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

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Date:   Thu, 12 Dec 2019 09:12:04 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Peter Xu <peterx@...hat.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 12/12/19 08:36, Michael S. Tsirkin wrote:
> On Thu, Dec 12, 2019 at 01:08:14AM +0100, Paolo Bonzini wrote:
>>>> I'd say it won't be a big issue on locking 1/2M of host mem for a
>>>> vm...
>>>> Also note that if dirty ring is enabled, I plan to evaporate the
>>>> dirty_bitmap in the next post. The old kvm->dirty_bitmap takes
>>>> $GUEST_MEM/32K*2 mem.  E.g., for 64G guest it's 64G/32K*2=4M.  If with
>>>> dirty ring of 8 vcpus, that could be 64K*8=0.5M, which could be even
>>>> less memory used.
>>>
>>> Right - I think Avi described the bitmap in kernel memory as one of
>>> design mistakes. Why repeat that with the new design?
>>
>> Do you have a source for that?
> 
> Nope, it was a private talk.
> 
>> At least the dirty bitmap has to be
>> accessed from atomic context so it seems unlikely that it can be moved
>> to user memory.
> 
> Why is that? We could surely do it from VCPU context?

Spinlock is taken.

>> The dirty ring could use user memory indeed, but it would be much harder
>> to set up (multiple ioctls for each ring?  what to do if userspace
>> forgets one? etc.).
> 
> Why multiple ioctls? If you do like virtio packed ring you just need the
> base and the size.

You have multiple rings, so multiple invocations of one ioctl.

Paolo