linux-kernel - Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

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Date:   Mon, 16 Dec 2019 16:31:50 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Christophe de Dinechin <christophe.de.dinechin@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

On 16/12/19 16:26, Peter Xu wrote:
> On Mon, Dec 16, 2019 at 10:29:36AM +0100, Paolo Bonzini wrote:
>> On 14/12/19 17:26, Peter Xu wrote:
>>> On Sat, Dec 14, 2019 at 08:57:26AM +0100, Paolo Bonzini wrote:
>>>> On 13/12/19 21:23, Peter Xu wrote:
>>>>>> What is the benefit of using u16 for that? That means with 4K pages, you
>>>>>> can share at most 256M of dirty memory each time? That seems low to me,
>>>>>> especially since it's sufficient to touch one byte in a page to dirty it.
>>>>>>
>>>>>> Actually, this is not consistent with the definition in the code ;-)
>>>>>> So I'll assume it's actually u32.
>>>>> Yes it's u32 now.  Actually I believe at least Paolo would prefer u16
>>>>> more. :)
>>>>
>>>> It has to be u16, because it overlaps the padding of the first entry.
>>>
>>> Hmm, could you explain?
>>>
>>> Note that here what Christophe commented is on dirty_index,
>>> reset_index of "struct kvm_dirty_ring", so imho it could really be
>>> anything we want as long as it can store a u32 (which is the size of
>>> the elements in kvm_dirty_ring_indexes).
>>>
>>> If you were instead talking about the previous union definition of
>>> "struct kvm_dirty_gfns" rather than "struct kvm_dirty_ring", iiuc I've
>>> moved those indices out of it and defined kvm_dirty_ring_indexes which
>>> we expose via kvm_run, so we don't have that limitation as well any
>>> more?
>>
>> Yeah, I meant that since the size has (had) to be u16 in the union, it
>> need not be bigger in kvm_dirty_ring.
>>
>> I don't think having more than 2^16 entries in the *per-CPU* ring buffer
>> makes sense; lagging in recording dirty memory by more than 256 MiB per
>> CPU would mean a large pause later on resetting the ring buffers (your
>> KVM_CLEAR_DIRTY_LOG patches found the sweet spot to be around 1 GiB for
>> the whole system).
> 
> That's right, 1G could probably be a "common flavor" for guests in
> that case.
> 
> Though I wanted to use u64 only because I wanted to prepare even
> better for future potential changes as long as it won't hurt much.

No u64, please.  u32 I can agree with, 16-bit *should* be enough but it
is a bit tight, so let's make it 32-bit if we drop the union idea.

Paolo