linux-kernel - Re: [PATCH v2 00/20] Introduce the TDP MMU

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Date:   Tue, 20 Oct 2020 10:07:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH v2 00/20] Introduce the TDP MMU

On 19/10/20 20:15, Ben Gardon wrote:
> When getting the dirty log, we
> follow the following steps:
> 1. Atomically get and clear an unsigned long of the dirty bitmap
> 2. For each GFN in the range of pages covered by the unsigned long mask:
>     3. Clear the dirty or writable bit on the SPTE
> 4. Copy the mask of dirty pages to be returned to userspace
> 
> If we mark the page as dirty in the dirty bitmap in step 3, we'll
> report the page as dirty twice - once in this dirty log call, and
> again in the next one. This can lead to unexpected behavior:
> 1. Pause all vCPUs
> 2. Get the dirty log <--- Returns all pages dirtied before the vCPUs were paused
> 3. Get the dirty log again <--- Unexpectedly returns a non-zero number
> of dirty pages even though no pages were actually dirtied

Got it, that might also fail the dirty_log_test.  Thanks!

Paolo