lkml.org 
[lkml]   [2021]   [Sep]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 2/7] KVM: X86: Synchronize the shadow pagetable before link it
From
Date


On 2021/9/4 00:40, Sean Christopherson wrote:
> On Sat, Sep 04, 2021, Lai Jiangshan wrote:
>>
>> On 2021/9/4 00:06, Sean Christopherson wrote:
>>
>>> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
>>> index 50ade6450ace..2ff123ec0d64 100644
>>> --- a/arch/x86/kvm/mmu/paging_tmpl.h
>>> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
>>> @@ -704,6 +704,9 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>>> access = gw->pt_access[it.level - 2];
>>> sp = kvm_mmu_get_page(vcpu, table_gfn, fault->addr,
>>> it.level-1, false, access);
>>> + if (sp->unsync_children &&
>>> + mmu_sync_children(vcpu, sp, false))
>>> + return RET_PF_RETRY;
>>
>> It was like my first (unsent) fix. Just return RET_PF_RETRY when break.
>>
>> And then I thought that it'd be better to retry fetching directly rather than
>> retry guest when the conditions are still valid/unchanged to avoid all the
>> next guest page walking and GUP(). Although the code does not check all
>> conditions such as interrupt event pending. (we can add that too)
>
> But not in a bug fix that needs to go to stable branches.

Good point, it is too complicated for a fix, I accept just "return RET_PF_RETRY".
(and don't need "SOME_ARBITRARY_THRESHOLD").

Is it Ok? I will update the patch as it.

>
>> I think it is a good design to allow break mmu_lock when mmu is handling
>> heavy work.
>
> I don't disagree in principle, but I question the relevance/need. I doubt this
> code is relevant to nested TDP performance as hypervisors generally don't do the
> type of PTE manipulations that would lead to linking an existing unsync sp. And
> for legacy shadow paging, my preference would be to put it into maintenance-only
> mode as much as possible. I'm not dead set against new features/functionality
> for shadow paging, but for something like dropping mmu_lock in the page fault path,
> IMO there needs to be performance numbers to justify such a change.
>

I understood the concern and the relevance/need.

\
 
 \ /
  Last update: 2021-09-03 19:01    [W:0.163 / U:0.052 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site