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Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking
To:     Peter Xu <peterx@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
References: <20191129213505.18472-1-peterx@redhat.com>
 <20191129213505.18472-5-peterx@redhat.com>
 <1355422f-ab62-9dc3-2b48-71a6e221786b@redhat.com>
 <a3e83e6b-4bfa-3a6b-4b43-5dd451e03254@redhat.com>
 <20191204195230.GF19939@xz-x1>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <efa1523f-2cff-8d65-7b43-4a19eff89051@redhat.com>
Date:   Thu, 5 Dec 2019 14:51:15 +0800
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On 2019/12/5 =E4=B8=8A=E5=8D=883:52, Peter Xu wrote:
> On Wed, Dec 04, 2019 at 12:04:53PM +0100, Paolo Bonzini wrote:
>> On 04/12/19 11:38, Jason Wang wrote:
>>>> +=C2=A0=C2=A0=C2=A0 entry =3D &ring->dirty_gfns[ring->dirty_index & (r=
ing->size - 1)];
>>>> +=C2=A0=C2=A0=C2=A0 entry->slot =3D slot;
>>>> +=C2=A0=C2=A0=C2=A0 entry->offset =3D offset;
>>>
>>> Haven't gone through the whole series, sorry if it was a silly question
>>> but I wonder things like this will suffer from similar issue on
>>> virtually tagged archs as mentioned in [1].
>> There is no new infrastructure to track the dirty pages---it's just a
>> different way to pass them to userspace.
>>
>>> Is this better to allocate the ring from userspace and set to KVM
>>> instead? Then we can use copy_to/from_user() friends (a little bit slow
>>> on recent CPUs).
>> Yeah, I don't think that would be better than mmap.
> Yeah I agree, because I didn't see how copy_to/from_user() helped to
> do icache/dcache flushings...


It looks to me one advantage is that exact the same VA is used by both=20
userspace and kernel so there will be no alias.

Thanks


>
> Some context here: Jason raised this question offlist first on whether
> we should also need these flush_dcache_cache() helpers for operations
> like kvm dirty ring accesses.  I feel like it should, however I've got
> two other questions, on:
>
>    - if we need to do flush_dcache_page() on kernel modified pages
>      (assuming the same page has mapped to userspace), then why don't
>      we need flush_cache_page() too on the page, where
>      flush_cache_page() is defined not-a-nop on those archs?
>
>    - assuming an arch has not-a-nop impl for flush_[d]cache_page(),
>      would atomic operations like cmpxchg really work for them
>      (assuming that ISAs like cmpxchg should depend on cache
>      consistency).
>
> Sorry I think these are for sure a bit out of topic for kvm dirty ring
> patchset, but since we're at it, I'm raising the questions up in case
> there're answers..
>
> Thanks,
>