Re: [patch 0/6] Guest page hinting version 7.

[Rik van Riel]

Dave Hansen wrote:
> On Fri, 2009-03-27 at 16:09 +0100, Martin Schwidefsky wrote:
> > If the host picks one of the
> > pages the guest can recreate, the host can throw it away instead of writing
> > it to the paging device. Simple and elegant.
>
> Heh, simple and elegant for the hypervisor.  But I'm not sure I'm going
> to call *anything* that requires a new CPU instruction elegant. ;)

I am convinced that it could be done with a guest-writable
"bitmap", with 2 bits per page.  That would make this scheme
useful for KVM, too.

> I don't see any description of it in there any more, but I thought this
> entire patch set was to get rid of the idiotic triple I/Os in the
> following scenario:

> I don't see that mentioned at all in the current description.
> Simplifying the hypervisor is hard to get behind, but cutting system I/O
> by 2/3 is a much nicer benefit for 1200 lines of invasive code. ;)

Cutting down on a fair bit of IO is absolutely worth
1200 lines of fairly well isolated code.

> Can we persuade the hypervisor to tell us which pages it decided to page
> out and just skip those when we're scanning the LRU?

The easiest "notification" points are in the page fault
handler and the page cache lookup code.

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