Re: [PATCH] memory: synchronize dirty bitmap before unmapping a range

Avi Kivity
2011-08-01T10:27:41Z

On 08/01/2011 01:21 PM, Jan Kiszka wrote:
> >
> > Note that drop/add is always paired (i.e. the guest never sees an
> > unmapped area), and we always map the full 64k even though cirrus code
> > manages each 32k bank individually. It looks optimal... we're probably
> > not testing the same thing (either qemu or guest code).
>
> This is what my instrumentation revealed:
>
> map_linear_vram_bank 0
> map 0 (actually perform the mapping)
> map_linear_vram_bank 1
> map 1
> 4 a0000 0 7fe863a62000 1 (KVM_SET_USER_MEMORY_REGION)
> 4 a0000 10000 7fe863a72000 1
> run (enter guest)
> map_linear_vram_bank 0
> map 0
> map_linear_vram_bank 1
> map 1
> 4 a0000 0 7fe863a72000 1
> 4 a0000 10000 7fe863a62000 1
> run
> map_linear_vram_bank 0
> map 0
> map_linear_vram_bank 1
> map 1
> 4 a0000 0 7fe863a62000 1
> run
> map_linear_vram_bank 0
> map 0
> map_linear_vram_bank 1
> map 1
> run
>
> So we suddenly get out of sync and enter the guest with an unmapped vram
> segment. I takes a long time (in number of map changes) until the region
> becomes mapped again.

I'll try to reproduce. Yes, it looks like a bug in the core, perhaps in
the symmetric-difference code.

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