Messages in this thread | | | Subject | Re: [PATCH 2/7] KVM: X86: Synchronize the shadow pagetable before link it | From | Lai Jiangshan <> | Date | Sat, 4 Sep 2021 01:00:33 +0800 |
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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.
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