nested VM, a page could be mapped 2M in EPT01 and 4K in EPT02, so we'd
>>> still need kvm_mmu_slot_gfn_write_protect calls for both levels.
>>> I don't think there's a case where we can return early here with the
>>> information that the first calls to kvm_mmu_slot_gfn_write_protect
>>> access.
>> Thanks for the detailed explanation.
>>
>>>
>>>> + *
>>>> + * 2. kvm_mmu_slot_gfn_write_protect() doesn't tell us whether the large
>>>> + * page mapping exist. If it exists but is clean, we can return early.
>>>> + * However, we have to do invasive change.
>>>
>>> What do you mean by invasive change?
>> We need the kvm_mmu_slot_gfn_write_protect to report whether all mapping are large
>> and clean, so we can return early. However it's not a part of semantics of this function.
>>
>> If this is the final code, compared to old code, we have an extra gfn_write_protect(),
>> I don't whether it's acceptable?
>
> Ah, I see. Please correct me if I'm wrong, but I think that in order
> to check that the only mappings on the GFN range are large, we'd still
> have to go over the rmap for the 4k mappings, at least for the legacy
> MMU. In that case, we're doing about as much work as the extra
> gfn_write_protect and I don't think that we'd get any efficiency gain
> for the change in semantics.
>
> Likewise for the TDP MMU, if the GFN range is mapped both large and
> 4k, it would have to be in different TDP structures, so the efficiency
> gains would again not be very big.
I am not familiar with the MMU virtualization of x86 arch, but I think
you are right.

>
> I'm really just guessing about those performance characteristics
> though. It would definitely help to have some performance data to back
> all this up. Even just a few runs of the dirty_log_perf_test (in
> selftests) could provide some interesting results, and I'd be happy to
> help review any improvements you might make to that test.
>
> Regardless, I'd be inclined to keep this change as simple as possible
> for now and the early return optimization could happen in a follow-up
> patch. I think the extra gfn_write_protect is acceptable, especially
> if you can show that it doesn't cause a big hit in performance when
> running the dirty_log_perf_test with 4k and 2m backing memory.
I tested it using dirty_log_perf_test, the result shows that performance
of clear_dirty_log different within 2%.

*Without this patch*

./dirty_log_perf_test -i 5 -v 16 -s anonymous

Testing guest mode: PA-bits:ANY, VA-bits:48, 4K pages
guest physical test memory offset: 0xffbfffff000
Populate memory time: 3.105203579s
Enabling dirty logging time: 0.000323444s
[...]
Get dirty log over 5 iterations took 0.000595033s. (Avg 0.000119006s/iteration)
Clear dirty log over 5 iterations took 0.713212922s. (Avg 0.142642584s/iteration)

./dirty_log_perf_test -i 5 -v 16 -s anonymous_hugetlb

Testing guest mode: PA-bits:ANY, VA-bits:48, 4K pages
guest physical test memory offset: 0xffbfffff000
Populate memory time: 3.922764235s
Enabling dirty logging time: 0.000316473s
[...]
Get dirty log over 5 iterations took 0.000485459s. (Avg 0.000097091s/iteration)
Clear dirty log over 5 iterations took 0.603749670s. (Avg 0.120749934s/iteration)


*With this patch*

./dirty_log_perf_test -i 5 -v 16 -s anonymous

Testing guest mode: PA-bits:ANY, VA-bits:48, 4K pages
guest physical test memory offset: 0xffbfffff000
Populate memory time: 3.244515198s
Enabling dirty logging time: 0.000280207s
[...]
Get dirty log over 5 iterations took 0.000484953s. (Avg 0.000096990s/iteration)
Clear dirty log over 5 iterations took 0.727620114s. (Avg 0.145524022s/iteration)

./dirty_log_perf_test -i 5 -v 16 -s anonymous_hugetlb

Testing guest mode: PA-bits:ANY, VA-bits:48, 4K pages
guest physical test memory offset: 0xffbfffff000
Populate memory time: 3.244294061s
Enabling dirty logging time: 0.000273590s
[...]
Get dirty log over 5 iterations took 0.000474244s. (Avg 0.000094848s/iteration)
Clear dirty log over 5 iterations took 0.600593672s. (Avg 0.120118734s/iteration)


I faced a problem that there is no huge page mapping when test with
"-s anonymous_hugetlb", both for TDP enabled or disabled.

However, these tests above can tell the fact that our optimization does little effect
on clear_dirty_log performance.

Thanks,
Keqian

>
>>
>> Thanks,
>> Keqian
>>
>>
>>>
>>>> + */
>>>> +
>>>> + /* Then we can handle the PT level pages */
>>>> if (kvm_x86_ops.cpu_dirty_log_size)
>>>> kvm_mmu_clear_dirty_pt_masked(kvm, slot, gfn_offset, mask);
>>>> else
>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>> index eca63625aee4..dfd676ffa7da 100644
>>>> --- a/arch/x86/kvm/x86.c
>>>> +++ b/arch/x86/kvm/x86.c
>>>> @@ -10888,36 +10888,19 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
>>>> */
>>>> kvm_mmu_zap_collapsible_sptes(kvm, new);
>>>> } else {
>>>> - /* By default, write-protect everything to log writes. */
>>>> - int level = PG_LEVEL_4K;
>>>> + /*
>>>> + * If we're with initial-all-set, we don't need to write protect
>>>> + * any page because they're reported as dirty already.
>>>> + */
>>>> + if (kvm_dirty_log_manual_protect_and_init_set(kvm))
>>>> + return;
>>>>
>>>> if (kvm_x86_ops.cpu_dirty_log_size) {
>>>> - /*
>>>> - * Clear all dirty bits, unless pages are treated as
>>>> - * dirty from the get-go.
>>>> - */
>>>> - if (!kvm_dirty_log_manual_protect_and_init_set(kvm))
>>>> - kvm_mmu_slot_leaf_clear_dirty(kvm, new);
>>>> -
>>>> - /*
>>>> - * Write-protect large pages on write so that dirty
>>>> - * logging happens at 4k granularity. No need to
>>>> - * write-protect small SPTEs since write accesses are
>>>> - * logged by the CPU via dirty bits.
>>>> - */
>>>> - level = PG_LEVEL_2M;
>>>> - } else if (kvm_dirty_log_manual_protect_and_init_set(kvm)) {
>>>> - /*
>>>> - * If we're with initial-all-set, we don't need
>>>> - * to write protect any small page because
>>>> - * they're reported as dirty already. However
>>>> - * we still need to write-protect huge pages
>>>> - * so that the page split can happen lazily on
>>>> - * the first write to the huge page.
>>>> - */
>>>> - level = PG_LEVEL_2M;
>>>> + kvm_mmu_slot_leaf_clear_dirty(kvm, new);
>>>> + kvm_mmu_slot_remove_write_access(kvm, new, PG_LEVEL_2M);
>>>> + } else {
>>>> + kvm_mmu_slot_remove_write_access(kvm, new, PG_LEVEL_4K);
>>>> }
>>>> - kvm_mmu_slot_remove_write_access(kvm, new, level);
>>>> }
>>>> }
>>>>
>>>> --
>>>> 2.23.0
>>>>
>>> .
>>>
> .
>

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