linux-kernel - Re: [PATCH 1/2] KVM: x86/mmu: Protect marking SPs unsync when using TDP MMU with spinlock

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Date:   Wed, 11 Aug 2021 13:47:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 1/2] KVM: x86/mmu: Protect marking SPs unsync when using
 TDP MMU with spinlock

On 11/08/21 00:45, Sean Christopherson wrote:
> Use an entirely new spinlock even though piggybacking tdp_mmu_pages_lock
> would functionally be ok.  Usurping the lock could degrade performance when
> building upper level page tables on different vCPUs, especially since the
> unsync flow could hold the lock for a comparatively long time depending on
> the number of indirect shadow pages and the depth of the paging tree.

If we are to introduce a new spinlock, do we need to make it conditional 
and pass it around like this?  It would be simpler to just take it 
everywhere (just like, in patch 2, passing "shared == true" to 
tdp_mmu_link_page is always safe anyway).

Paolo