linux-kernel - Re: [PATCH v3 00/21] KVM: Dirty ring interface

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Date:   Fri, 10 Jan 2020 10:10:53 -0500
From:   Peter Xu <peterx@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christophe de Dinechin <dinechin@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yan Zhao <yan.y.zhao@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Kevin Kevin <kevin.tian@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v3 00/21] KVM: Dirty ring interface

On Thu, Jan 09, 2020 at 05:28:36PM -0500, Michael S. Tsirkin wrote:
> On Thu, Jan 09, 2020 at 02:39:49PM -0500, Peter Xu wrote:
> > On Thu, Jan 09, 2020 at 02:08:52PM -0500, Michael S. Tsirkin wrote:
> > > On Thu, Jan 09, 2020 at 12:08:49PM -0500, Peter Xu wrote:
> > > > On Thu, Jan 09, 2020 at 11:40:23AM -0500, Michael S. Tsirkin wrote:
> > > > 
> > > > [...]
> > > > 
> > > > > > > I know it's mostly relevant for huge VMs, but OTOH these
> > > > > > > probably use huge pages.
> > > > > > 
> > > > > > Yes huge VMs could benefit more, especially if the dirty rate is not
> > > > > > that high, I believe.  Though, could you elaborate on why huge pages
> > > > > > are special here?
> > > > > > 
> > > > > > Thanks,
> > > > > 
> > > > > With hugetlbfs there are less bits to test: e.g. with 2M pages a single
> > > > > bit set marks 512 pages as dirty.  We do not take advantage of this
> > > > > but it looks like a rather obvious optimization.
> > > > 
> > > > Right, but isn't that the trade-off between granularity of dirty
> > > > tracking and how easy it is to collect the dirty bits?  Say, it'll be
> > > > merely impossible to migrate 1G-huge-page-backed guests if we track
> > > > dirty bits using huge page granularity, since each touch of guest
> > > > memory will cause another 1G memory to be transferred even if most of
> > > > the content is the same.  2M can be somewhere in the middle, but still
> > > > the same write amplify issue exists.
> > > >
> > > 
> > > OK I see I'm unclear.
> > > 
> > > IIUC at the moment KVM never uses huge pages if any part of the huge page is
> > > tracked.
> > 
> > To be more precise - I think it's per-memslot.  Say, if the memslot is
> > dirty tracked, then no huge page on the host on that memslot (even if
> > guest used huge page over that).
> 
> Yea ... so does it make sense to make this implementation detail
> leak through UAPI?

I think that's not a leak of internal implementation detail, we just
define the interface as that the address for each kvm_dirty_gfn is
always host page aligned (by default it means no huge page) and point
to a single host page, that's all.  Host page size is always there for
userspace after all so imho it's fine.  Thanks,

-- 
Peter Xu