Re: [PATCH] KVM: x86/mmu: optimizing the code in mmu_try_to_unsync_pages

[Yun Lu <luyun_611@163.com>]

On 2022/5/20 下午10:47, Sean Christopherson wrote:

> On Fri, May 20, 2022, Yuan Yao wrote:
>> On Fri, May 20, 2022 at 02:09:07PM +0800, Yun Lu wrote:
>>> There is no need to check can_unsync and prefetch in the loop
>>> every time, just move this check before the loop.
>>>
>>> Signed-off-by: Yun Lu <luyun@kylinos.cn>
>>> ---
>>>   arch/x86/kvm/mmu/mmu.c | 12 ++++++------
>>>   1 file changed, 6 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>> index 311e4e1d7870..e51e7735adca 100644
>>> --- a/arch/x86/kvm/mmu/mmu.c
>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>> @@ -2534,6 +2534,12 @@ int mmu_try_to_unsync_pages(struct kvm *kvm, const struct kvm_memory_slot *slot,
>>>   	if (kvm_slot_page_track_is_active(kvm, slot, gfn, KVM_PAGE_TRACK_WRITE))
>>>   		return -EPERM;
>>>
>>> +	if (!can_unsync)
>>> +		return -EPERM;
>>> +
>>> +	if (prefetch)
>>> +		return -EEXIST;
>>> +
>>>   	/*
>>>   	 * The page is not write-tracked, mark existing shadow pages unsync
>>>   	 * unless KVM is synchronizing an unsync SP (can_unsync = false).  In
>>> @@ -2541,15 +2547,9 @@ int mmu_try_to_unsync_pages(struct kvm *kvm, const struct kvm_memory_slot *slot,
>>>   	 * allowing shadow pages to become unsync (writable by the guest).
>>>   	 */
>>>   	for_each_gfn_indirect_valid_sp(kvm, sp, gfn) {
>>> -		if (!can_unsync)
>>> -			return -EPERM;
>>> -
>>>   		if (sp->unsync)
>>>   			continue;
>>>
>>> -		if (prefetch)
>>> -			return -EEXIST;
>>> -
>> Consider the case that for_each_gfn_indirect_valid_sp() loop is
>> not triggered, means the gfn is not MMU page table page:
>>
>> The old behavior when : return 0;
>> The new behavior with this change: returrn -EPERM / -EEXIST;
>>
>> It at least breaks FNAME(sync_page) -> make_spte(prefetch = true, can_unsync = false)
>> which removes PT_WRITABLE_MASK from last level mapping unexpectedly.
> Yep, the flags should be queried if and only if there's at least one valid, indirect
> SP for th gfn.  And querying whether there's such a SP is quite expesnive and requires
> looping over a list, so checking every iteration of the loop is far cheaper.  E.g. each
> check is a single uop on modern CPUs as both gcc and clang are smart enough to stash
> the flags in registers so that there's no reload from memory on each loop.  And that
> also means the CPU can more than likely correctly predict subsequent iterations.
OK, it's my mistake.  Thanks for your answers.