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Re: [Qemu-devel] directory hierarchy
From: |
Avi Kivity |
Subject: |
Re: [Qemu-devel] directory hierarchy |
Date: |
Sun, 23 Sep 2012 10:25:29 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 |
On 09/22/2012 04:15 PM, Blue Swirl wrote:
> >
> >> This could have nice cleanup effects though and for example enable
> >> generic 'info vmtree' to discover VA->PA mappings for any target
> >> instead of current MMU table walkers.
> >
> > How? That's in a hardware defined format that's completely invisible to
> > the memory API.
>
> It's invisible now, but target-specific code could grab the mappings
> and feed them to memory API. Memory API would just see the per-CPU
> virtual memory as address spaces that map to physical memory address
> space.
>
> For RAM backed MMU tables like x86 and Sparc32, writes to page table
> memory areas would need to be tracked like SMC. For in-MMU TLBs, this
> would not be needed.
>
> Again, if performance would degrade, this would not be worthwhile. I'd
> expect VA->PA mappings to change at least at context switch rate +
> page fault rate + mmap/exec activity so this could amount to thousands
> of changes per second per CPU.
>
> In theory KVM could use memory API as CPU type agnostic way to
> exchange this information, I'd expect that KVM exit rate is not nearly
> as big and in many cases exchange of mapping information would not be
> needed. It would not improve performance there either.
>
First, the memory API does not operate at that level. It handles (guest
physical) -> (host virtual | io callback) translations. These are
(guest virtual) -> (guest physical translations).
Second, the memory API is machine-wide and designed for coarse maps.
Processor memory maps are per-cpu and page-grained. (the memory API
actually needs to efficiently support page-grained maps (for iommus) and
per-cpu maps (smm), but that's another story).
Third, we know from the pre-npt/ept days that tracking all mappings
destroys performance. It's much better to do this on demand.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
- Re: [Qemu-devel] directory hierarchy, (continued)
- Re: [Qemu-devel] directory hierarchy, Stefan Weil, 2012/09/14
- Re: [Qemu-devel] directory hierarchy, Kevin Wolf, 2012/09/14
- Re: [Qemu-devel] directory hierarchy, Blue Swirl, 2012/09/14
- Re: [Qemu-devel] directory hierarchy, Paolo Bonzini, 2012/09/14
- Re: [Qemu-devel] directory hierarchy, Avi Kivity, 2012/09/19
- Re: [Qemu-devel] directory hierarchy, Blue Swirl, 2012/09/19
- Re: [Qemu-devel] directory hierarchy, Avi Kivity, 2012/09/20
- Re: [Qemu-devel] directory hierarchy, Blue Swirl, 2012/09/22
- Re: [Qemu-devel] directory hierarchy,
Avi Kivity <=
- Re: [Qemu-devel] directory hierarchy, Blue Swirl, 2012/09/23
- Re: [Qemu-devel] directory hierarchy, Avi Kivity, 2012/09/24
Re: [Qemu-devel] directory hierarchy, Anthony Liguori, 2012/09/16