linux-kernel - Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

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Date:   Tue, 19 Feb 2019 15:35:29 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com,
        pagupta@...hat.com, wei.w.wang@...el.com,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Rik van Riel <riel@...riel.com>, dodgen@...gle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        dhildenb@...hat.com, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

On Tue, Feb 19, 2019 at 09:21:20PM +0100, David Hildenbrand wrote:
> On 19.02.19 21:17, Michael S. Tsirkin wrote:
> > On Tue, Feb 19, 2019 at 09:02:52PM +0100, David Hildenbrand wrote:
> >> On 19.02.19 20:58, Michael S. Tsirkin wrote:
> >>> On Tue, Feb 19, 2019 at 10:06:35AM -0800, Alexander Duyck wrote:
> >>>>> I tend to like an asynchronous reporting approach as discussed in this
> >>>>> thread, we would have to see if Nitesh could get it implemented.
> >>>>
> >>>> I agree it would be great if it could work. However I have concerns
> >>>> given that work on this patch set dates back to 2017, major issues
> >>>> such as working around device assignment have yet to be addressed,
> >>>
> >>> BTW for device assignment to work, your idea of sending
> >>> data directly to kvm won't work, will it?
> >>> You need to update userspace so it can update VFIO right?
> >>> Another blocker for assignment is ability to make holes
> >>> an an existing mapping - supported by hardware but
> >>> not by IOMMU drivers.
> >>
> >> I had the exact same thought and then realized that we decided to block
> >> the balloon in user space until we figured out how to handle this properly.
> >>
> >> I wonder if MADV_FREE behaves differently compared to MADV_DONTNEED when
> >> finding pinned pages, but I doubt it. Most probably we'll have to
> >> disable hinting for device assignments as well.
> > 
> > OK but let's recognize it as a bug not a feature.
> > 
> 
> Yes, btw interesting read: https://lwn.net/Articles/198380/

There's also slideware from Rik van Riel circa 2011. His idea
was tagging pages in guest memory, freeing a page involves
- drop the EPT PTE
- check page is still free
- unpin page

This way you can hint without exits at all if you like.

> 
> "Pages which have been locked into memory pose an extra challenge here -
> they can be part of the page cache, but they still shouldn't be taken
> away by the host system. So such pages cannot be marked as "volatile."
> The problem is that figuring out if a page is locked is harder than it
> might seem; it can involve scanning a list of virtual memory area (VMA)
> structures, which is slow. So the hinting patches add a new flag to the
> address_space structure to note that somebody has locked pages from that
> address space in memory."
> 
> I assume locked here actually means pinned.

Locked seems to mean mlock there.

This seems to also resemble Xen's tmem a bit.


> -- 
> 
> Thanks,
> 
> David / dhildenb